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Adobe PDF Library 11.0; modified using iText® 5.5.0 ©2000-2013 iText Group NV (Safari Books Online, LLC; licensed version)


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HOLISTICA Vol 10, Issue 2, 2019, pp. 115-128                                      DOI:10.2478/hjbpa-2019-0020 
 
Cyber Security 
 
Rohit, KALAKUNTLA, 
Information Technology, University of the Cumberlands, USA 
rkalakuntla2612@ucumberlands.edu 
Anvesh Babu, VANAMALA, 
Information Technology, University of the Cumberlands, USA 
avanamala1554@ucumberlands.edu 
Ranjith Reddy, KOLIPYAKA, 
Information Technology, University of the Cumberlands 
rkolipyaka8395@ucumberlands.edu 
 
 
Abstract 
Cyber Security accepts a vigorous role in the area of information technology. Safeguarding the 
information has become an enormous problem in the current day. The cybersecurity the main thing 
that originates in mind is 'cyber crimes' which are aggregate colossally daily. Different governments 
and organizations are taking numerous measures to keep these cyber wrongdoings. Other than 
different measures cybersecurity is as yet a significant worry to many. This paper mostly emphases 
on cyber security and cyber terrorism. The significant trends of cybersecurity and the consequence 
of cybersecurity discuss in it.  The cyber-terrorism could make associations lose billions of dollars in 
the region of organizations. The paper also explains the components of cyber terrorism and 
motivation of it. Two case studies related to cybersecurity also provide in this paper. Some solution 
about cyber security and cyber terrorism also explain in it. 
 
Keywords: Cybersecurity; cyberspace; cyber terrorism; Information security. 
 
 
1. Introduction 
Today an individual can receive and send any information may be video, or 
an email or only through the click of a button but did s/he ever ponder how safe 
this information transmitted to another individual strongly with no spillage of 
data? The proper response lies in cybersecurity. Today more than 61% of full 
industry exchanges are done on the internet, so this area prerequisite high quality 
of security for direct and best exchanges. Thus, cybersecurity has become a most 
recent issue (Dervojeda, et. all., 2014). The extent of cybersecurity does not 
merely restrict to verifying the data in IT industry yet also to different fields like 
cyberspace and so forth. Improving cybersecurity and ensuring that necessary 
data systems are vital to each country's security and financial prosperity. 
Unauthentifiziert   | Heruntergeladen  01.09.19 21:18   UTC


key: chroma:document
string_value:
HOLISTICA Vol 10, Issue 2, 2019, pp. 115-128                                      DOI:10.2478/hjbpa-2019-0020 
 
Cyber Security 
 
Rohit, KALAKUNTLA, 
Information Technology, University of the Cumberlands, USA 
rkalakuntla2612@ucumberlands.edu 
Anvesh Babu, VANAMALA, 
Information Technology, University of the Cumberlands, USA 
avanamala1554@ucumberlands.edu


key: chroma:document
string_value:
Ranjith Reddy, KOLIPYAKA, 
Information Technology, University of the Cumberlands 
rkolipyaka8395@ucumberlands.edu 
 
 
Abstract 
Cyber Security accepts a vigorous role in the area of information technology. Safeguarding the 
information has become an enormous problem in the current day. The cybersecurity the main thing


key: chroma:document
string_value:
that originates in mind is 'cyber crimes' which are aggregate colossally daily. Different governments 
and organizations are taking numerous measures to keep these cyber wrongdoings. Other than 
different measures cybersecurity is as yet a significant worry to many. This paper mostly emphases


key: chroma:document
string_value:
on cyber security and cyber terrorism. The significant trends of cybersecurity and the consequence 
of cybersecurity discuss in it.  The cyber-terrorism could make associations lose billions of dollars in 
the region of organizations. The paper also explains the components of cyber terrorism and


key: chroma:document
string_value:
motivation of it. Two case studies related to cybersecurity also provide in this paper. Some solution 
about cyber security and cyber terrorism also explain in it. 
 
Keywords: Cybersecurity; cyberspace; cyber terrorism; Information security. 
 
 
1. Introduction 
Today an individual can receive and send any information may be video, or


key: chroma:document
string_value:
an email or only through the click of a button but did s/he ever ponder how safe 
this information transmitted to another individual strongly with no spillage of 
data? The proper response lies in cybersecurity. Today more than 61% of full 
industry exchanges are done on the internet, so this area prerequisite high quality


key: chroma:document
string_value:
of security for direct and best exchanges. Thus, cybersecurity has become a most 
recent issue (Dervojeda, et. all., 2014). The extent of cybersecurity does not 
merely restrict to verifying the data in IT industry yet also to different fields like 
cyberspace and so forth. Improving cybersecurity and ensuring that necessary


key: chroma:document
string_value:
data systems are vital to each country's security and financial prosperity. 
Unauthentifiziert   | Heruntergeladen  01.09.19 21:18   UTC


key: full_page_content
string_value:
HOLISTICA Vol 10, Issue 2, 2019 
 
Creating the Internet safer (and safeguarding Internet clients) has become 
to be essential to the improvement of new management just as a legislative 
strategy. The encounter against cybercrime needs an extensive and more secure 
practice (Gross, Canetti & Vashdi, 2017). The particular estimates alone cannot 
keep any crime; it is essential that law authorization offices are allowable to 
investigation and indict cybercrime efficiently. Nowadays numerous countries and 
administrations are compelling strict rules on cyber safeties to keep the loss of 
some vital data. Each should be equipped on this cybersecurity and save 
themselves from these increasing cybercrimes. 
Cyber-security is both about the insecurity made by and through this new 
space and about the practices or procedures to make it (progressively) secure 
(Kumar, & Somani, 2018). It alludes to a lot of exercises and measures, both 
specialized and non-specialized, expected to ensure the bioelectrical condition 
and the information it contains and transports from all possible threats. This 
research aims to gather all the information and overview related to cyber-crime 
and provide the historical facts and perform reports on the analyzed data of 
different attacks reported everywhere in the last five years. Based on the analyzed 
information, we would like to provide all the countermeasures that organizations 
may undertake in order to ensure improved security that would support in 
defending the organizations from being attacked by the hackers and provide a 
cyber-security to avoid all risks. 
 
2. Purpose 
      The paper provides information about cyber security and cyber 
terrorism. It covers various information about these topics in its subsections. 
Trends of cybersecurity and the role of social media in cybersecurity define in this 
paper. The paper provides some necessary information about cyber terrorism. The 
components of “cyber terrorism” and the consequences of this terrorism also 
explain in this paper. There are some examples of case studies those related to 
cybersecurity. The paper also provides some solutions regarding cyber security 
and cyber terrorism. It provides some techniques for preventing cyber terrorism. 
The future study and scope of cybersecurity define in it.  
Cybersecurity has become a major concern over the last 10 year in the IT 
world. In the present world, everybody is facing a lot of problems with cyber-
crime. As hackers are hacking major sensitive information from government and 
some enterprise organizations the individuals are very much worried as cyber-
security assault can bring about everything from wholesale fraud, to blackmail big 
companies. They are many varieties of cyber-crimes emerging where everyone 
 
116 
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key: chroma:document
string_value:
HOLISTICA Vol 10, Issue 2, 2019 
 
Creating the Internet safer (and safeguarding Internet clients) has become 
to be essential to the improvement of new management just as a legislative 
strategy. The encounter against cybercrime needs an extensive and more secure 
practice (Gross, Canetti & Vashdi, 2017). The particular estimates alone cannot


key: chroma:document
string_value:
keep any crime; it is essential that law authorization offices are allowable to 
investigation and indict cybercrime efficiently. Nowadays numerous countries and 
administrations are compelling strict rules on cyber safeties to keep the loss of 
some vital data. Each should be equipped on this cybersecurity and save 
themselves from these increasing cybercrimes.


key: chroma:document
string_value:
Cyber-security is both about the insecurity made by and through this new 
space and about the practices or procedures to make it (progressively) secure 
(Kumar, & Somani, 2018). It alludes to a lot of exercises and measures, both 
specialized and non-specialized, expected to ensure the bioelectrical condition


key: chroma:document
string_value:
and the information it contains and transports from all possible threats. This 
research aims to gather all the information and overview related to cyber-crime 
and provide the historical facts and perform reports on the analyzed data of 
different attacks reported everywhere in the last five years. Based on the analyzed


key: chroma:document
string_value:
information, we would like to provide all the countermeasures that organizations 
may undertake in order to ensure improved security that would support in 
defending the organizations from being attacked by the hackers and provide a 
cyber-security to avoid all risks. 
 
2. Purpose 
      The paper provides information about cyber security and cyber


key: chroma:document
string_value:
terrorism. It covers various information about these topics in its subsections. 
Trends of cybersecurity and the role of social media in cybersecurity define in this 
paper. The paper provides some necessary information about cyber terrorism. The 
components of “cyber terrorism” and the consequences of this terrorism also


key: chroma:document
string_value:
explain in this paper. There are some examples of case studies those related to 
cybersecurity. The paper also provides some solutions regarding cyber security 
and cyber terrorism. It provides some techniques for preventing cyber terrorism. 
The future study and scope of cybersecurity define in it.  
Cybersecurity has become a major concern over the last 10 year in the IT


key: chroma:document
string_value:
world. In the present world, everybody is facing a lot of problems with cyber-
crime. As hackers are hacking major sensitive information from government and 
some enterprise organizations the individuals are very much worried as cyber-
security assault can bring about everything from wholesale fraud, to blackmail big


key: chroma:document
string_value:
companies. They are many varieties of cyber-crimes emerging where everyone 
 
116 
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key: full_page_content
string_value:
HOLISTICA Vol 10, Issue 2, 2019  
 
 
needs to be aware of the scams and they are different measures and tools which 
can be used for avoiding the cyber-crimes. Every organization wants to secure 
their confidential data from getting hacked.  Getting hacked is not just about losing 
the confidential data but losing the relationship with customers in the market 
(Bendovschi, 2015). 
The Internet is today's fastest growing infrastructure. In today's technical 
environment many new technologies are changing mankind. But due to these 
emerging technologies, we are unable to protect our private information in an 
efficient way, so the cyber-crimes are drastically increasing on daily basis. Majority 
of the transactions both commercial and personal are done using the means online 
transaction, so it is important to have an expertise who require a high quality of 
security maintaining a better transparency to everyone and having safer 
transactions. So cybersecurity is the latest issue. Advanced technologies like cloud 
services, mobiles, E-commerce, internet banking and many more they require a 
high standards and safer process of security. All the tools and technologies 
involved for these transactions hold the most sensitive and cruicial user 
information. So providing the necessary security to them is very important. 
Improving the cybersecurity and safeguarding the sensitive data and 
infrastructures are important to every countries top priority security 
(Panchanatham, 2015). 
 
3. Trends of Cyber Security  
     Cyber Security assumes a critical role in the area of data technology. 
Safeguarding the data have become the greatest difficulty in the current day. The 
cybersecurity the main thing that raids a chord is cybercrimes which are increasing 
tremendously step by step (Samuel, & Osman, 2014). Different administrations 
and organizations are taking many measures to keep these cybercrimes. 
Additional the different measures cybersecurity is as yet an enormous worry to 
numerous. Some main trends that are changing cybersecurity give as follows: 
 
3.1. 
Web servers 
The risk of assaults on web applications to separate information or to 
circulate malicious code perseveres. Cybercriminals convey their code using good 
web servers they have traded off. In any case, information taking attacks, a 
considerable lot of which get the deliberation of media, are also a significant risk. 
Currently, individuals need a more unusual accentuation on securing web servers 
 
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key: chroma:document
string_value:
HOLISTICA Vol 10, Issue 2, 2019  
 
 
needs to be aware of the scams and they are different measures and tools which 
can be used for avoiding the cyber-crimes. Every organization wants to secure 
their confidential data from getting hacked.  Getting hacked is not just about losing 
the confidential data but losing the relationship with customers in the market 
(Bendovschi, 2015).


key: chroma:document
string_value:
The Internet is today's fastest growing infrastructure. In today's technical 
environment many new technologies are changing mankind. But due to these 
emerging technologies, we are unable to protect our private information in an 
efficient way, so the cyber-crimes are drastically increasing on daily basis. Majority


key: chroma:document
string_value:
of the transactions both commercial and personal are done using the means online 
transaction, so it is important to have an expertise who require a high quality of 
security maintaining a better transparency to everyone and having safer 
transactions. So cybersecurity is the latest issue. Advanced technologies like cloud


key: chroma:document
string_value:
services, mobiles, E-commerce, internet banking and many more they require a 
high standards and safer process of security. All the tools and technologies 
involved for these transactions hold the most sensitive and cruicial user 
information. So providing the necessary security to them is very important. 
Improving the cybersecurity and safeguarding the sensitive data and


key: chroma:document
string_value:
infrastructures are important to every countries top priority security 
(Panchanatham, 2015). 
 
3. Trends of Cyber Security  
     Cyber Security assumes a critical role in the area of data technology. 
Safeguarding the data have become the greatest difficulty in the current day. The 
cybersecurity the main thing that raids a chord is cybercrimes which are increasing


key: chroma:document
string_value:
tremendously step by step (Samuel, & Osman, 2014). Different administrations 
and organizations are taking many measures to keep these cybercrimes. 
Additional the different measures cybersecurity is as yet an enormous worry to 
numerous. Some main trends that are changing cybersecurity give as follows: 
 
3.1. 
Web servers


key: chroma:document
string_value:
3.1. 
Web servers 
The risk of assaults on web applications to separate information or to 
circulate malicious code perseveres. Cybercriminals convey their code using good 
web servers they have traded off. In any case, information taking attacks, a 
considerable lot of which get the deliberation of media, are also a significant risk.


key: chroma:document
string_value:
Currently, individuals need a more unusual accentuation on securing web servers 
 
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key: full_page_content
string_value:
HOLISTICA Vol 10, Issue 2, 2019 
 
as well as web applications (Bendovschi, 2015). Web servers are mainly the pre-
eminent stage for these cybercriminals to take the information. Thus, one should 
reliably utilize an additional secure program, mainly amid vital exchanges all 
together not to fall as a quarry for these defilements. 
 
3.2. 
Mobile Networks 
The risk of assaults on web applications to separate information or to 
circulate malicious code perseveres. Cybercriminals convey their code using good 
web servers they have traded off. In any case, information taking attacks, a 
considerable lot of which get the deliberation of media, are also a significant risk. 
Currently, individuals need a more unusual accentuation on securing web servers 
as well as web applications (Bendovschi, 2015). Web servers are mainly the pre-
eminent stage for these cybercriminals to take the information. Thus, one should 
reliably utilize an additional secure program, mainly amid vital exchanges all 
together not to fall as a quarry for these defilements.  
 
3.3. 
Encryption 
It is the method toward encoding messages so programmers cannot 
scrutinize it. In encryption, the message is encoded by encryption, changing it into 
a stirred-up figure content. It commonly completes with the use of an “encryption 
key,” that demonstrates how the message is to encode. Encryption at the earliest 
reference point level secures information protection and its respectability 
(Sharma, 2012). Additional use of encryption obtains more problems in 
cybersecurity. Encryption is used to ensure the information in travel, for instance, 
the information being exchanged using systems (for example the Internet, online 
business), mobile phones, wireless radios and so on.  
 
3.4. 
ADP’s and targeted attacks 
Advanced Persistent Threat (APT) is a whole of the dimension of cybercrime 
ware. For quite a long time network security capacities. For example, IPS or web 
filtering have had a key influence in distinguishing such focused-on assaults 
(Bendovschi, 2015). As attackers become bolder and utilize increasingly dubious 
methods, network security must incorporate with other security benefits to 
identify assaults. Thus, one must recover our security procedures to counteract 
more dangers coming later on. Subsequently the above is a portion of the patterns 
 
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key: chroma:document
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HOLISTICA Vol 10, Issue 2, 2019 
 
as well as web applications (Bendovschi, 2015). Web servers are mainly the pre-
eminent stage for these cybercriminals to take the information. Thus, one should 
reliably utilize an additional secure program, mainly amid vital exchanges all 
together not to fall as a quarry for these defilements. 
 
3.2. 
Mobile Networks


key: chroma:document
string_value:
Mobile Networks 
The risk of assaults on web applications to separate information or to 
circulate malicious code perseveres. Cybercriminals convey their code using good 
web servers they have traded off. In any case, information taking attacks, a 
considerable lot of which get the deliberation of media, are also a significant risk.


key: chroma:document
string_value:
Currently, individuals need a more unusual accentuation on securing web servers 
as well as web applications (Bendovschi, 2015). Web servers are mainly the pre-
eminent stage for these cybercriminals to take the information. Thus, one should 
reliably utilize an additional secure program, mainly amid vital exchanges all 
together not to fall as a quarry for these defilements.


key: chroma:document
string_value:
3.3. 
Encryption 
It is the method toward encoding messages so programmers cannot 
scrutinize it. In encryption, the message is encoded by encryption, changing it into 
a stirred-up figure content. It commonly completes with the use of an “encryption 
key,” that demonstrates how the message is to encode. Encryption at the earliest


key: chroma:document
string_value:
reference point level secures information protection and its respectability 
(Sharma, 2012). Additional use of encryption obtains more problems in 
cybersecurity. Encryption is used to ensure the information in travel, for instance, 
the information being exchanged using systems (for example the Internet, online 
business), mobile phones, wireless radios and so on.  
 
3.4.


key: chroma:document
string_value:
3.4. 
ADP’s and targeted attacks 
Advanced Persistent Threat (APT) is a whole of the dimension of cybercrime 
ware. For quite a long time network security capacities. For example, IPS or web 
filtering have had a key influence in distinguishing such focused-on assaults 
(Bendovschi, 2015). As attackers become bolder and utilize increasingly dubious


key: chroma:document
string_value:
methods, network security must incorporate with other security benefits to 
identify assaults. Thus, one must recover our security procedures to counteract 
more dangers coming later on. Subsequently the above is a portion of the patterns 
 
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HOLISTICA Vol 10, Issue 2, 2019  
 
 
changing the essence of cybersecurity on the planet. The top network threats are 
showing in figure 1. 
 
Figure 1. Threats for Cyber Security 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
4. Role of Social Media in Cyber Security 
Social media has turned into a lifestyle for some individuals. We use it to stay 
in contact, plan occasions, share our photographs and comment on recent 
developments. It has replaced email and telephone requires a ton of us. However, 
similarly as with whatever else on the web, it is imperative to know about the 
dangers. PCs, cell phones, and different gadgets are priceless assets that furnish 
people of any age with the extraordinary capacity to connect and collaborate with 
whatever remains of the world. Individuals can do this in various ways, including 
the utilization of social media or networking sites.  
Courtesy of social media, people can share musings, pictures, exercises, or 
any part of their lives (Gross, Canetti & Vashdi, 2017). They can bring an unknown 
look into the lives of others, regardless of whether they live nearby or over the 
globe. Unfortunately, these networks additionally represent security toward one's 
PC, protection, and even their security. Social media collection among faculty is 
soaring as is the risk of assault (Sharma, 2012). Since social media sites are nearly 
utilized by the majority of them reliably, it has become an excellent stage for 
cybercriminals for hacking private data and taking significant data. 
 
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HOLISTICA Vol 10, Issue 2, 2019  
 
 
changing the essence of cybersecurity on the planet. The top network threats are 
showing in figure 1. 
 
Figure 1. Threats for Cyber Security 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
4. Role of Social Media in Cyber Security 
Social media has turned into a lifestyle for some individuals. We use it to stay


key: chroma:document
string_value:
in contact, plan occasions, share our photographs and comment on recent 
developments. It has replaced email and telephone requires a ton of us. However, 
similarly as with whatever else on the web, it is imperative to know about the 
dangers. PCs, cell phones, and different gadgets are priceless assets that furnish


key: chroma:document
string_value:
people of any age with the extraordinary capacity to connect and collaborate with 
whatever remains of the world. Individuals can do this in various ways, including 
the utilization of social media or networking sites.  
Courtesy of social media, people can share musings, pictures, exercises, or 
any part of their lives (Gross, Canetti & Vashdi, 2017). They can bring an unknown


key: chroma:document
string_value:
look into the lives of others, regardless of whether they live nearby or over the 
globe. Unfortunately, these networks additionally represent security toward one's 
PC, protection, and even their security. Social media collection among faculty is 
soaring as is the risk of assault (Sharma, 2012). Since social media sites are nearly


key: chroma:document
string_value:
utilized by the majority of them reliably, it has become an excellent stage for 
cybercriminals for hacking private data and taking significant data. 
 
119 
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key: full_page_content
string_value:
HOLISTICA Vol 10, Issue 2, 2019 
 
The organizations need to assure they are likewise as fast in recognizing 
dangers, reacting increasingly, and keeping away from a rupture of any sort. 
Subsequently, individuals must take suitable measures particularly in managing 
social media to keep the loss of their data. The capacity of persons to impart data 
to a group of persons of millions is at the core of the exact test that social media 
offerings to organizations (Cabaj, Kotulski, Księżopolski, & Mazurczyk, 2018). 
Nevertheless, enabling anyone to disperse financially delicate data, social media 
additionally gives a comparable ability to range false data. It can be merely being 
as harming. The rapid spread of incorrect information by social media is among 
the growing dangers. Though social media can utilize for cybercrimes, these 
organizations cannot stand to quit utilizing social media as it assumes an essential 
role in the attention of an organization. In its place, they should have 
arrangements that will inform them of the risk to fix it before any actual harm is 
done Dervojeda, Verzijl, Nagtegaal, Lengton, & Rouwmaat, 2014). Anyway, 
organizations should understand this and observe the meaning of breaking down 
the data chiefly in social deliberations and give good security plans to avoid 
dangers. One must contract with social media by using specific plans and the right 
technologies. 
 
5. Cyber Terrorism 
The term “terrorism” can allude to the illegal utilization of power or 
viciousness against people in order to threaten an administration or its residents 
and associations which might be to accomplish a political or a malicious site [10]. 
Terrorism has transformed from the conventional structure to the cyber type of 
innovation supported terrorism recognized as cyber terrorism. It stays vital issues 
of the present society. Not just that the battle against terrorism is falling behind, 
current cybercrime assaults are ending up progressively forceful and 
confrontational (Sharma, 2012). This terrorism is the utilization of cyber word to 
dispatch an assault to the essential foundations that the presence of associations 
and countries entirely depended after that can prompt its shut down.  
 
5.1. 
Components of Cyber Terrorism 
A few attacks as cyber terrorism have a few parts which have been 
distinguished by numerous observational researchers in the exploration network. 
As indicated by Samuel and Osman (2014) in their hypothetical model recognize 
the five sections that a “cyber-terrorism” classified they are; the objective of the 
violence, inspiration and dedication towards the mission to be accomplished when 
such incident takes place, impact, instruments are utilized to dispatch such assault 
 
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HOLISTICA Vol 10, Issue 2, 2019 
 
The organizations need to assure they are likewise as fast in recognizing 
dangers, reacting increasingly, and keeping away from a rupture of any sort. 
Subsequently, individuals must take suitable measures particularly in managing 
social media to keep the loss of their data. The capacity of persons to impart data


key: chroma:document
string_value:
to a group of persons of millions is at the core of the exact test that social media 
offerings to organizations (Cabaj, Kotulski, Księżopolski, & Mazurczyk, 2018). 
Nevertheless, enabling anyone to disperse financially delicate data, social media 
additionally gives a comparable ability to range false data. It can be merely being


key: chroma:document
string_value:
as harming. The rapid spread of incorrect information by social media is among 
the growing dangers. Though social media can utilize for cybercrimes, these 
organizations cannot stand to quit utilizing social media as it assumes an essential 
role in the attention of an organization. In its place, they should have


key: chroma:document
string_value:
arrangements that will inform them of the risk to fix it before any actual harm is 
done Dervojeda, Verzijl, Nagtegaal, Lengton, & Rouwmaat, 2014). Anyway, 
organizations should understand this and observe the meaning of breaking down 
the data chiefly in social deliberations and give good security plans to avoid


key: chroma:document
string_value:
dangers. One must contract with social media by using specific plans and the right 
technologies. 
 
5. Cyber Terrorism 
The term “terrorism” can allude to the illegal utilization of power or 
viciousness against people in order to threaten an administration or its residents 
and associations which might be to accomplish a political or a malicious site [10].


key: chroma:document
string_value:
Terrorism has transformed from the conventional structure to the cyber type of 
innovation supported terrorism recognized as cyber terrorism. It stays vital issues 
of the present society. Not just that the battle against terrorism is falling behind, 
current cybercrime assaults are ending up progressively forceful and


key: chroma:document
string_value:
confrontational (Sharma, 2012). This terrorism is the utilization of cyber word to 
dispatch an assault to the essential foundations that the presence of associations 
and countries entirely depended after that can prompt its shut down.  
 
5.1. 
Components of Cyber Terrorism 
A few attacks as cyber terrorism have a few parts which have been


key: chroma:document
string_value:
distinguished by numerous observational researchers in the exploration network. 
As indicated by Samuel and Osman (2014) in their hypothetical model recognize 
the five sections that a “cyber-terrorism” classified they are; the objective of the 
violence, inspiration and dedication towards the mission to be accomplished when


key: chroma:document
string_value:
such incident takes place, impact, instruments are utilized to dispatch such assault 
 
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HOLISTICA Vol 10, Issue 2, 2019  
 
 
and attacking’s, area which is nature just as the strategy for activity. It can 
confidently know by knowing the profile of activities that drive the actions of the 
culprits (Kumar, & Somani, 2018).  
The critical issue in “cyber terrorism” is the motivation to complete such an 
action on the Internet, that outcomes in savagery/damage to people and their 
property Dervojeda, Verzijl, Nagtegaal,  Lengton, & Rouwmaat, 2014). It is by a 
portion of the segments. The terrorists of the world proceed the upside of the 
cyber world with solid incentive as a stage with which they can use to dispatch 
more unusual outbreak. Yunos and Ahmad (2014) said that with the utilization of 
Information and correspondence innovation, a terrorist could present more 
noteworthy harms or exact the republic with troublesome conditions because of 
the interruption of necessary administrations that the “cyberspace terrorist” 
causes more damage and devastation by the cyberspace than done the 
conventional strategy for terrorism. 
 
5.2. 
Motivating Factor of Cyber Terrorism 
The motivating factors of cyber terrorism give as follows: 
5.2.1. Websites’ Supportive Nature: 
The internet has viewed as a medium that is exceptionally tremendous, and 
that can in the meantime draw in light of a legitimate concern for some individuals 
to join some group of interest. The cyberterrorist prefers the utilization of the 
website as a result of its robust nature in that it can refer a message to a great 
many individuals inside a twinkle of an eye; they consider it to be a stage that is 
anything but difficult to select absorbed individuals.  
5.2.2. Anonymity Nature of the internet: 
Anonymity is the pivotal element that each evil culprit leans towards with 
the goal that their character could not be recognizable after playing out their 
devilish act.  The Internet is a sheltered domain just as concealing stage for the 
terrorist as they can stay unknown so that their personality cannot be known. 
5.2.3. Hacking: 
The overall term of all kinds of unapproved access to any "computer system" 
network organize is hacking that can occur in any structure all things measured as 
"cyber murder." A large number of these hackers make use of a "brute force‟ 
which is the combinations of every single imaginable letter just as numbers and 
images till they get the password Sreenu, & Krishna, 2017). 
 
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and attacking’s, area which is nature just as the strategy for activity. It can 
confidently know by knowing the profile of activities that drive the actions of the 
culprits (Kumar, & Somani, 2018).  
The critical issue in “cyber terrorism” is the motivation to complete such an


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action on the Internet, that outcomes in savagery/damage to people and their 
property Dervojeda, Verzijl, Nagtegaal,  Lengton, & Rouwmaat, 2014). It is by a 
portion of the segments. The terrorists of the world proceed the upside of the 
cyber world with solid incentive as a stage with which they can use to dispatch


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more unusual outbreak. Yunos and Ahmad (2014) said that with the utilization of 
Information and correspondence innovation, a terrorist could present more 
noteworthy harms or exact the republic with troublesome conditions because of 
the interruption of necessary administrations that the “cyberspace terrorist” 
causes more damage and devastation by the cyberspace than done the


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conventional strategy for terrorism. 
 
5.2. 
Motivating Factor of Cyber Terrorism 
The motivating factors of cyber terrorism give as follows: 
5.2.1. Websites’ Supportive Nature: 
The internet has viewed as a medium that is exceptionally tremendous, and 
that can in the meantime draw in light of a legitimate concern for some individuals


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to join some group of interest. The cyberterrorist prefers the utilization of the 
website as a result of its robust nature in that it can refer a message to a great 
many individuals inside a twinkle of an eye; they consider it to be a stage that is 
anything but difficult to select absorbed individuals.  
5.2.2. Anonymity Nature of the internet:


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Anonymity is the pivotal element that each evil culprit leans towards with 
the goal that their character could not be recognizable after playing out their 
devilish act.  The Internet is a sheltered domain just as concealing stage for the 
terrorist as they can stay unknown so that their personality cannot be known. 
5.2.3. Hacking:


key: chroma:document
string_value:
5.2.3. Hacking: 
The overall term of all kinds of unapproved access to any "computer system" 
network organize is hacking that can occur in any structure all things measured as 
"cyber murder." A large number of these hackers make use of a "brute force‟ 
which is the combinations of every single imaginable letter just as numbers and


key: chroma:document
string_value:
images till they get the password Sreenu, & Krishna, 2017). 
 
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5.2.4. Computer Viruses: 
These viruses are here and there scattered on a system to in other to do 
hurtful exercises. These may be to fill in as an administrative agent, create 
information or even split down the system. 
5.2.5. Password Sniffing: 
The “Cyber terrorist” may use one of the technique such as password sniff 
as procedures to complete their “cyber-attack” on different countries and many 
big organizations to see their downfall and have control over their systems. The 
password sniffer is programming which uses to screen organize and in the 
meantime catch all the password that passes the system connector. 
 
5.3. 
Consequences of “Cyber Terrorism” 
Cyber terrorism is an original type of cyber danger and attack that has many 
outcomes connected to it when propelled against any countries and associations. 
Some consequences of cyber terrorism define as follows: 
5.3.1. Data Intrusion:  
The cyber terrorism can annihilate information honesty with the goal that 
the information could never again be trusted, pulverizing its classification as 
intruding on its accessibility. The expanding rate of this cyber terrorism in 
encroaching associations and country's information has produced a ton of 
difficulties which has come about in loss of vitals and critical information that is 
typically difficult to recover (Sutton, 2017). 
5.3.2. The attack on Businesses:   
The cyber-terrorism could make associations lose billions of dollars in the 
region of organizations. The data arrangement of a bank can be attacked or hack 
through the terrorists who will prompt unapproved access to such financial 
balance and make them lose gigantic millions of dollars which can create such 
bank to keep running into bankruptcy (Gade, & Reddy, 2014). 
5.3.3. Loss of Life:  
Cyber terrorism has guaranteed many acquitted lives and in the meantime 
render numerous homes to a condition of the problem that is occasionally coming 
about to mental injury to the influenced families. The “cyber-terrorism” can in one 
method or alternate prompts the death toll just as causing severe harms. It has 
shown in an attack on the PCs utilization, networks’ as well as attacks that have 
 
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5.2.4. Computer Viruses: 
These viruses are here and there scattered on a system to in other to do 
hurtful exercises. These may be to fill in as an administrative agent, create 
information or even split down the system. 
5.2.5. Password Sniffing: 
The “Cyber terrorist” may use one of the technique such as password sniff


key: chroma:document
string_value:
as procedures to complete their “cyber-attack” on different countries and many 
big organizations to see their downfall and have control over their systems. The 
password sniffer is programming which uses to screen organize and in the 
meantime catch all the password that passes the system connector. 
 
5.3. 
Consequences of “Cyber Terrorism”


key: chroma:document
string_value:
Cyber terrorism is an original type of cyber danger and attack that has many 
outcomes connected to it when propelled against any countries and associations. 
Some consequences of cyber terrorism define as follows: 
5.3.1. Data Intrusion:  
The cyber terrorism can annihilate information honesty with the goal that


key: chroma:document
string_value:
the information could never again be trusted, pulverizing its classification as 
intruding on its accessibility. The expanding rate of this cyber terrorism in 
encroaching associations and country's information has produced a ton of 
difficulties which has come about in loss of vitals and critical information that is 
typically difficult to recover (Sutton, 2017).


key: chroma:document
string_value:
5.3.2. The attack on Businesses:   
The cyber-terrorism could make associations lose billions of dollars in the 
region of organizations. The data arrangement of a bank can be attacked or hack 
through the terrorists who will prompt unapproved access to such financial 
balance and make them lose gigantic millions of dollars which can create such


key: chroma:document
string_value:
bank to keep running into bankruptcy (Gade, & Reddy, 2014). 
5.3.3. Loss of Life:  
Cyber terrorism has guaranteed many acquitted lives and in the meantime 
render numerous homes to a condition of the problem that is occasionally coming 
about to mental injury to the influenced families. The “cyber-terrorism” can in one


key: chroma:document
string_value:
method or alternate prompts the death toll just as causing severe harms. It has 
shown in an attack on the PCs utilization, networks’ as well as attacks that have 
 
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come about to the different types of blasts of a few plane accidents issues 
everywhere throughout the world which that has asserted numerous life (Cabaj, 
Kotulski, Księżopolski,  & Mazurczyk, 2018). 
5.3.4. Consumer Trust in Doubt: 
The development of any organizations and its support relies upon the trust 
that its purchaser has on such association as trust can see tools that fortify 
association and certainty among associations and clients.  
 
6. Case Study Examples 
6.1. 
Cyber Security in E-Governance case study 
E-Governance is the extension of the efforts completed through the 
governments to recover relations with their nationals. With its instilled 
straightforwardness and receptiveness, given the standards of the Internet, E-
Governance conveys governments all the more near their residents. Existing and 
potential dangers in the circle of cybersecurity are among the most genuine 
difficulties of the 21st century. To ensure E-Governance extends there is a 
requirement for data security best practices (Hua, & Bapna, 2013). Security 
policies, practices, and techniques must be set up just as the use of security 
technology. It helps to ensure e-Government systems against attack, recognizes 
great exercises administrations and to have a demonstrated alternate course of 
action set up. An open private organization is a vital part of cybersecurity in E-
Governance. These associations can conveniently go up against coordination 
issues. Powerful cyber-crime prevention and arraignment activities in all the ICT 
appropriate conditions. 
 
6.2. 
Kaspersky Kidnapping Case 
The “highest-profile” cyber surveillance, stalking, and kidnapping case 
included Ivan Kaspersky, child of the administrator and CEO of Russia-based 
Kaspersky Lab, a standout amongst the most unmistakable cybersecurity firms on 
the planet. Ivan Kaspersky was abducted for payoff in 2011 while strolling to work 
from his Moscow loft. As indicated by Russian media sources, beginners – a more 
seasoned obligated couple – organized the plot and enrolled their child and two 
of his companions as "muscle" for the plot (Cabaj, Kotulski, Księżopolski,  & 
Mazurczyk, 2018). The abductors stalked Kaspersky and his sweetheart for a while 
preceding the seizing, deciding his conduct standards and finding that he did not 
 
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HOLISTICA Vol 10, Issue 2, 2019  
 
 
come about to the different types of blasts of a few plane accidents issues 
everywhere throughout the world which that has asserted numerous life (Cabaj, 
Kotulski, Księżopolski,  & Mazurczyk, 2018). 
5.3.4. Consumer Trust in Doubt: 
The development of any organizations and its support relies upon the trust


key: chroma:document
string_value:
that its purchaser has on such association as trust can see tools that fortify 
association and certainty among associations and clients.  
 
6. Case Study Examples 
6.1. 
Cyber Security in E-Governance case study 
E-Governance is the extension of the efforts completed through the 
governments to recover relations with their nationals. With its instilled


key: chroma:document
string_value:
straightforwardness and receptiveness, given the standards of the Internet, E-
Governance conveys governments all the more near their residents. Existing and 
potential dangers in the circle of cybersecurity are among the most genuine 
difficulties of the 21st century. To ensure E-Governance extends there is a


key: chroma:document
string_value:
requirement for data security best practices (Hua, & Bapna, 2013). Security 
policies, practices, and techniques must be set up just as the use of security 
technology. It helps to ensure e-Government systems against attack, recognizes 
great exercises administrations and to have a demonstrated alternate course of


key: chroma:document
string_value:
action set up. An open private organization is a vital part of cybersecurity in E-
Governance. These associations can conveniently go up against coordination 
issues. Powerful cyber-crime prevention and arraignment activities in all the ICT 
appropriate conditions. 
 
6.2. 
Kaspersky Kidnapping Case 
The “highest-profile” cyber surveillance, stalking, and kidnapping case


key: chroma:document
string_value:
included Ivan Kaspersky, child of the administrator and CEO of Russia-based 
Kaspersky Lab, a standout amongst the most unmistakable cybersecurity firms on 
the planet. Ivan Kaspersky was abducted for payoff in 2011 while strolling to work 
from his Moscow loft. As indicated by Russian media sources, beginners – a more


key: chroma:document
string_value:
seasoned obligated couple – organized the plot and enrolled their child and two 
of his companions as "muscle" for the plot (Cabaj, Kotulski, Księżopolski,  & 
Mazurczyk, 2018). The abductors stalked Kaspersky and his sweetheart for a while 
preceding the seizing, deciding his conduct standards and finding that he did not 
 
123


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HOLISTICA Vol 10, Issue 2, 2019 
 
have a protective security detail. The hijackers supposedly acquired all the 
required data from Kaspersky's client profile on Vkontakte, a famous Russian 
social systems administration site. Kaspersky was compelled to call his dad to 
transfer the payoff requests (Gade, & Reddy, 2014). The abductors may have 
utilized similar wireless to make food deliveries or had geolocation administrations 
empowered. 
 
6.3. 
Uber Case study 
Data breaches happen every day, in too many places, but the risk of data 
breach doesn't necessarily depend on the number, it may also depend on the risk 
and damage it causes the company's revenue and impact on the users or account 
holders, one of the biggest recent data breaches is Uber. 
6.3.1. Impact:  
One of the recent major cyber-attack is data breach of personal information 
of around 57million Uber users and 600,000 Uber drivers got revealed. 
 
6.3.2. Details:  
The worst part of this attack is how the Uber handled the issue, this is a 
lesson to most companies what not to do. In late 2016 just two hackers were able 
to steel the Users personal data with includes names, phone numbers and email 
addresses.  They were able to steal the 600,000 driver's license information. 
Hackers got access to the Uber's GitHub account through a third-party cloud-
based service. With the details found from the GitHub, Hackers found a way to 
access Uber user data in AWS. Ubers paid those two hackers $100, 000 to 
permanently destroy the whole data they obtained and not letting the users or 
the regulators about stolen information.  
But also, Uber confirmed that data was destroyed with the assurance they 
received from the hackers. According to US Law enforcement, any breach should 
be reported to the authorities and not pay hackers. And this kind of approach from 
Uber led other hackers to blackmail Netflix, where Hackers frightened to release 
TV shows unless the company paid the money hackers requested. Almost 49 states 
have this law enforcement where a security breach should be notified, after the 
court hearings Uber agreed to pay 20million to settle FTC charges. Not only the US 
but also other major countries like UK, Italy, and the Philippines reacted on this 
 
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HOLISTICA Vol 10, Issue 2, 2019 
 
have a protective security detail. The hijackers supposedly acquired all the 
required data from Kaspersky's client profile on Vkontakte, a famous Russian 
social systems administration site. Kaspersky was compelled to call his dad to 
transfer the payoff requests (Gade, & Reddy, 2014). The abductors may have


key: chroma:document
string_value:
utilized similar wireless to make food deliveries or had geolocation administrations 
empowered. 
 
6.3. 
Uber Case study 
Data breaches happen every day, in too many places, but the risk of data 
breach doesn't necessarily depend on the number, it may also depend on the risk 
and damage it causes the company's revenue and impact on the users or account


key: chroma:document
string_value:
holders, one of the biggest recent data breaches is Uber. 
6.3.1. Impact:  
One of the recent major cyber-attack is data breach of personal information 
of around 57million Uber users and 600,000 Uber drivers got revealed. 
 
6.3.2. Details:  
The worst part of this attack is how the Uber handled the issue, this is a


key: chroma:document
string_value:
lesson to most companies what not to do. In late 2016 just two hackers were able 
to steel the Users personal data with includes names, phone numbers and email 
addresses.  They were able to steal the 600,000 driver's license information. 
Hackers got access to the Uber's GitHub account through a third-party cloud-


key: chroma:document
string_value:
based service. With the details found from the GitHub, Hackers found a way to 
access Uber user data in AWS. Ubers paid those two hackers $100, 000 to 
permanently destroy the whole data they obtained and not letting the users or 
the regulators about stolen information.  
But also, Uber confirmed that data was destroyed with the assurance they


key: chroma:document
string_value:
received from the hackers. According to US Law enforcement, any breach should 
be reported to the authorities and not pay hackers. And this kind of approach from 
Uber led other hackers to blackmail Netflix, where Hackers frightened to release 
TV shows unless the company paid the money hackers requested. Almost 49 states


key: chroma:document
string_value:
have this law enforcement where a security breach should be notified, after the 
court hearings Uber agreed to pay 20million to settle FTC charges. Not only the US 
but also other major countries like UK, Italy, and the Philippines reacted on this 
 
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issue. Uber's breach is different from the regular breaches, the company tried to 
cover up the breach and not alert the authorities and the users. 
6.3.3. Uber's plan after the breach:  
Khrosrowshabi the new CEO of Uber received few disputed problems only 
with respect to its legal issue also criticism for sexual harassment, underpaying the 
drivers and few more. 
 
6.3.4. Solutions 
Some solutions regarding cyber security and cyber terrorism describe here: 
• Cyber Security Techniques 
Some techniques can use to improve cybersecurity.                                                                  
• 
Access control and “password security”: The idea of password and user 
name has a primary method for ensuring data. It may be the principal 
measures concerning cybersecurity. 
• 
Data’s Authentication:  The documents that we get should dependably be 
validated be before transferring. It should check if it has begun from a 
critical and dependable source and that they are not modified (Gade, & 
Reddy, 2014). Verifying of these records is typically done by the “anti-
virus” software present in the gadgets. Subsequently, a decent “anti-
virus” software is likewise necessary to shield the gadgets from viruses. 
• 
Anti-virus software: It is a PC program that classifies, avoids, and makes a 
move to harm or evacuate noxious software programs, for instance, 
viruses as well as worms. Most "antivirus programs" comprise an "auto-
update" feature that authorizes the program to download profiles of new 
viruses with the objective that it can chequer for the new viruses when 
they find. 
• 
Malware scanners: This is software that typically filters each of the 
records and archives current in the framework for vindictive code or 
destructive viruses [10]. Viruses, worms, as well as Trojan horses, are 
instances of “malicious software” that regularly assemble and alluded to 
as malware. 
• 
Firewall: A “software program” or an equipment that helps monitor 
hackers, infections, and all types of worms which endeavour to achieve 
PC over the Internet. All data which is transmitting to and fro over the 
web go through the firewall contemporary, which looks at every 
 
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issue. Uber's breach is different from the regular breaches, the company tried to 
cover up the breach and not alert the authorities and the users. 
6.3.3. Uber's plan after the breach:  
Khrosrowshabi the new CEO of Uber received few disputed problems only 
with respect to its legal issue also criticism for sexual harassment, underpaying the


key: chroma:document
string_value:
drivers and few more. 
 
6.3.4. Solutions 
Some solutions regarding cyber security and cyber terrorism describe here: 
• Cyber Security Techniques 
Some techniques can use to improve cybersecurity.                                                                  
• 
Access control and “password security”: The idea of password and user


key: chroma:document
string_value:
name has a primary method for ensuring data. It may be the principal 
measures concerning cybersecurity. 
• 
Data’s Authentication:  The documents that we get should dependably be 
validated be before transferring. It should check if it has begun from a 
critical and dependable source and that they are not modified (Gade, &


key: chroma:document
string_value:
Reddy, 2014). Verifying of these records is typically done by the “anti-
virus” software present in the gadgets. Subsequently, a decent “anti-
virus” software is likewise necessary to shield the gadgets from viruses. 
• 
Anti-virus software: It is a PC program that classifies, avoids, and makes a 
move to harm or evacuate noxious software programs, for instance,


key: chroma:document
string_value:
viruses as well as worms. Most "antivirus programs" comprise an "auto-
update" feature that authorizes the program to download profiles of new 
viruses with the objective that it can chequer for the new viruses when 
they find. 
• 
Malware scanners: This is software that typically filters each of the 
records and archives current in the framework for vindictive code or


key: chroma:document
string_value:
destructive viruses [10]. Viruses, worms, as well as Trojan horses, are 
instances of “malicious software” that regularly assemble and alluded to 
as malware. 
• 
Firewall: A “software program” or an equipment that helps monitor 
hackers, infections, and all types of worms which endeavour to achieve 
PC over the Internet. All data which is transmitting to and fro over the


key: chroma:document
string_value:
web go through the firewall contemporary, which looks at every 
 
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HOLISTICA Vol 10, Issue 2, 2019 
 
individual message and hinders which do not meet the security 
requirements and classify them as threat and try to block from the system 
and monitor the activities. Henceforth firewalls assume an essential job 
in recognizing the malware.  
6.4. 
Prevention of Cyber Terrorism 
The capacity to prevent cyber terrorism lies with the capacity to securely 
verify cyberspace. Cybersecurity has an intriguing parallel to terrorism. Both are 
lopsided. Guaranteeing the security of information, data, and correspondence is 
impressively harder than hacking into a framework. The attacker has an 
inalienable preferred standpoint in both regular terrorism and cyber-attacks. On 
account of state-supported attacks, the difficulties are of a lot higher greatness 
(Cabaj, Kotulski, Księżopolski, & Mazurczyk, 2018). Governments should guarantee 
that their rules smear to cybercrimes and be wholly actualized and hold fast to; it 
is essential that the countries of the biosphere take measures to guarantee that 
its punitive and technical law is satisfactory to address the difficulties presented 
by cybercrimes (Kumar, & Somani, 2018).  
The availability, confidentiality and the integrity of information in any 
associations are essential which endeavors must be set up to guarantee that they 
are exceptionally secure because it is the significant cyber resource that makes 
each association stand and in the meantime depended upon. The information has 
entered by the “cyber-terrorist” is something beyond records which may 
incorporate messages, web applications, web pages, and just as some 
indispensable operating systems. (Kumar, & Somani, 2018) 
 
7. Future Study and Scope 
      This paper will help to advance the scientific interests in the exploration 
of cybersecurity, particularly to respond to the procedural questions of the 
prediction of future data and actions significant to security patterns. This study 
sets the background to begin executing rules for all intentions as indicated through 
the usual security issues and answers for data systems. This paper consolidates 
many procedures connected and may be improved to serve cybersecurity 
regarding anticipating the operational legitimacy of the methodologies of 
assessment benchmarks. Finally, the emphasis on limiting, recouping, and 
disposing of weakness is the primary, basic patterns, and reactions to the constant 
expanding progress (Panchanatham, 2015). 
 
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individual message and hinders which do not meet the security 
requirements and classify them as threat and try to block from the system 
and monitor the activities. Henceforth firewalls assume an essential job 
in recognizing the malware.  
6.4. 
Prevention of Cyber Terrorism


key: chroma:document
string_value:
The capacity to prevent cyber terrorism lies with the capacity to securely 
verify cyberspace. Cybersecurity has an intriguing parallel to terrorism. Both are 
lopsided. Guaranteeing the security of information, data, and correspondence is 
impressively harder than hacking into a framework. The attacker has an


key: chroma:document
string_value:
inalienable preferred standpoint in both regular terrorism and cyber-attacks. On 
account of state-supported attacks, the difficulties are of a lot higher greatness 
(Cabaj, Kotulski, Księżopolski, & Mazurczyk, 2018). Governments should guarantee 
that their rules smear to cybercrimes and be wholly actualized and hold fast to; it


key: chroma:document
string_value:
is essential that the countries of the biosphere take measures to guarantee that 
its punitive and technical law is satisfactory to address the difficulties presented 
by cybercrimes (Kumar, & Somani, 2018).  
The availability, confidentiality and the integrity of information in any 
associations are essential which endeavors must be set up to guarantee that they


key: chroma:document
string_value:
are exceptionally secure because it is the significant cyber resource that makes 
each association stand and in the meantime depended upon. The information has 
entered by the “cyber-terrorist” is something beyond records which may 
incorporate messages, web applications, web pages, and just as some 
indispensable operating systems. (Kumar, & Somani, 2018)


key: chroma:document
string_value:
7. Future Study and Scope 
      This paper will help to advance the scientific interests in the exploration 
of cybersecurity, particularly to respond to the procedural questions of the 
prediction of future data and actions significant to security patterns. This study 
sets the background to begin executing rules for all intentions as indicated through


key: chroma:document
string_value:
the usual security issues and answers for data systems. This paper consolidates 
many procedures connected and may be improved to serve cybersecurity 
regarding anticipating the operational legitimacy of the methodologies of 
assessment benchmarks. Finally, the emphasis on limiting, recouping, and


key: chroma:document
string_value:
disposing of weakness is the primary, basic patterns, and reactions to the constant 
expanding progress (Panchanatham, 2015). 
 
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      Over the next five years, cyber-crime may create severe damage in 
information technology. According to the researchers they have estimated an 
approximate close to 6 trillion dollars loss. So, there would be a very bright scope 
for people who work and resolve the issues related to cyber-crime and provide all 
the necessary security measures. Big organizations like CISCO which is completely 
related to networking technology which is one of the top organization has 
approximately millions of openings related to cybersecurity because which is the 
future for the safety of Information technology. They are also wide opportunities 
in government-related fields and also defence field to save the countries secure 
data from cyber attackers. 
 
8. Conclusion 
      Cyber-security is both about the insecurity made by and through this new 
space and about the practices or procedures to make it (progressively) secure. 
Exertion to verify the cyberspace should give a definitive need else the 
"information technology" will not be viably used by clients. The terrorist of things 
to come will win the wars without discharging a shot just by crushing the country's 
necessary substructure if steps are not taken to handle the pervasiveness of the 
expansion in such a cyber-attack. They can bring an unknown look into the lives of 
others, regardless of whether they live nearby or over the globe.  
      The “cyber-terrorism” can in one method or alternate prompts the death 
toll just as causing severe harms. Though social media can utilize for cybercrimes, 
these organizations cannot stand to quit utilizing social media as it assumes an 
essential role in the attention of an organization. Cyber terrorism has guaranteed 
numerous innocent lives and in the meantime render numerous homes to a 
condition of the problem that is occasionally coming about to mental injury to the 
influenced families. Cyber terrorism stays vital issues of the present society. Not 
just that the battle against Cyber terrorism is falling behind, current cybercrime 
assaults are ending up progressively forceful and confrontational. Cybersecurity 
has an intriguing parallel to terrorism. Guaranteeing the security of information, 
data, and correspondence is impressively harder than hacking into a system. 
 
9. References 
[1] Bendovschi, A. (2015). Cyber-Attacks – Trends, Patterns and Security Countermeasures. 
Procedia Economics and Finance, 24-31. doi:10.1016/S2212-5671(15)01077- 
[2] Cabaj, K., Kotulski, Z., Księżopolski, B., & Mazurczyk, W. (2018). Cybersecurity: trends, 
issues, and challenges. EURASIP Journal on Information Security. doi:10.1186/s13635-
018-0080-0 
 
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      Over the next five years, cyber-crime may create severe damage in 
information technology. According to the researchers they have estimated an 
approximate close to 6 trillion dollars loss. So, there would be a very bright scope 
for people who work and resolve the issues related to cyber-crime and provide all


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the necessary security measures. Big organizations like CISCO which is completely 
related to networking technology which is one of the top organization has 
approximately millions of openings related to cybersecurity because which is the 
future for the safety of Information technology. They are also wide opportunities


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in government-related fields and also defence field to save the countries secure 
data from cyber attackers. 
 
8. Conclusion 
      Cyber-security is both about the insecurity made by and through this new 
space and about the practices or procedures to make it (progressively) secure. 
Exertion to verify the cyberspace should give a definitive need else the


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"information technology" will not be viably used by clients. The terrorist of things 
to come will win the wars without discharging a shot just by crushing the country's 
necessary substructure if steps are not taken to handle the pervasiveness of the 
expansion in such a cyber-attack. They can bring an unknown look into the lives of


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others, regardless of whether they live nearby or over the globe.  
      The “cyber-terrorism” can in one method or alternate prompts the death 
toll just as causing severe harms. Though social media can utilize for cybercrimes, 
these organizations cannot stand to quit utilizing social media as it assumes an


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essential role in the attention of an organization. Cyber terrorism has guaranteed 
numerous innocent lives and in the meantime render numerous homes to a 
condition of the problem that is occasionally coming about to mental injury to the 
influenced families. Cyber terrorism stays vital issues of the present society. Not


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just that the battle against Cyber terrorism is falling behind, current cybercrime 
assaults are ending up progressively forceful and confrontational. Cybersecurity 
has an intriguing parallel to terrorism. Guaranteeing the security of information, 
data, and correspondence is impressively harder than hacking into a system. 
 
9. References


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9. References 
[1] Bendovschi, A. (2015). Cyber-Attacks – Trends, Patterns and Security Countermeasures. 
Procedia Economics and Finance, 24-31. doi:10.1016/S2212-5671(15)01077- 
[2] Cabaj, K., Kotulski, Z., Księżopolski, B., & Mazurczyk, W. (2018). Cybersecurity: trends, 
issues, and challenges. EURASIP Journal on Information Security. doi:10.1186/s13635-
018-0080-0 
 
127


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HOLISTICA Vol 10, Issue 2, 2019 
 
[3] Dervojeda, K., Verzijl, D., Nagtegaal, F., Lengton, M., & Rouwmaat, E. (2014). Innovative 
Business Models: Supply chain finance. Netherlands: Business Innovation Observatory; 
European Union. 
[4] Gade, N. R., & Reddy, U. G. (2014). A Study Of Cyber Security Challenges And Its Emerging 
Trends 
On 
Latest 
Technologies. 
Retrieved 
from 
https://www.researchgate.net/publication/260126665_A_Study_Of_Cyber_Security_Ch
allenges_And_Its_Emerging_Trends_On_Latest_Technologies 
[5] Gross, M. L., Canetti, D., & Vashdi, D. R. (2017). Cyberterrorism: its effects on 
psychological well-being, public confidence and political attitudes. Journal of 
Cybersecurity, 3(1), 49–58. doi:10.1093/cybsec/tyw018 
[6] Hua, J., & Bapna, S. (2013). The economic impact of cyber terrorism. The Journal of 
Strategic Information Systems, 22(2), pp. 175-186. 
[7] Kumar, S., & Somani, V. (2018). Social Media Security Risks, Cyber Threats And Risks 
Prevention And Mitigation Techniques. International Journal of Advance Research in 
Computer Science and Management, 4(4), pp. 125-129. 
[8] Panchanatham, D. N. (2015). A case study on Cyber Security in E-Governance. 
International Research Journal of Engineering and Technology. 
[9] Samuel, K. O., & Osman, W. R. (2014). Cyber Terrorism Attack of The Contemporary 
Information Technology Age: Issues, Consequences and Panacea. International Journal of 
Computer Science and Mobile Computing, 3(5), pp. 1082-1090. 
[10] Sharma, R. (2012). Study of Latest Emerging Trends on Cyber Security and its challenges 
to Society. International Journal of Scientific & Engineering Research, 3(6). 
[11] Sreenu, M., & Krishna, D. V. (2017). A General Study on Cyber-Attacks on Social Networks. 
IOSR Journal of Computer Engineering (IOSR-JCE), 19(5), pp. 01-04. 
[12] Sutton, D. (2017). Cyber Security : A Practitioner’s Guide. Swindon, UK: BCS, the Chartered 
Institute for IT.  
 
128 
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HOLISTICA Vol 10, Issue 2, 2019 
 
[3] Dervojeda, K., Verzijl, D., Nagtegaal, F., Lengton, M., & Rouwmaat, E. (2014). Innovative 
Business Models: Supply chain finance. Netherlands: Business Innovation Observatory; 
European Union. 
[4] Gade, N. R., & Reddy, U. G. (2014). A Study Of Cyber Security Challenges And Its Emerging 
Trends 
On 
Latest 
Technologies. 
Retrieved 
from


key: chroma:document
string_value:
Retrieved 
from 
https://www.researchgate.net/publication/260126665_A_Study_Of_Cyber_Security_Ch
allenges_And_Its_Emerging_Trends_On_Latest_Technologies 
[5] Gross, M. L., Canetti, D., & Vashdi, D. R. (2017). Cyberterrorism: its effects on 
psychological well-being, public confidence and political attitudes. Journal of 
Cybersecurity, 3(1), 49–58. doi:10.1093/cybsec/tyw018


key: chroma:document
string_value:
[6] Hua, J., & Bapna, S. (2013). The economic impact of cyber terrorism. The Journal of 
Strategic Information Systems, 22(2), pp. 175-186. 
[7] Kumar, S., & Somani, V. (2018). Social Media Security Risks, Cyber Threats And Risks 
Prevention And Mitigation Techniques. International Journal of Advance Research in 
Computer Science and Management, 4(4), pp. 125-129.


key: chroma:document
string_value:
[8] Panchanatham, D. N. (2015). A case study on Cyber Security in E-Governance. 
International Research Journal of Engineering and Technology. 
[9] Samuel, K. O., & Osman, W. R. (2014). Cyber Terrorism Attack of The Contemporary 
Information Technology Age: Issues, Consequences and Panacea. International Journal of 
Computer Science and Mobile Computing, 3(5), pp. 1082-1090.


key: chroma:document
string_value:
[10] Sharma, R. (2012). Study of Latest Emerging Trends on Cyber Security and its challenges 
to Society. International Journal of Scientific & Engineering Research, 3(6). 
[11] Sreenu, M., & Krishna, D. V. (2017). A General Study on Cyber-Attacks on Social Networks. 
IOSR Journal of Computer Engineering (IOSR-JCE), 19(5), pp. 01-04.


key: chroma:document
string_value:
[12] Sutton, D. (2017). Cyber Security : A Practitioner’s Guide. Swindon, UK: BCS, the Chartered 
Institute for IT.  
 
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1 
 
 
SCHOOL OF COMPUTING 
DEPARTMENT OF INFORMATION TECHNOLOGY 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
UNIT – I – Cyber Security – SITA1602


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2 
 
I. INTRODUCTION 
Basic Concepts – Security Architecture, Attacks, Services, Mechanisms, Model - Cryptography 
Basics - Symmetric Ciphers – Transposition, Substitution, Rotor Machines – Block Cipher – Data 
Encryption Standard – Confidentiality using Symmetric Encryption 
 
1.1 Introduction to Cyber Security 
• 
Cyber Security is a process that's designed to protect networks and devices from external 
threats. Businesses typically employ Cyber Security professionals to protect their confidential 
information, maintain employee productivity, and enhance customer confidence in products 
and services. Cyber security is the protection of Internet-connected systems, including 
hardware, software, and data from cyber-attacks.  
• 
It is made up of two words one is cyber and other is security. 
• 
Cyber is related to the technology which contains systems, network and programs or data. 
• 
Whereas security related to the protection which includes systems security, network security 
and application and information security. 
1.2 Need of Cyber Security 
• 
Cyber security becomes so important in our predominant digital world 
• 
Cyber-attacks can be extremely expensive for businesses to endure.  
• 
In addition to financial damage suffered by the business, a data breach can also inflict untold 
reputational damage.   
• 
Cyber-attacks these days are becoming progressively destructive. Cybercriminals are using 
more sophisticated ways to initiate cyber-attacks. 
• 
Regulations such as GDPR are forcing organizations into taking better care of the personal 
data they hold. 
1.3 Security Goals


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2 
 
I. INTRODUCTION 
Basic Concepts – Security Architecture, Attacks, Services, Mechanisms, Model - Cryptography 
Basics - Symmetric Ciphers – Transposition, Substitution, Rotor Machines – Block Cipher – Data 
Encryption Standard – Confidentiality using Symmetric Encryption 
 
1.1 Introduction to Cyber Security 
•


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• 
Cyber Security is a process that's designed to protect networks and devices from external 
threats. Businesses typically employ Cyber Security professionals to protect their confidential 
information, maintain employee productivity, and enhance customer confidence in products 
and services. Cyber security is the protection of Internet-connected systems, including


key: chroma:document
string_value:
hardware, software, and data from cyber-attacks.  
• 
It is made up of two words one is cyber and other is security. 
• 
Cyber is related to the technology which contains systems, network and programs or data. 
• 
Whereas security related to the protection which includes systems security, network security 
and application and information security. 
1.2 Need of Cyber Security 
•


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• 
Cyber security becomes so important in our predominant digital world 
• 
Cyber-attacks can be extremely expensive for businesses to endure.  
• 
In addition to financial damage suffered by the business, a data breach can also inflict untold 
reputational damage.   
• 
Cyber-attacks these days are becoming progressively destructive. Cybercriminals are using


key: chroma:document
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more sophisticated ways to initiate cyber-attacks. 
• 
Regulations such as GDPR are forcing organizations into taking better care of the personal 
data they hold. 
1.3 Security Goals


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3 
 
 
Fig. 1 Security Goals 
1.3.1 Confidentiality 
• 
Confidentiality is about preventing the disclosure of data to unauthorized parties. 
• 
 It also means trying to keep the identity of authorized parties involved in sharing and holding 
data private and anonymous. 
• 
Often confidentiality is compromised by cracking poorly encrypted data, Man-in-the-middle 
(MITM) attacks, disclosing sensitive data 
• 
Standard measures to establish confidentiality include: 
• 
Data encryption  
• 
Two-factor authentication 
• 
Biometric verification  
• 
Security tokens  
1.3.2 Integrity 
• Integrity refers to protecting information from being modified by unauthorized parties. 
Standard measures to guarantee integrity include:  
- 
Cryptographic checksums  
- 
Using file permissions  
- 
Uninterrupted power supplies  
- 
Data backups  
1.3.3 Availability 
• 
Availability is making sure that authorized parties are able to access the information when 
needed.  
o Standard measures to guarantee availability include:  
Security Goals
Confidentiality
Integrity
Availability


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3 
 
 
Fig. 1 Security Goals 
1.3.1 Confidentiality 
• 
Confidentiality is about preventing the disclosure of data to unauthorized parties. 
• 
 It also means trying to keep the identity of authorized parties involved in sharing and holding 
data private and anonymous. 
• 
Often confidentiality is compromised by cracking poorly encrypted data, Man-in-the-middle


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(MITM) attacks, disclosing sensitive data 
• 
Standard measures to establish confidentiality include: 
• 
Data encryption  
• 
Two-factor authentication 
• 
Biometric verification  
• 
Security tokens  
1.3.2 Integrity 
• Integrity refers to protecting information from being modified by unauthorized parties. 
Standard measures to guarantee integrity include:  
-


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- 
Cryptographic checksums  
- 
Using file permissions  
- 
Uninterrupted power supplies  
- 
Data backups  
1.3.3 Availability 
• 
Availability is making sure that authorized parties are able to access the information when 
needed.  
o Standard measures to guarantee availability include:  
Security Goals
Confidentiality
Integrity
Availability


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4 
 
o Backing up data to external drives 
o Implementing firewalls 
o Having backup power supplies  
o Data redundancy 
Table 1: CIA Comparison 
 
1.4 History of Cyber Security 
1940s: The time before crime 
• 
For nearly two decades after the creation of the world’s first digital computer in 1943, 
carrying out cyberattacks was tricky.  
• 
Access to the giant electronic machines was limited to small numbers of people and they 
weren’t networked. 
• 
Only a few people knew how to work them so the threat was almost non-existent. 
1950s: The phone phreaks 
• 
Fraudulent manipulation of telephone signaling in order to make free phone calls. 
1960s: All quiet on the Western Front 
1970s: Computer security is born 
• 
Cybersecurity proper began in 1972 with a research project on ARPANET (The Advanced 
Research Projects Agency Network), a precursor to the internet. 
• 
Researcher Bob Thomas created a computer program called Creeper that could move across 
ARPANET’s network, leaving a breadcrumb trail wherever it went.  
• 
It read: ‘I’m the creeper, catch me if you can’. Ray Tomlinson – the inventor of email – wrote 
the program Reaper, which chased and deleted Creeper.


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4 
 
o Backing up data to external drives 
o Implementing firewalls 
o Having backup power supplies  
o Data redundancy 
Table 1: CIA Comparison 
 
1.4 History of Cyber Security 
1940s: The time before crime 
• 
For nearly two decades after the creation of the world’s first digital computer in 1943, 
carrying out cyberattacks was tricky.  
•


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• 
Access to the giant electronic machines was limited to small numbers of people and they 
weren’t networked. 
• 
Only a few people knew how to work them so the threat was almost non-existent. 
1950s: The phone phreaks 
• 
Fraudulent manipulation of telephone signaling in order to make free phone calls. 
1960s: All quiet on the Western Front 
1970s: Computer security is born 
•


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• 
Cybersecurity proper began in 1972 with a research project on ARPANET (The Advanced 
Research Projects Agency Network), a precursor to the internet. 
• 
Researcher Bob Thomas created a computer program called Creeper that could move across 
ARPANET’s network, leaving a breadcrumb trail wherever it went.  
•


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• 
It read: ‘I’m the creeper, catch me if you can’. Ray Tomlinson – the inventor of email – wrote 
the program Reaper, which chased and deleted Creeper.


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5 
 
• 
 Reaper was not only the very first example of antivirus software, but it was also the first self-
replicating program, making it the first-ever computer worm. 
1980s: From ARPANET to internet 
• 
The 1980s brought an increase in high-profile attacks, including those at National CSS, 
AT&T, and Los Alamos National Laboratory 
• 
Despite this, in 1986, German hacker Marcus Hess used an internet gateway in Berkeley, CA, 
to piggyback onto the ARPANET. He hacked 400 military computers, including mainframes 
at the Pentagon, intending to sell information to the KGB. 
1987: The birth of cybersecurity 
• 
1987 was the birth year of commercial antivirus, although there are competing claims for the 
innovator of the first antivirus product. 
• 
Andreas Luning and Kai Figge released their first antivirus product for the Atari ST – which 
also 
saw 
the 
release 
of 
Ultimate 
Virus 
Killer 
(UVK) 
Three 
Czechoslovakians 
created 
the 
first 
version 
of NOD antivirus 
In the U.S., John McAfee founded McAfee (then part of Intel Security), and released 
VirusScan. 
1990s: The world goes online 
• 
The first polymorphic viruses were created (code that mutates while keeping the 
original algorithm intact to avoid detection) 
• 
British computer magazine PC Today released an edition with a free disc that ‘accidentally’ 
contained the DiskKiller virus, infecting tens of thousands of computers 
• 
EICAR (European Institute for Computer Antivirus Research) was established towards the 
end of the 1990s, email was proliferating and while it promised to revolutionize 
communication, it also opened up a new entry point for viruses. 
2000s: Threats diversify and multiply 
2010s: The next generation 
• 
2012: Saudi hacker 0XOMAR publishes the details of more than 400,000 credit cards online 
• 
2013: Former CIA employee for the US Government Edward Snowden copied and leaked 
classified information from the National Security Agency (NSA)


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5 
 
• 
 Reaper was not only the very first example of antivirus software, but it was also the first self-
replicating program, making it the first-ever computer worm. 
1980s: From ARPANET to internet 
• 
The 1980s brought an increase in high-profile attacks, including those at National CSS, 
AT&T, and Los Alamos National Laboratory 
•


key: chroma:document
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• 
Despite this, in 1986, German hacker Marcus Hess used an internet gateway in Berkeley, CA, 
to piggyback onto the ARPANET. He hacked 400 military computers, including mainframes 
at the Pentagon, intending to sell information to the KGB. 
1987: The birth of cybersecurity 
• 
1987 was the birth year of commercial antivirus, although there are competing claims for the


key: chroma:document
string_value:
innovator of the first antivirus product. 
• 
Andreas Luning and Kai Figge released their first antivirus product for the Atari ST – which 
also 
saw 
the 
release 
of 
Ultimate 
Virus 
Killer 
(UVK) 
Three 
Czechoslovakians 
created 
the 
first 
version 
of NOD antivirus 
In the U.S., John McAfee founded McAfee (then part of Intel Security), and released 
VirusScan.


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string_value:
VirusScan. 
1990s: The world goes online 
• 
The first polymorphic viruses were created (code that mutates while keeping the 
original algorithm intact to avoid detection) 
• 
British computer magazine PC Today released an edition with a free disc that ‘accidentally’ 
contained the DiskKiller virus, infecting tens of thousands of computers 
•


key: chroma:document
string_value:
• 
EICAR (European Institute for Computer Antivirus Research) was established towards the 
end of the 1990s, email was proliferating and while it promised to revolutionize 
communication, it also opened up a new entry point for viruses. 
2000s: Threats diversify and multiply 
2010s: The next generation 
•


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• 
2012: Saudi hacker 0XOMAR publishes the details of more than 400,000 credit cards online 
• 
2013: Former CIA employee for the US Government Edward Snowden copied and leaked 
classified information from the National Security Agency (NSA)


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6 
 
• 
2013-2014: Malicious hackers broke into Yahoo, compromising the accounts and personal 
information of its 3 billion users. Yahoo was subsequently fined $35 million for failing to 
disclose the news 
• 
2017: WannaCry ransomware infects 230,000 computers in one day 
• 
2019: Multiple DDoS attacks forced New Zealand's stock market to temporarily shut down 
• 
According to a security research firm, 81 global firms from 81 countries reported data 
breaches in the first half of 2020 alone.  
• 
80% of firms have seen an increase in cyber-attacks this year. Coronavirus is alone blamed 
for a 238% rise in cyber-attacks on banks. Phishing attacks have seen a dramatic increase of 
600% since the end of February.  
• 
Due to pandemic, ransomware attacks rose 148% in March and the average ransomware 
payment rose by 33% to $111,605 as compared to Q4 2019. 
1.5 Layers of Cyber Security 
 
Fig. 2 Layers of Cyber Security 
1: Mission Critical Assets – This is the data you need to protect  
2: Data Security – Data security controls protect the storage and transfer of data. 
3: Application Security – Applications security controls protect access to an application, an 
application’s access to your mission critical assets, and the internal security of the application.  
4: Endpoint Security – Endpoint security controls protect the connection between devices and the 
network.  
5: Network Security – Network security controls protect an organization’s network and prevent 
unauthorized access of the network.


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6 
 
• 
2013-2014: Malicious hackers broke into Yahoo, compromising the accounts and personal 
information of its 3 billion users. Yahoo was subsequently fined $35 million for failing to 
disclose the news 
• 
2017: WannaCry ransomware infects 230,000 computers in one day 
• 
2019: Multiple DDoS attacks forced New Zealand's stock market to temporarily shut down 
•


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• 
According to a security research firm, 81 global firms from 81 countries reported data 
breaches in the first half of 2020 alone.  
• 
80% of firms have seen an increase in cyber-attacks this year. Coronavirus is alone blamed 
for a 238% rise in cyber-attacks on banks. Phishing attacks have seen a dramatic increase of 
600% since the end of February.  
•


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• 
Due to pandemic, ransomware attacks rose 148% in March and the average ransomware 
payment rose by 33% to $111,605 as compared to Q4 2019. 
1.5 Layers of Cyber Security 
 
Fig. 2 Layers of Cyber Security 
1: Mission Critical Assets – This is the data you need to protect  
2: Data Security – Data security controls protect the storage and transfer of data.


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3: Application Security – Applications security controls protect access to an application, an 
application’s access to your mission critical assets, and the internal security of the application.  
4: Endpoint Security – Endpoint security controls protect the connection between devices and the 
network.


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network.  
5: Network Security – Network security controls protect an organization’s network and prevent 
unauthorized access of the network.


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7 
 
6: Perimeter Security – Perimeter security controls include both the physical and digital security 
methodologies that protect the business overall. 
 7: The Human Layer – Humans are the weakest link in any cyber security posture. Human security 
controls include phishing simulations and access management controls that protect mission critical 
assets from a wide variety of human threats, including cyber criminals, malicious insiders, and 
negligent users 
 
1.6 Taxonomy of Attacks with respect to Security Goals 
 
 
Fig.3 Taxonomy of Attacks 
1.6.1 Attacks Threatening Confidentiality 
(a) Snooping 
 - It refers to unauthorized access to or interception of data. 
 - The file transferred through the internet may contain confidential information. 
 - Unauthorized entity may intercept the transmission.  
 - To prevent this, the data can be made unintelligible using encipherment techniques


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7 
 
6: Perimeter Security – Perimeter security controls include both the physical and digital security 
methodologies that protect the business overall. 
 7: The Human Layer – Humans are the weakest link in any cyber security posture. Human security 
controls include phishing simulations and access management controls that protect mission critical


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assets from a wide variety of human threats, including cyber criminals, malicious insiders, and 
negligent users 
 
1.6 Taxonomy of Attacks with respect to Security Goals 
 
 
Fig.3 Taxonomy of Attacks 
1.6.1 Attacks Threatening Confidentiality 
(a) Snooping 
 - It refers to unauthorized access to or interception of data.


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- The file transferred through the internet may contain confidential information. 
 - Unauthorized entity may intercept the transmission.  
 - To prevent this, the data can be made unintelligible using encipherment techniques


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8 
 
 
Fig.4 Snooping 
(b) Traffic Analysis 
• Although encipherment of data may make it non-intelligible for the interceptor, she can obtain 
some other type information by monitoring online traffic.  
• For example, she can find the electronic address (such as the e-mail address) of the sender or the 
receiver. She can collect pairs of requests and responses to help her guess the nature of transaction. 
 
Fig.5 Traffic Analysis 
1.6.2 Attacks Threatening Integrity 
• 
The integrity of data can be threatened by several kinds of attacks: modification, 
masquerading, replaying, and repudiation. 
(a) Modification


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8 
 
 
Fig.4 Snooping 
(b) Traffic Analysis 
• Although encipherment of data may make it non-intelligible for the interceptor, she can obtain 
some other type information by monitoring online traffic.  
• For example, she can find the electronic address (such as the e-mail address) of the sender or the


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receiver. She can collect pairs of requests and responses to help her guess the nature of transaction. 
 
Fig.5 Traffic Analysis 
1.6.2 Attacks Threatening Integrity 
• 
The integrity of data can be threatened by several kinds of attacks: modification, 
masquerading, replaying, and repudiation. 
(a) Modification


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9 
 
❑  After intercepting or accessing information, the attacker modifies the information to make 
it beneficial to herself.  
❑  For example, a customer sends a message to a bank to do some transaction.  
❑  The attacker intercepts the message and changes the type of transaction to benefit herself. 
 
Fig. 6 Modification 
(b) Masquerading  
➢  Masquerading, or spoofing, happens when the attacker impersonates somebody else.  
➢  For example, an attacker might steal the bank card and PIN of a bank customer and pretend 
that she is that customer.  
➢  Sometimes the attacker pretends instead to be the receiver entity.  
➢  For example, a user tries to contact a bank, but another site pretends that it is the bank and 
obtains some information from the user.  
 
Fig.7 Masquerading


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9 
 
❑  After intercepting or accessing information, the attacker modifies the information to make 
it beneficial to herself.  
❑  For example, a customer sends a message to a bank to do some transaction.  
❑  The attacker intercepts the message and changes the type of transaction to benefit herself. 
 
Fig. 6 Modification 
(b) Masquerading


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(b) Masquerading  
➢  Masquerading, or spoofing, happens when the attacker impersonates somebody else.  
➢  For example, an attacker might steal the bank card and PIN of a bank customer and pretend 
that she is that customer.  
➢  Sometimes the attacker pretends instead to be the receiver entity.


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➢  For example, a user tries to contact a bank, but another site pretends that it is the bank and 
obtains some information from the user.  
 
Fig.7 Masquerading


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10 
 
(c) Replaying  
➢  Replaying is another attack.  
➢  The attacker obtains a copy of a message sent by a user and later tries to replay it.  
➢  For example, a person sends a request to her bank to ask for payment to the attacker, who 
has done a job for her.  
➢  The attacker intercepts the message and sends it again to receive another payment from the 
bank.  
 
Fig.8 Replay 
(d) Repudiation  
➢  This type of attack is different from others because it is performed by one of the two parties 
in the communication: the sender or the receiver.  
➢  The sender of the message might later deny that she has sent the message; the receiver of the 
message might later deny that he has received the message.  
➢  An example of denial by the sender would be a bank customer asking her bank to send some 
money to a third party but later denying that she has made such a request.  
➢  An example of denial by the receiver could occur when a person buys a product from a 
manufacturer and pays for it electronically, but the manufacturer later denies having received 
the payment and asks to be paid 
1.6.3 Attacks Threatening Availability 
 
(a) Denial of Service


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10 
 
(c) Replaying  
➢  Replaying is another attack.  
➢  The attacker obtains a copy of a message sent by a user and later tries to replay it.  
➢  For example, a person sends a request to her bank to ask for payment to the attacker, who 
has done a job for her.  
➢  The attacker intercepts the message and sends it again to receive another payment from the 
bank.


key: chroma:document
string_value:
bank.  
 
Fig.8 Replay 
(d) Repudiation  
➢  This type of attack is different from others because it is performed by one of the two parties 
in the communication: the sender or the receiver.  
➢  The sender of the message might later deny that she has sent the message; the receiver of the 
message might later deny that he has received the message.


key: chroma:document
string_value:
➢  An example of denial by the sender would be a bank customer asking her bank to send some 
money to a third party but later denying that she has made such a request.  
➢  An example of denial by the receiver could occur when a person buys a product from a 
manufacturer and pays for it electronically, but the manufacturer later denies having received


key: full_page_content
string_value:
11 
 
✓  Denial of service (DoS) is a very common attack. It may slow down or totally interrupt the 
service of a system.  
✓  The attacker can use several strategies to achieve this. She might send so many bogus 
requests to a server that the server crashes because of the heavy load.  
✓  The attacker might intercept and delete a server’s response to a client, making the client to 
believe that the server is not responding.  
✓  The attacker may also intercept requests from the clients, causing the clients to send requests 
many times and overload the system. 
 
Fig. 9 Denial of Service 
1.7 Passive Vs Active Attacks 
(a) Passive Attacks 
▪ In a passive attack, the attacker’s goal is just to obtain information. This means that the 
attack does not modify data or harm the system.  
▪  The system continues with its normal operation.  
▪  However, the attack may harm the sender or the receiver of the message.  
▪  Attacks that threaten confidentiality, snooping and traffic analysis are passive attacks.  
▪  The revealing of the information may harm the sender or receiver of the message, but the 
system is not affected.  
▪  For this reason, it is difficult to detect this type of attack until the sender or receiver finds 
out about the leaking of confidential information.


key: chroma:document
string_value:
11 
 
✓  Denial of service (DoS) is a very common attack. It may slow down or totally interrupt the 
service of a system.  
✓  The attacker can use several strategies to achieve this. She might send so many bogus 
requests to a server that the server crashes because of the heavy load.


key: chroma:document
string_value:
✓  The attacker might intercept and delete a server’s response to a client, making the client to 
believe that the server is not responding.  
✓  The attacker may also intercept requests from the clients, causing the clients to send requests 
many times and overload the system. 
 
Fig. 9 Denial of Service 
1.7 Passive Vs Active Attacks 
(a) Passive Attacks


key: chroma:document
string_value:
▪ In a passive attack, the attacker’s goal is just to obtain information. This means that the 
attack does not modify data or harm the system.  
▪  The system continues with its normal operation.  
▪  However, the attack may harm the sender or the receiver of the message.  
▪  Attacks that threaten confidentiality, snooping and traffic analysis are passive attacks.


key: chroma:document
string_value:
▪  The revealing of the information may harm the sender or receiver of the message, but the 
system is not affected.  
▪  For this reason, it is difficult to detect this type of attack until the sender or receiver finds 
out about the leaking of confidential information.


key: full_page_content
string_value:
12 
 
▪  Passive attacks, however, can be prevented by encipherment of the data. 
(b) Active Attacks 
▪ An active attack may change the data or harm the system.  
▪  Attacks that threaten the integrity and availability are active attacks.  
▪  Active attacks are normally easier to detect than to prevent, because an attacker can launch 
them in a variety of ways. 
Table 2. Passive Vs Active Attacks 
 
1.8 Security Services 
❖ Authentication 
❖  Access Control 
❖  Data Confidentiality 
❖  Data Integrity 
❖  Non-repudiation 
 
Fig.10 Security Services 
(a) Authentication 
• 
Authentication is the process of verifying whether someone (or something) is, in fact, who 
(or what) it is declared to be.


key: chroma:document
string_value:
12 
 
▪  Passive attacks, however, can be prevented by encipherment of the data. 
(b) Active Attacks 
▪ An active attack may change the data or harm the system.  
▪  Attacks that threaten the integrity and availability are active attacks.  
▪  Active attacks are normally easier to detect than to prevent, because an attacker can launch 
them in a variety of ways.


key: chroma:document
string_value:
Table 2. Passive Vs Active Attacks 
 
1.8 Security Services 
❖ Authentication 
❖  Access Control 
❖  Data Confidentiality 
❖  Data Integrity 
❖  Non-repudiation 
 
Fig.10 Security Services 
(a) Authentication 
• 
Authentication is the process of verifying whether someone (or something) is, in fact, who 
(or what) it is declared to be.


key: full_page_content
string_value:
13 
 
• 
Authentication: Verifying the identity of a user, process, or device, often as a prerequisite to 
allowing access to resources in an information system. 
• 
This process for establishing your identity to gain access to a system is typically two-steps: 
you must first identify yourself (i.e. user ID, account number or email address), and then you 
have to prove that you are who you say you are (authenticate yourself). 
(b) Access Control 
• The prevention of unauthorized use of a resource (i.e., this service controls who can have 
access to a resource, under what conditions access can occur, and what those accessing the 
resource are allowed to do). 
(c) Confidentiality 
o The protection of data from unauthorized disclosure. 
o The other aspect of confidentiality is the protection of traffic flow from analysis. This 
requires that an attacker not be able to observe the source and destination, frequency, length, 
or other characteristics of the traffic on a communications facility. 
(d) Integrity 
• 
An integrity service deals with a stream of messages and assures that messages are received 
as sent with no duplication, insertion, modification, reordering, or replays. 
(e) Nonrepudiation 
• 
Nonrepudiation prevents either sender or receiver from denying a transmitted message.  
• 
Thus, when a message is sent, the receiver can prove that the alleged sender in fact sent the 
message.  
• 
Similarly, when a message is received, the sender can prove that the alleged receiver in fact 
received the message. 
1.9 Security Mechanisms 
• Encipherment, hiding or covering data, can provide confidentiality. It can also be used to 
complement other mechanisms to provide other services. Today two techniques 
cryptography and steganography are used for enciphering. 
• Data integrity mechanism appends to the data a short check value that has been created by a 
specific process from the data itself. The receiver receives the data and the check value. He 
creates a new check value from the received data and compares the newly created check 
value with the one received. If the two check values are the same, the integrity of data has 
been preserved.


key: chroma:document
string_value:
13 
 
• 
Authentication: Verifying the identity of a user, process, or device, often as a prerequisite to 
allowing access to resources in an information system. 
• 
This process for establishing your identity to gain access to a system is typically two-steps: 
you must first identify yourself (i.e. user ID, account number or email address), and then you


key: chroma:document
string_value:
have to prove that you are who you say you are (authenticate yourself). 
(b) Access Control 
• The prevention of unauthorized use of a resource (i.e., this service controls who can have 
access to a resource, under what conditions access can occur, and what those accessing the 
resource are allowed to do). 
(c) Confidentiality


key: chroma:document
string_value:
o The protection of data from unauthorized disclosure. 
o The other aspect of confidentiality is the protection of traffic flow from analysis. This 
requires that an attacker not be able to observe the source and destination, frequency, length, 
or other characteristics of the traffic on a communications facility. 
(d) Integrity 
•


key: chroma:document
string_value:
(d) Integrity 
• 
An integrity service deals with a stream of messages and assures that messages are received 
as sent with no duplication, insertion, modification, reordering, or replays. 
(e) Nonrepudiation 
• 
Nonrepudiation prevents either sender or receiver from denying a transmitted message.  
•


key: chroma:document
string_value:
• 
Thus, when a message is sent, the receiver can prove that the alleged sender in fact sent the 
message.  
• 
Similarly, when a message is received, the sender can prove that the alleged receiver in fact 
received the message. 
1.9 Security Mechanisms 
• Encipherment, hiding or covering data, can provide confidentiality. It can also be used to


key: chroma:document
string_value:
complement other mechanisms to provide other services. Today two techniques 
cryptography and steganography are used for enciphering. 
• Data integrity mechanism appends to the data a short check value that has been created by a 
specific process from the data itself. The receiver receives the data and the check value. He


key: chroma:document
string_value:
creates a new check value from the received data and compares the newly created check 
value with the one received. If the two check values are the same, the integrity of data has 
been preserved.


key: full_page_content
string_value:
14 
 
• A digital signature is a means by which the sender can electronically sign the data and the 
receiver can electronically verify the signature. The sender uses a process that involves 
showing that she owns a private key related to the public key that she has announced publicly. 
The receiver uses the sender’s public key to prove that the message is indeed signed by the 
sender who claims to have sent the message. 
• Authentication Exchange In authentication exchange, two entities exchange some messages 
to prove their identity to each other. For example, one entity can prove that she knows a secret 
that only she is supposed to know. 
• Traffic padding means inserting some bogus data into the data traffic to thwart the adversary’s 
attempt to use the traffic analysis. 
• Routing control means selecting and continuously changing different available routes 
between the sender and the receiver to prevent the opponent from eavesdropping on a 
particular route. 
• Notarization means selecting a third trusted party to control the communication between two 
entities. This can be done, for example, to prevent repudiation. The receiver can involve a 
trusted party to store the sender request in order to prevent the sender from later denying that 
she has made such a request. 
• Access control uses methods to prove that a user has access right to the data or resources 
owned by a system. Examples of proofs are passwords and PINs.  
 
Fig. 11 Security Mechanisms 
1.10 Model of Network Security


key: chroma:document
string_value:
14 
 
• A digital signature is a means by which the sender can electronically sign the data and the 
receiver can electronically verify the signature. The sender uses a process that involves 
showing that she owns a private key related to the public key that she has announced publicly. 
The receiver uses the sender’s public key to prove that the message is indeed signed by the


key: chroma:document
string_value:
sender who claims to have sent the message. 
• Authentication Exchange In authentication exchange, two entities exchange some messages 
to prove their identity to each other. For example, one entity can prove that she knows a secret 
that only she is supposed to know. 
• Traffic padding means inserting some bogus data into the data traffic to thwart the adversary’s


key: chroma:document
string_value:
attempt to use the traffic analysis. 
• Routing control means selecting and continuously changing different available routes 
between the sender and the receiver to prevent the opponent from eavesdropping on a 
particular route. 
• Notarization means selecting a third trusted party to control the communication between two


key: chroma:document
string_value:
entities. This can be done, for example, to prevent repudiation. The receiver can involve a 
trusted party to store the sender request in order to prevent the sender from later denying that 
she has made such a request. 
• Access control uses methods to prove that a user has access right to the data or resources 
owned by a system. Examples of proofs are passwords and PINs.


key: full_page_content
string_value:
15 
 
 
Fig.12 Model of Network Security 
Basic Tasks 
This general model shows that there are four basic tasks in designing a particular security service: 
1. Design an algorithm for performing the security-related transformation. The algorithm should 
be such that an opponent cannot defeat its purpose. 
2. Generate the secret information to be used with the algorithm. 
3. Develop methods for the distribution and sharing of the secret information. 
4. Specify a protocol to be used by the two principals that makes use of the security algorithm 
and the secret information to achieve a particular security service. 
Network Access Security Model 
 
Fig.13 Network Access Security Model


key: chroma:document
string_value:
15 
 
 
Fig.12 Model of Network Security 
Basic Tasks 
This general model shows that there are four basic tasks in designing a particular security service: 
1. Design an algorithm for performing the security-related transformation. The algorithm should 
be such that an opponent cannot defeat its purpose. 
2. Generate the secret information to be used with the algorithm.


key: chroma:document
string_value:
3. Develop methods for the distribution and sharing of the secret information. 
4. Specify a protocol to be used by the two principals that makes use of the security algorithm 
and the secret information to achieve a particular security service. 
Network Access Security Model 
 
Fig.13 Network Access Security Model


key: full_page_content
string_value:
16 
 
1.11 Symmetric Encryption 
A symmetric encryption scheme has five ingredients  
• Plaintext: This is the original message or data that is fed into the algorithm as input. 
•Encryption algorithm: The encryption algorithm performs various substitutions and 
transformations on the plaintext. 
• Secret key: The secret key is also input to the algorithm. The exact substitutions and transformations 
performed by the algorithm depend on the key. 
• Ciphertext: This is the scrambled message produced as output. It depends on the plaintext and the 
secret key. For a given message, two different keys will produce two different ciphertexts. 
• Decryption algorithm: This is essentially the encryption algorithm run in reverse. It takes the 
ciphertext and the same secret key and produces the original plaintext. 
 
Fig.14 Simplified Model of Symmetric Encryption 
Encryption, is the process of changing information in such a way as to make it unreadable by 
anyone except those possessing special knowledge (usually referred to as a "key") that allows 
them to change the information back to its original, readable form.  
(a) Basic Terms 
▪ Plaintext - original message 
▪  Ciphertext - coded message 
▪  Cipher - algorithm for transforming plaintext to cipher text 
▪  Key - info used in cipher known only to sender/receiver info used in cipher known only to 
sender/receiver


key: chroma:document
string_value:
16 
 
1.11 Symmetric Encryption 
A symmetric encryption scheme has five ingredients  
• Plaintext: This is the original message or data that is fed into the algorithm as input. 
•Encryption algorithm: The encryption algorithm performs various substitutions and 
transformations on the plaintext.


key: chroma:document
string_value:
• Secret key: The secret key is also input to the algorithm. The exact substitutions and transformations 
performed by the algorithm depend on the key. 
• Ciphertext: This is the scrambled message produced as output. It depends on the plaintext and the 
secret key. For a given message, two different keys will produce two different ciphertexts.


key: chroma:document
string_value:
• Decryption algorithm: This is essentially the encryption algorithm run in reverse. It takes the 
ciphertext and the same secret key and produces the original plaintext. 
 
Fig.14 Simplified Model of Symmetric Encryption 
Encryption, is the process of changing information in such a way as to make it unreadable by


key: chroma:document
string_value:
anyone except those possessing special knowledge (usually referred to as a "key") that allows 
them to change the information back to its original, readable form.  
(a) Basic Terms 
▪ Plaintext - original message 
▪  Ciphertext - coded message 
▪  Cipher - algorithm for transforming plaintext to cipher text


key: chroma:document
string_value:
▪  Key - info used in cipher known only to sender/receiver info used in cipher known only to 
sender/receiver


key: full_page_content
string_value:
17 
 
▪  Encipher (encrypt) Encipher (encrypt) - converting plaintext to ciphertext  
▪  Decipher (decrypt) - recovering ciphertext from plaintext  
▪  Cryptography  - study of encryption principles/methods study of encryption 
principles/methods  
▪  Cryptanalysis (code breaking) - study of principles/ methods of deciphering ciphertext 
without knowing key 
▪  Cryptology - field of both cryptography and cryptanalysis 
(b) Cryptography – Classification 
1. By type of encryption operations used: 
➢ Substitution  
➢ Transposition  
➢ Product 
2. By number of keys used By number of keys used: 
➢ Single-key or private key  
➢ Two-key or public 
3. By the way in which plaintext is processed: 
➢ Block  
➢ Stream 
1.12 Substitution 
❑ A substitution technique is one in which the letters of plaintext are replaced by other letters 
or by numbers or symbols.  
❑  If the plaintext is viewed as a sequence of bits, then substitution involves replacing 
plaintext bit patterns with cipher text bit patterns. 
❑  Caesar Cipher.  
❑  Playfair Cipher.  
❑  One-Time Pad. 
❑ Hill Cipher 
❑ Monoalphabetic Ciphers


key: chroma:document
string_value:
17 
 
▪  Encipher (encrypt) Encipher (encrypt) - converting plaintext to ciphertext  
▪  Decipher (decrypt) - recovering ciphertext from plaintext  
▪  Cryptography  - study of encryption principles/methods study of encryption 
principles/methods  
▪  Cryptanalysis (code breaking) - study of principles/ methods of deciphering ciphertext 
without knowing key


key: chroma:document
string_value:
▪  Cryptology - field of both cryptography and cryptanalysis 
(b) Cryptography – Classification 
1. By type of encryption operations used: 
➢ Substitution  
➢ Transposition  
➢ Product 
2. By number of keys used By number of keys used: 
➢ Single-key or private key  
➢ Two-key or public 
3. By the way in which plaintext is processed: 
➢ Block  
➢ Stream 
1.12 Substitution


key: chroma:document
string_value:
1.12 Substitution 
❑ A substitution technique is one in which the letters of plaintext are replaced by other letters 
or by numbers or symbols.  
❑  If the plaintext is viewed as a sequence of bits, then substitution involves replacing 
plaintext bit patterns with cipher text bit patterns. 
❑  Caesar Cipher.  
❑  Playfair Cipher.  
❑  One-Time Pad. 
❑ Hill Cipher


key: full_page_content
string_value:
18 
 
❑ Polyalphabetic Ciphers 
1.12.1 Caesar Cipher: 
The earliest known use of a substitution cipher and the simplest was by Julius Caesar. 
 
 
If n=3 
 
Plain Text: meet me after the toga party 
Considering n=3 
Cipher Text: PHHW PH DIWHU WKH WRJD SDUWB 
Weakness: Total 26 keys 
Brute Force Cryptanalysis 
• 
If it is known that a given ciphertext is a Caesar cipher, then a brute-force cryptanalysis is 
easily performed: simply try all the 25 possible keys. 
• 
Three important characteristics of this problem enabled us to use a bruteforce cryptanalysis: 
1. The encryption and decryption algorithms are known. 
2. There are only 25 keys to try. 
3. The language of the plaintext is known and easily recognizable.


key: chroma:document
string_value:
18 
 
❑ Polyalphabetic Ciphers 
1.12.1 Caesar Cipher: 
The earliest known use of a substitution cipher and the simplest was by Julius Caesar. 
 
 
If n=3 
 
Plain Text: meet me after the toga party 
Considering n=3 
Cipher Text: PHHW PH DIWHU WKH WRJD SDUWB 
Weakness: Total 26 keys 
Brute Force Cryptanalysis 
•


key: chroma:document
string_value:
• 
If it is known that a given ciphertext is a Caesar cipher, then a brute-force cryptanalysis is 
easily performed: simply try all the 25 possible keys. 
• 
Three important characteristics of this problem enabled us to use a bruteforce cryptanalysis: 
1. The encryption and decryption algorithms are known. 
2. There are only 25 keys to try.


key: full_page_content
string_value:
19 
 
 
Fig.15 Brute Force Cryptanalysis 
1.12.2 Playfair Cipher 
➢ The best-known multiple letter encryption cipher is the Playfair, which treats digrams in the 
plaintext as single units and translates these units into cipher text digrams.  
➢ The Playfair algorithm is based on the use of 5x5 matrix of letters constructed using a 
keyword. 
➢ Let the keyword be “monarchy‟.  
➢ The matrix is constructed by filling in the letters of the keyword (minus duplicates) from left 
to right and from top to bottom, and then filling in the remainder of the matrix with the 
remaining letters in alphabetical order. 
➢ The letter „i‟ and „j‟ count as one letter.  
Rules: 
Plaintext is encrypted two letters at a time according to the following rules: 
❑  Repeating plaintext letters that would fall in the same pair are separated with a filler letter 
such as „x‟.


key: chroma:document
string_value:
19 
 
 
Fig.15 Brute Force Cryptanalysis 
1.12.2 Playfair Cipher 
➢ The best-known multiple letter encryption cipher is the Playfair, which treats digrams in the 
plaintext as single units and translates these units into cipher text digrams.  
➢ The Playfair algorithm is based on the use of 5x5 matrix of letters constructed using a 
keyword. 
➢ Let the keyword be “monarchy‟.


key: chroma:document
string_value:
➢ The matrix is constructed by filling in the letters of the keyword (minus duplicates) from left 
to right and from top to bottom, and then filling in the remainder of the matrix with the 
remaining letters in alphabetical order. 
➢ The letter „i‟ and „j‟ count as one letter.  
Rules: 
Plaintext is encrypted two letters at a time according to the following rules:


key: chroma:document
string_value:
❑  Repeating plaintext letters that would fall in the same pair are separated with a filler letter 
such as „x‟.


key: full_page_content
string_value:
20 
 
❑  Plaintext letters that fall in the same row of the matrix are each replaced by the letter to the 
right, with the first element of the row following the last. 
❑  Plaintext letters that fall in the same column are replaced by the letter beneath, with the top 
element of the column following the last. 
❑  Otherwise, each plaintext letter is replaced by the letter that lies in its own row and the column 
occupied by the other plaintext letter. 
(Ex): 
Keyword : MONARCHY 
M 
O 
N 
A 
R 
C 
H 
Y 
B 
D 
E 
F 
G 
I/J 
K 
L 
P 
Q 
S 
T 
U 
V 
W 
X 
Z 
 
Plaintext = meet me at the school house 
Splitting two letters as a unit =>  
me et me at th es ch ox ol ho us ex  
Corresponding cipher text =>  
CL KL CL RS PD IL HY AV MP HF XL IU 
Since there are 26 letters, 26x26 = 676 diagrams are possible, so identification of individual digram 
is more difficult. Frequency analysis is much more difficult.


key: chroma:document
string_value:
20 
 
❑  Plaintext letters that fall in the same row of the matrix are each replaced by the letter to the 
right, with the first element of the row following the last. 
❑  Plaintext letters that fall in the same column are replaced by the letter beneath, with the top 
element of the column following the last.


key: chroma:document
string_value:
❑  Otherwise, each plaintext letter is replaced by the letter that lies in its own row and the column 
occupied by the other plaintext letter. 
(Ex): 
Keyword : MONARCHY 
M 
O 
N 
A 
R 
C 
H 
Y 
B 
D 
E 
F 
G 
I/J 
K 
L 
P 
Q 
S 
T 
U 
V 
W 
X 
Z 
 
Plaintext = meet me at the school house 
Splitting two letters as a unit =>  
me et me at th es ch ox ol ho us ex


key: chroma:document
string_value:
Corresponding cipher text =>  
CL KL CL RS PD IL HY AV MP HF XL IU 
Since there are 26 letters, 26x26 = 676 diagrams are possible, so identification of individual digram 
is more difficult. Frequency analysis is much more difficult.


key: full_page_content
string_value:
21 
 
1.12.3 Hill Cipher 
• 
The Hill Cipher was invented by Lester S. Hill in 1929. 
• 
The encryption algorithm takes ‘m’ successive plain text letters and substitutes for them ‘m’ 
ciphertext letters. 
Steps: 
• 
Select a message to encrypt 
• 
 Select a key 
• 
If we encrypt 2 letters at a time, then key size is 4. 
• 
Assign a numerical equivalent to each letter. 
• 
Convert the key to matrix 
• 
Convert the message to a ‘n’ component vector 
• 
Encryption - Ciphertext = K * P mod 26 
• 
Decryption Plaintext = K-1 * C mod 26 
Example : 
 
Hill Cipher 
Plain Text  : welcome 
we lc om ex 
Group the plain text as digrams ( 2 letter combinations) 
Key : dbgf 
K = ⌈𝑑 𝑏
𝑔 𝑓⌉ = ⌈3 1
6 5⌉ 
Represent the plaintext in matrix form 
⌈𝑤
𝑒⌉⌈𝑙
𝑐⌉⌈𝑜
𝑚⌉⌈𝑒
𝑥⌉


key: chroma:document
string_value:
21 
 
1.12.3 Hill Cipher 
• 
The Hill Cipher was invented by Lester S. Hill in 1929. 
• 
The encryption algorithm takes ‘m’ successive plain text letters and substitutes for them ‘m’ 
ciphertext letters. 
Steps: 
• 
Select a message to encrypt 
• 
 Select a key 
• 
If we encrypt 2 letters at a time, then key size is 4. 
• 
Assign a numerical equivalent to each letter. 
•


key: chroma:document
string_value:
• 
Convert the key to matrix 
• 
Convert the message to a ‘n’ component vector 
• 
Encryption - Ciphertext = K * P mod 26 
• 
Decryption Plaintext = K-1 * C mod 26 
Example : 
 
Hill Cipher 
Plain Text  : welcome 
we lc om ex 
Group the plain text as digrams ( 2 letter combinations) 
Key : dbgf 
K = ⌈𝑑 𝑏
𝑔 𝑓⌉ = ⌈3 1
6 5⌉ 
Represent the plaintext in matrix form 
⌈𝑤
𝑒⌉⌈𝑙
𝑐⌉⌈𝑜
𝑚⌉⌈𝑒


key: full_page_content
string_value:
22 
 
⌈22
4 ⌉⌈11
2 ⌉⌈14
12⌉⌈4
23⌉ 
Encryption 
C = K * P mod 26 
1. For 22
4  
C = ⌈3 1
6 5⌉ * ⌈22
4 ⌉  mod 26 
   = ⌈3 ∗22 + 1 ∗4
6 ∗22 + 5 ∗4⌉ mod 26 
   =⌈70
152⌉ mod 26 
 = ⌈18
22⌉ = ⌈𝑠
𝑤⌉ 
2. For 11
2  
C = ⌈3 1
6 5⌉ * ⌈11
2 ⌉  mod 26 
   = ⌈3 ∗11 + 1 ∗2
6 ∗11 + 5 ∗2⌉ mod 26 
   = ⌈35
76⌉ mod 26 
   = ⌈9
24⌉ = ⌈𝑗
𝑦⌉ 
3. For 14
12 
C = ⌈3 1
6 5⌉ * ⌈14
12⌉  mod 26 
  = ⌈3 ∗14 + 1 ∗12
6 ∗14 + 5 ∗12⌉ mod 26 
 = ⌈54
144⌉ mod 26 
 = ⌈2
14⌉ = ⌈𝑐
𝑜⌉ 
4. For 4
23


key: chroma:document
string_value:
22 
 
⌈22
4 ⌉⌈11
2 ⌉⌈14
12⌉⌈4
23⌉ 
Encryption 
C = K * P mod 26 
1. For 22
4  
C = ⌈3 1
6 5⌉ * ⌈22
4 ⌉  mod 26 
   = ⌈3 ∗22 + 1 ∗4
6 ∗22 + 5 ∗4⌉ mod 26 
   =⌈70
152⌉ mod 26 
 = ⌈18
22⌉ = ⌈𝑠
𝑤⌉ 
2. For 11
2  
C = ⌈3 1
6 5⌉ * ⌈11
2 ⌉  mod 26 
   = ⌈3 ∗11 + 1 ∗2
6 ∗11 + 5 ∗2⌉ mod 26 
   = ⌈35
76⌉ mod 26 
   = ⌈9
24⌉ = ⌈𝑗
𝑦⌉ 
3. For 14
12 
C = ⌈3 1
6 5⌉ * ⌈14
12⌉  mod 26


key: chroma:document
string_value:
12⌉  mod 26 
  = ⌈3 ∗14 + 1 ∗12
6 ∗14 + 5 ∗12⌉ mod 26 
 = ⌈54
144⌉ mod 26 
 = ⌈2
14⌉ = ⌈𝑐
𝑜⌉ 
4. For 4
23


key: full_page_content
string_value:
23 
 
C = ⌈3 1
6 5⌉ * ⌈4
23⌉  mod 26 
         = ⌈3 ∗4 + 1 ∗23
6 ∗4 + 5 ∗23⌉ mod 26 
         = ⌈35
139⌉ mod 26 
         = ⌈9
9⌉ = ⌈𝑗
𝑗⌉ 
 
 Cipher Text : swjycojj 
 
Decryption 
Plain Text = K-1 * C mod 26 
To compute K-1 
K-1 = 
𝟏
|𝑲|  𝒂𝒅𝒋(𝑲) 
 
 
|K|  = ⌈3 1
6 5⌉ = 3*5 – 6*1 = 15-6 = 9 
 
Determine the multiplicative inverse 
9 * mod 26 = 1 
Find the multiplicative inverse of 9 w.r.t mod 26, such that the outcome 
is 1. 
9 * 1  mod 26 = 9 
9 * 2 mod 26 = 18 
9 * 3 mod 26 = 1


key: chroma:document
string_value:
23 
 
C = ⌈3 1
6 5⌉ * ⌈4
23⌉  mod 26 
         = ⌈3 ∗4 + 1 ∗23
6 ∗4 + 5 ∗23⌉ mod 26 
         = ⌈35
139⌉ mod 26 
         = ⌈9
9⌉ = ⌈𝑗
𝑗⌉ 
 
 Cipher Text : swjycojj 
 
Decryption 
Plain Text = K-1 * C mod 26 
To compute K-1 
K-1 = 
𝟏
|𝑲|  𝒂𝒅𝒋(𝑲) 
 
 
|K|  = ⌈3 1
6 5⌉ = 3*5 – 6*1 = 15-6 = 9 
 
Determine the multiplicative inverse 
9 * mod 26 = 1


key: chroma:document
string_value:
9 * mod 26 = 1 
Find the multiplicative inverse of 9 w.r.t mod 26, such that the outcome 
is 1. 
9 * 1  mod 26 = 9 
9 * 2 mod 26 = 18 
9 * 3 mod 26 = 1


key: full_page_content
string_value:
24 
 
Ans : 3 
𝟏
|𝑲| = 3 
𝐾= ⌈3 1
6 5⌉ 
adj (K) = ⌈5   −1
−6     3⌉ 
 
K-1 = 
𝟏
|𝑲|  𝒂𝒅𝒋(𝑲) 
     = 3 * ⌈5   −1
−6     3⌉ 
      =  ⌈15   −3
−18     9⌉ 
In order to eliminate the negative sign, add 26 to the negative numbers 
K-1    = ⌈15   23
8     9 ⌉ 
 
Cipher Text : swjycojj 
Represent the given ciphertext as pairs in matrix form 
⌈𝑠
𝑤⌉⌈𝑗
𝑦⌉⌈𝑐
𝑜⌉⌈𝑗
𝑗⌉ 
1. For ⌈𝑠
𝑤⌉= ⌈18
22⌉ 
Plain Text = K-1 * C mod 26 
 
 
 = ⌈15   23
8     9 ⌉ * ⌈18
22⌉ mod 26 
                        = ⌈15 ∗18 + 23 ∗22
8 ∗18 + 9 ∗22 ⌉ mod 26 
                       = ⌈776
342⌉ mod 26


key: chroma:document
string_value:
24 
 
Ans : 3 
𝟏
|𝑲| = 3 
𝐾= ⌈3 1
6 5⌉ 
adj (K) = ⌈5   −1
−6     3⌉ 
 
K-1 = 
𝟏
|𝑲|  𝒂𝒅𝒋(𝑲) 
     = 3 * ⌈5   −1
−6     3⌉ 
      =  ⌈15   −3
−18     9⌉ 
In order to eliminate the negative sign, add 26 to the negative numbers 
K-1    = ⌈15   23
8     9 ⌉ 
 
Cipher Text : swjycojj 
Represent the given ciphertext as pairs in matrix form 
⌈𝑠
𝑤⌉⌈𝑗
𝑦⌉⌈𝑐
𝑜⌉⌈𝑗
𝑗⌉ 
1. For ⌈𝑠
𝑤⌉= ⌈18
22⌉


key: chroma:document
string_value:
𝑤⌉= ⌈18
22⌉ 
Plain Text = K-1 * C mod 26 
 
 
 = ⌈15   23
8     9 ⌉ * ⌈18
22⌉ mod 26 
                        = ⌈15 ∗18 + 23 ∗22
8 ∗18 + 9 ∗22 ⌉ mod 26 
                       = ⌈776
342⌉ mod 26


key: full_page_content
string_value:
25 
 
                       = ⌈22
4 ⌉ = ⌈𝑤
𝑒⌉ 
2. For ⌈𝑗
𝑦⌉= ⌈9
24⌉ 
Plain Text = K-1 * C mod 26 
= ⌈15   23
8     9 ⌉ * ⌈9
24⌉ mod 26 
 
 
= ⌈687
288⌉ mod 26 
 
 
= ⌈11
2 ⌉ = ⌈𝑙
𝑐⌉ 
 
 
3. For ⌈𝑐
𝑜⌉ = ⌈2
14⌉ 
Plain Text = K-1 * C mod 26 
                   = ⌈15   23
8     9 ⌉ * ⌈2
14⌉ mod 26 
  
      = ⌈352
142⌉ mod 26 
                   = ⌈14
12⌉ = ⌈𝑜
𝑚⌉ 
4. For ⌈𝑗
𝑗⌉ = ⌈9
9⌉ 
 
Plain Text = K-1 * C mod 26 
  
=  ⌈15   23
8     9 ⌉ * ⌈9
9⌉ mod 26 
  
= ⌈4
23⌉ = ⌈𝑒
𝑥⌉ 
Plaintext : welcome


key: chroma:document
string_value:
25 
 
                       = ⌈22
4 ⌉ = ⌈𝑤
𝑒⌉ 
2. For ⌈𝑗
𝑦⌉= ⌈9
24⌉ 
Plain Text = K-1 * C mod 26 
= ⌈15   23
8     9 ⌉ * ⌈9
24⌉ mod 26 
 
 
= ⌈687
288⌉ mod 26 
 
 
= ⌈11
2 ⌉ = ⌈𝑙
𝑐⌉ 
 
 
3. For ⌈𝑐
𝑜⌉ = ⌈2
14⌉ 
Plain Text = K-1 * C mod 26 
                   = ⌈15   23
8     9 ⌉ * ⌈2
14⌉ mod 26 
  
      = ⌈352
142⌉ mod 26 
                   = ⌈14
12⌉ = ⌈𝑜
𝑚⌉ 
4. For ⌈𝑗
𝑗⌉ = ⌈9


key: chroma:document
string_value:
4. For ⌈𝑗
𝑗⌉ = ⌈9
9⌉ 
 
Plain Text = K-1 * C mod 26 
  
=  ⌈15   23
8     9 ⌉ * ⌈9
9⌉ mod 26 
  
= ⌈4
23⌉ = ⌈𝑒
𝑥⌉ 
Plaintext : welcome


key: full_page_content
string_value:
26 
 
3 letter combinations 
Consider the plaintext =”Act” 
Select a 3 X 3 matrix as the key. 
K= 
6
24
1
13
16
10
20
17
15
 
Represent the plain text in numerical form 
P = 
0
2
19
 
Encryption 
C = K * P mod 26 
    =
6
24
1
13
16
10
20
17
15
    * ⌈
0
2
19
⌉    mod 26 
 
=  ⌈
6 ∗0 + 24 ∗2 + 1 ∗19
13 ∗0 + 16 ∗2 + 10 ∗19
20 ∗0 + 17 ∗2 + 15 ∗19
⌉ mod 26 
 
= ⌈
67
222
319
⌉    mod 26 
 
= ⌈
15
14
7
⌉ = ⌈
𝑃
𝑂
𝐻
⌉ 
 
 
Decryption 
Plain Text = K-1 * C mod 26 
To compute K-1


key: chroma:document
string_value:
26 
 
3 letter combinations 
Consider the plaintext =”Act” 
Select a 3 X 3 matrix as the key. 
K= 
6
24
1
13
16
10
20
17
15
 
Represent the plain text in numerical form 
P = 
0
2
19
 
Encryption 
C = K * P mod 26 
    =
6
24
1
13
16
10
20
17
15
    * ⌈
0
2
19
⌉    mod 26 
 
=  ⌈
6 ∗0 + 24 ∗2 + 1 ∗19
13 ∗0 + 16 ∗2 + 10 ∗19
20 ∗0 + 17 ∗2 + 15 ∗19
⌉ mod 26 
 
= ⌈
67
222
319


key: chroma:document
string_value:
= ⌈
67
222
319
⌉    mod 26 
 
= ⌈
15
14
7
⌉ = ⌈
𝑃
𝑂
𝐻
⌉ 
 
 
Decryption 
Plain Text = K-1 * C mod 26 
To compute K-1


key: full_page_content
string_value:
27 
 
K-1 = 
𝟏
|𝑲|  𝒂𝒅𝒋(𝑲) 
|𝐾| = ⌈
6
24
1
13
16
10
20
17
15
⌉ 
    
      = 6 (16*15 – 17*10) – 24(13*15 – 20*10) + 1(13*17 – 20*16) 
      = 6 (240-170) – 24(195-200) +1(221-320) 
      = 6(70) -24(-5) +1(-99) 
      = 420 + 120 – 99 
      = 441 
𝒂𝒅𝒋(𝑲) 
 
K = ⌈
6
24
1
13
16
10
20
17
15
⌉ 
 
 KT = ⌈
6
13
20
24
16
17
1
10
15
⌉ 
     
       = ⌈
16 ∗15 −10 ∗17
−(24 ∗15 −1 ∗17)
24 ∗10 −1 ∗16
−(13 ∗15 −10 ∗20)
6 ∗15 −1 ∗20
−(6 ∗10 −1 ∗13)
13 ∗17 −16 ∗20
−(6 ∗17 −24 ∗20)
(6 ∗16 −24 ∗13)
⌉ 
 
adj(K)   =  ⌈
70
−343
224
5
70
−47
−99
378
−216
⌉


key: chroma:document
string_value:
27 
 
K-1 = 
𝟏
|𝑲|  𝒂𝒅𝒋(𝑲) 
|𝐾| = ⌈
6
24
1
13
16
10
20
17
15
⌉ 
    
      = 6 (16*15 – 17*10) – 24(13*15 – 20*10) + 1(13*17 – 20*16) 
      = 6 (240-170) – 24(195-200) +1(221-320) 
      = 6(70) -24(-5) +1(-99) 
      = 420 + 120 – 99 
      = 441 
𝒂𝒅𝒋(𝑲) 
 
K = ⌈
6
24
1
13
16
10
20
17
15
⌉ 
 
 KT = ⌈
6
13
20
24
16
17
1
10
15
⌉ 
     
       = ⌈
16 ∗15 −10 ∗17
−(24 ∗15 −1 ∗17)


key: chroma:document
string_value:
−(24 ∗15 −1 ∗17)
24 ∗10 −1 ∗16
−(13 ∗15 −10 ∗20)
6 ∗15 −1 ∗20
−(6 ∗10 −1 ∗13)
13 ∗17 −16 ∗20
−(6 ∗17 −24 ∗20)
(6 ∗16 −24 ∗13)
⌉ 
 
adj(K)   =  ⌈
70
−343
224
5
70
−47
−99
378
−216
⌉


key: full_page_content
string_value:
28 
 
 
Plain Text = K-1 * C mod 26 
 
 
K-1     =
1
441  ⌈
70
−343
224
5
70
−47
−99
378
−216
⌉ 
 
P = 
1
441  ⌈
70
−343
224
5
70
−47
−99
378
−216
⌉∗⌈
15
14
7
⌉ mod 26 
 
    =  
1
441  ⌈
−2184
726
2295
⌉ mod 26 
 
 
Multiplicative inverse for 
1
441 
 
441 *1 mod 26 = 25 
… 
441 *25 mod 26 =1 
Ans : 25 
 
= 25  ⌈
−2184
726
2295
⌉ mod 26


key: full_page_content
string_value:
29 
 
= ⌈
−54600
18150
57375
⌉ mod 26 
⌈
0
2
19
⌉  = ⌈
𝐴
𝐶
𝑇
⌉ 
 
1.12.4 One-Time Pad 
▪ It is an unbreakable cryptosystem. It represents the message as a sequence of 0s and 1s.  
▪ This can be accomplished by writing all numbers in binary, for example, or by using ASCII. 
The key is a random sequence of 0‟s and 1‟s of same length as the message. 
▪ Once a key is used, it is discarded and never used again. The system can be expressed as 
follows: 
▪ Thus the cipher text is generated by performing the bitwise XOR of the plaintext and the 
key. Decryption uses the same key.  
▪ Because of the properties of XOR, decryption simply involves the same bitwise operation 
 
 
Advantage: 
 Encryption method is completely unbreakable for a ciphertext only attack. 
 Disadvantages 
 It requires a very long key which is expensive to produce and expensive to transmit. 
 Once a key is used, it is dangerous to reuse it for a second message; any knowledge on the first 
message would give knowledge of the second. 
1.12.5 Monoalphabetic Ciphers 
• 
With only 25 possible keys, the Caesar cipher is far from secure.


key: chroma:document
string_value:
29 
 
= ⌈
−54600
18150
57375
⌉ mod 26 
⌈
0
2
19
⌉  = ⌈
𝐴
𝐶
𝑇
⌉ 
 
1.12.4 One-Time Pad 
▪ It is an unbreakable cryptosystem. It represents the message as a sequence of 0s and 1s.  
▪ This can be accomplished by writing all numbers in binary, for example, or by using ASCII. 
The key is a random sequence of 0‟s and 1‟s of same length as the message.


key: chroma:document
string_value:
▪ Once a key is used, it is discarded and never used again. The system can be expressed as 
follows: 
▪ Thus the cipher text is generated by performing the bitwise XOR of the plaintext and the 
key. Decryption uses the same key.  
▪ Because of the properties of XOR, decryption simply involves the same bitwise operation 
 
 
Advantage:


key: chroma:document
string_value:
Advantage: 
 Encryption method is completely unbreakable for a ciphertext only attack. 
 Disadvantages 
 It requires a very long key which is expensive to produce and expensive to transmit. 
 Once a key is used, it is dangerous to reuse it for a second message; any knowledge on the first 
message would give knowledge of the second. 
1.12.5 Monoalphabetic Ciphers 
•


key: full_page_content
string_value:
30 
 
• 
A dramatic increase in the key space can be achieved by allowing an arbitrary substitution. 
• 
A permutation of a finite set of elements is an ordered sequence of all the elements of, with 
each element appearing exactly once. For example, if, there are six permutations of : 
 
• 
In general, there are ! permutations of a set of elements, because the first element can be 
chosen in one of n ways 
• 
If, instead, the “cipher” line can be any permutation of the 26 alphabetic characters, then there 
are 26! or greater than   
  possible keys. 
• 
This is 10 orders of magnitude greater than the key space for DES and would seem to 
eliminate brute-force techniques for cryptanalysis.  
• 
Such an approach is referred to as a monoalphabetic substitution cipher, because a single 
cipher alphabet (mapping from plain alphabet to cipher alphabet) is used per message. 
1.12.6 Polyalphabetic Ciphers 
• Vigenere Cipher is a method of encrypting alphabetic text. It uses a simple form 
of polyalphabetic substitution.  
• A polyalphabetic cipher is any cipher based on substitution, using multiple substitution 
alphabets.  
• The encryption of the original text is done using the Vigenère square or Vigenère table. 
• The table consists of the alphabets written out 26 times in different rows, each alphabet shifted 
cyclically to the left compared to the previous alphabet, corresponding to the 26 
possible Caesar Ciphers. 
• At different points in the encryption process, the cipher uses a different alphabet from one of 
the rows. 
• The alphabet used at each point depends on a repeating keyword. 
1.13 Transposition (Permutation) Ciphers 
➢ Rearrange the letter order without altering the actual letters. 
Rail Fence Cipher: 
Meet me after the toga party


key: chroma:document
string_value:
30 
 
• 
A dramatic increase in the key space can be achieved by allowing an arbitrary substitution. 
• 
A permutation of a finite set of elements is an ordered sequence of all the elements of, with 
each element appearing exactly once. For example, if, there are six permutations of : 
 
• 
In general, there are ! permutations of a set of elements, because the first element can be


key: chroma:document
string_value:
chosen in one of n ways 
• 
If, instead, the “cipher” line can be any permutation of the 26 alphabetic characters, then there 
are 26! or greater than   
  possible keys. 
• 
This is 10 orders of magnitude greater than the key space for DES and would seem to 
eliminate brute-force techniques for cryptanalysis.  
•


key: chroma:document
string_value:
• 
Such an approach is referred to as a monoalphabetic substitution cipher, because a single 
cipher alphabet (mapping from plain alphabet to cipher alphabet) is used per message. 
1.12.6 Polyalphabetic Ciphers 
• Vigenere Cipher is a method of encrypting alphabetic text. It uses a simple form 
of polyalphabetic substitution.


key: chroma:document
string_value:
• A polyalphabetic cipher is any cipher based on substitution, using multiple substitution 
alphabets.  
• The encryption of the original text is done using the Vigenère square or Vigenère table. 
• The table consists of the alphabets written out 26 times in different rows, each alphabet shifted 
cyclically to the left compared to the previous alphabet, corresponding to the 26


key: chroma:document
string_value:
possible Caesar Ciphers. 
• At different points in the encryption process, the cipher uses a different alphabet from one of 
the rows. 
• The alphabet used at each point depends on a repeating keyword. 
1.13 Transposition (Permutation) Ciphers 
➢ Rearrange the letter order without altering the actual letters. 
Rail Fence Cipher: 
Meet me after the toga party


key: full_page_content
string_value:
31 
 
➢ Write message out diagonally as, 
 
Ciphertext:  
MEMATRHTGPRYETEFETEOAAT 
Row Transposition Ciphers: 
Write letters in rows, reorder the columns according to the key before reading off .  
• 
Key: 4312567 
 
Product Ciphers 
❑ Use several ciphers in succession to make harder, but: 
➢ Two substitutions make a more complex substitution 
➢ Two transpositions make more complex transposition 
➢  But a substitution followed by a transposition makes a new much harder cipher 
❑ This is a bridge from classical to modern ciphers


key: chroma:document
string_value:
31 
 
➢ Write message out diagonally as, 
 
Ciphertext:  
MEMATRHTGPRYETEFETEOAAT 
Row Transposition Ciphers: 
Write letters in rows, reorder the columns according to the key before reading off .  
• 
Key: 4312567 
 
Product Ciphers 
❑ Use several ciphers in succession to make harder, but: 
➢ Two substitutions make a more complex substitution


key: chroma:document
string_value:
➢ Two transpositions make more complex transposition 
➢  But a substitution followed by a transposition makes a new much harder cipher 
❑ This is a bridge from classical to modern ciphers


key: full_page_content
string_value:
32 
 
 
1.14 Block Cipher Vs Stream Cipher 
• 
A stream cipher is one that encrypts a digital data stream one bit or one byte at a time. 
Examples of classical stream ciphers are the autokeyed Vigenère cipher and the Vernam 
cipher. 
• 
A block cipher is one in which a block of plaintext is treated as a whole and used to 
produce a ciphertext block of equal length. Typically, a block size of 64 or 128 bits is used. 
Block Cipher 
• 
A block cipher is an encryption/decryption scheme in which a block of plaintext is treated 
as a whole and used to produce a ciphertext block of equal length. 
• 
Many block ciphers have a Feistel structure.  
• 
Such a structure consists of a number of identical rounds of processing.  
• 
In each round, a substitution is performed on one half of the data being processed, followed 
by a permutation that interchanges the two halves.  
• 
The original key is expanded so that a different key is used for each round.


key: chroma:document
string_value:
32 
 
 
1.14 Block Cipher Vs Stream Cipher 
• 
A stream cipher is one that encrypts a digital data stream one bit or one byte at a time. 
Examples of classical stream ciphers are the autokeyed Vigenère cipher and the Vernam 
cipher. 
• 
A block cipher is one in which a block of plaintext is treated as a whole and used to


key: chroma:document
string_value:
produce a ciphertext block of equal length. Typically, a block size of 64 or 128 bits is used. 
Block Cipher 
• 
A block cipher is an encryption/decryption scheme in which a block of plaintext is treated 
as a whole and used to produce a ciphertext block of equal length. 
• 
Many block ciphers have a Feistel structure.  
•


key: chroma:document
string_value:
• 
Such a structure consists of a number of identical rounds of processing.  
• 
In each round, a substitution is performed on one half of the data being processed, followed 
by a permutation that interchanges the two halves.  
• 
The original key is expanded so that a different key is used for each round.


key: full_page_content
string_value:
33 
 
 
Fig. 16 Stream Cipher 
 
Fig. 17 Block Cipher 
1.15 Feistel Cipher Structure 
• 
A block cipher operates on a plaintext block of n bits to produce a ciphertext block of n bits. 
There are 2n possible different plaintext blocks and, for the encryption to be reversible (i.e., 
for decryption to be possible), each must produce a unique ciphertext block. Such a 
transformation is called reversible, or nonsingular. 
• 
The following examples illustrate nonsingular and singular transformations for n=2. 
 
Fig. 18 Reversible vs Irreversible Mapping


key: chroma:document
string_value:
33 
 
 
Fig. 16 Stream Cipher 
 
Fig. 17 Block Cipher 
1.15 Feistel Cipher Structure 
• 
A block cipher operates on a plaintext block of n bits to produce a ciphertext block of n bits. 
There are 2n possible different plaintext blocks and, for the encryption to be reversible (i.e., 
for decryption to be possible), each must produce a unique ciphertext block. Such a


key: chroma:document
string_value:
transformation is called reversible, or nonsingular. 
• 
The following examples illustrate nonsingular and singular transformations for n=2. 
 
Fig. 18 Reversible vs Irreversible Mapping


key: full_page_content
string_value:
34 
 
• 
Feistel proposed the use of a cipher that alternates substitutions and permutations, where 
these terms are defined as follows: 
• 
Substitution: Each plaintext element or group of elements is uniquely replaced by a 
corresponding ciphertext element or group of elements. 
• 
Permutation: A sequence of plaintext elements is replaced by a permutation of that 
sequence. That is, no elements are added or deleted or replaced in the sequence, 
rather the order in which the elements appear in the sequence is changed 
• 
In fact, Feistel’s is a practical application of a proposal by Claude Shannon to develop a 
product cipher that alternates confusion and diffusion functions 
• 
The terms diffusion and confusion were introduced by Claude Shannon to capture the two 
basic building blocks for any cryptographic system. 
• 
In diffusion, the statistical structure of the plaintext is dissipated into long-range statistics of 
the ciphertext. This is achieved by having each plaintext digit affect the value of many 
ciphertext digits; generally, this is equivalent to having each ciphertext digit be affected by 
many plaintext digits. 
• 
confusion seeks to make the relationship between the statistics of the ciphertext and the 
value of the encryption key as complex as possible, again to thwart attempts to discover the 
key. 
• 
The inputs to the encryption algorithm are a plaintext block of length 2W bits and a key. 
• 
The plaintext block is divided into two halves, L0 and R0  
• 
The two halves of the data pass through rounds of processing and then combine to produce 
the ciphertext block 
• 
Each round has as inputs Li-1 and Ri-1 derived from the previous round, as well as a subkey 
Ki derived from the overall K. 
• 
16 rounds are used, although any number of rounds could be implemented. 
• 
A substitution is performed on the left half of the data. This is done by applying a round 
function F to the right half of the data and then taking the exclusive-OR of the output of that 
function and the left half of the data.


key: chroma:document
string_value:
34 
 
• 
Feistel proposed the use of a cipher that alternates substitutions and permutations, where 
these terms are defined as follows: 
• 
Substitution: Each plaintext element or group of elements is uniquely replaced by a 
corresponding ciphertext element or group of elements. 
• 
Permutation: A sequence of plaintext elements is replaced by a permutation of that


key: chroma:document
string_value:
sequence. That is, no elements are added or deleted or replaced in the sequence, 
rather the order in which the elements appear in the sequence is changed 
• 
In fact, Feistel’s is a practical application of a proposal by Claude Shannon to develop a 
product cipher that alternates confusion and diffusion functions 
•


key: chroma:document
string_value:
• 
The terms diffusion and confusion were introduced by Claude Shannon to capture the two 
basic building blocks for any cryptographic system. 
• 
In diffusion, the statistical structure of the plaintext is dissipated into long-range statistics of 
the ciphertext. This is achieved by having each plaintext digit affect the value of many


key: chroma:document
string_value:
ciphertext digits; generally, this is equivalent to having each ciphertext digit be affected by 
many plaintext digits. 
• 
confusion seeks to make the relationship between the statistics of the ciphertext and the 
value of the encryption key as complex as possible, again to thwart attempts to discover the 
key. 
•


key: chroma:document
string_value:
key. 
• 
The inputs to the encryption algorithm are a plaintext block of length 2W bits and a key. 
• 
The plaintext block is divided into two halves, L0 and R0  
• 
The two halves of the data pass through rounds of processing and then combine to produce 
the ciphertext block 
• 
Each round has as inputs Li-1 and Ri-1 derived from the previous round, as well as a subkey


key: chroma:document
string_value:
Ki derived from the overall K. 
• 
16 rounds are used, although any number of rounds could be implemented. 
• 
A substitution is performed on the left half of the data. This is done by applying a round 
function F to the right half of the data and then taking the exclusive-OR of the output of that 
function and the left half of the data.


key: full_page_content
string_value:
35 
 
• 
Permutation is performed that consists of the interchange of the two halves of the data 
• 
This structure is a particular form of the substitution-permutation network (SPN) proposed 
by Shannon 
 
Fig.18 Fiestel Cipher Structure 
Design Features 
• 
The exact realization of a Feistel network depends on the choice of the following parameters 
and design features: 
• 
Block size: Larger block sizes mean greater security (all other things being equal) but reduced 
encryption/decryption speed for a given algorithm 
• 
Key size: Larger key size means greater security but may decrease encryption/ decryption 
speed.


key: chroma:document
string_value:
35 
 
• 
Permutation is performed that consists of the interchange of the two halves of the data 
• 
This structure is a particular form of the substitution-permutation network (SPN) proposed 
by Shannon 
 
Fig.18 Fiestel Cipher Structure 
Design Features 
• 
The exact realization of a Feistel network depends on the choice of the following parameters 
and design features: 
•


key: chroma:document
string_value:
• 
Block size: Larger block sizes mean greater security (all other things being equal) but reduced 
encryption/decryption speed for a given algorithm 
• 
Key size: Larger key size means greater security but may decrease encryption/ decryption 
speed.


key: full_page_content
string_value:
36 
 
• 
Number of rounds: The essence of the Feistel cipher is that a single round offers inadequate 
security but that multiple rounds offer increasing security. A typical size is 16 rounds. 
• 
Subkey generation algorithm: Greater complexity in this algorithm should lead to greater 
difficulty of cryptanalysis. 
• 
Round function F: Again, greater complexity generally means greater resistance to 
cryptanalysis. 
Feistel Decryption Algorithm 
• 
The process of decryption with a Feistel cipher is essentially the same as the encryption 
process.  
• 
The rule is as follows:  
• 
Use the ciphertext as input to the algorithm, but use the subkeys Ki  in reverse order. 
1.16 Data Encryption Standard (DES) 
• 
The Data Encryption Standard (DES) has been the most widely used symmetric encryption 
algorithm until recently. It exhibits the classic Feistel structure.  
• 
DES uses a 64-bit block and a 56-bit key. 
• 
Two important methods of cryptanalysis are differential cryptanalysis and linear 
cryptanalysis.  
• 
DES has been shown to be highly resistant to these two types of attack. 
DES Encryption 
• 
As with any encryption scheme, there are two inputs to the encryption function: the plaintext 
to be encrypted and the key. In this case, the plaintext must be 64 bits in length and the key is 
56 bits in length. 
• 
With the exception of the initial and final permutations, DES has the exact structure of a 
Feistel cipher,


key: chroma:document
string_value:
36 
 
• 
Number of rounds: The essence of the Feistel cipher is that a single round offers inadequate 
security but that multiple rounds offer increasing security. A typical size is 16 rounds. 
• 
Subkey generation algorithm: Greater complexity in this algorithm should lead to greater 
difficulty of cryptanalysis. 
•


key: chroma:document
string_value:
• 
Round function F: Again, greater complexity generally means greater resistance to 
cryptanalysis. 
Feistel Decryption Algorithm 
• 
The process of decryption with a Feistel cipher is essentially the same as the encryption 
process.  
• 
The rule is as follows:  
• 
Use the ciphertext as input to the algorithm, but use the subkeys Ki  in reverse order.


key: chroma:document
string_value:
1.16 Data Encryption Standard (DES) 
• 
The Data Encryption Standard (DES) has been the most widely used symmetric encryption 
algorithm until recently. It exhibits the classic Feistel structure.  
• 
DES uses a 64-bit block and a 56-bit key. 
• 
Two important methods of cryptanalysis are differential cryptanalysis and linear 
cryptanalysis.  
•


key: chroma:document
string_value:
cryptanalysis.  
• 
DES has been shown to be highly resistant to these two types of attack. 
DES Encryption 
• 
As with any encryption scheme, there are two inputs to the encryption function: the plaintext 
to be encrypted and the key. In this case, the plaintext must be 64 bits in length and the key is 
56 bits in length. 
•


key: chroma:document
string_value:
• 
With the exception of the initial and final permutations, DES has the exact structure of a 
Feistel cipher,


key: full_page_content
string_value:
37 
 
 
Fig. 19 Data Encryption Standard 
Single Round of DES Algorithm 
 
Fig. 20 Single Round of DES Algorithm


key: full_page_content
string_value:
42 
 
• 
A desirable property of any encryption algorithm is that a small change in either the plaintext 
or the key should produce a significant change in the ciphertext.  
• 
In particular, a change in one bit of the plaintext or one bit of the key should produce a change 
in many bits of the ciphertext.  
• 
This is referred to as the avalanche effect.  
• 
If the change were small, this might provide a way to reduce the size of the plaintext or key 
space to be searched. 
 
Fig. 24 Avalanche Effect in DES (Change in Plaintext) 
 
Fig. 25 Avalanche Effect in DES (Change in Key)


key: chroma:document
string_value:
42 
 
• 
A desirable property of any encryption algorithm is that a small change in either the plaintext 
or the key should produce a significant change in the ciphertext.  
• 
In particular, a change in one bit of the plaintext or one bit of the key should produce a change 
in many bits of the ciphertext.  
• 
This is referred to as the avalanche effect.  
•


key: chroma:document
string_value:
• 
If the change were small, this might provide a way to reduce the size of the plaintext or key 
space to be searched. 
 
Fig. 24 Avalanche Effect in DES (Change in Plaintext) 
 
Fig. 25 Avalanche Effect in DES (Change in Key)


key: full_page_content
string_value:
43 
 
Strength of DES 
• 
The Use of 56-Bit Keys 
• 
With a key length of 56 bits, there are 256 possible keys, which is approximately 7.2 
X 1016 keys 
• 
Thus, on the face of it, a brute-force attack appears impractical 
• 
Assuming that, on average, half the key space has to be searched, a single machine 
performing one DES encryption per microsecond would take more than a thousand 
years to break the cipher. 
• 
The Nature of the DES Algorithm 
• 
Another concern is the possibility that cryptanalysis is possible by exploiting the 
characteristics of the DES algorithm. The focus of concern has been on the eight 
substitution tables, or S-boxes, that are used in each iteration. Because the design 
criteria for these boxes, and indeed for the entire algorithm, were not made public, 
there is a suspicion that the boxes were constructed in such a way that cryptanalysis 
• For most of its life, the prime concern with DES has been its vulnerability to brute-
force attack because of its relatively short (56 bits) key length. However, there has 
also been interest in finding cryptanalytic attacks on DES. With the increasing 
popularity of block ciphers with longer key lengths, including triple DES, brute- force 
attacks have become increasingly impractical. 
Confidentiality using Symmetric Encryption 
• 
Placement of Encryption Function 
• 
What to encrypt and where to place the encrypted function 
• 
Encryption placement – end to end encryption and link encryption 
• 
There are number of locations where attacks can occur. 
• 
And, it is not under the physical control of the end user.


key: chroma:document
string_value:
43 
 
Strength of DES 
• 
The Use of 56-Bit Keys 
• 
With a key length of 56 bits, there are 256 possible keys, which is approximately 7.2 
X 1016 keys 
• 
Thus, on the face of it, a brute-force attack appears impractical 
• 
Assuming that, on average, half the key space has to be searched, a single machine


key: chroma:document
string_value:
performing one DES encryption per microsecond would take more than a thousand 
years to break the cipher. 
• 
The Nature of the DES Algorithm 
• 
Another concern is the possibility that cryptanalysis is possible by exploiting the 
characteristics of the DES algorithm. The focus of concern has been on the eight


key: chroma:document
string_value:
substitution tables, or S-boxes, that are used in each iteration. Because the design 
criteria for these boxes, and indeed for the entire algorithm, were not made public, 
there is a suspicion that the boxes were constructed in such a way that cryptanalysis 
• For most of its life, the prime concern with DES has been its vulnerability to brute-


key: chroma:document
string_value:
force attack because of its relatively short (56 bits) key length. However, there has 
also been interest in finding cryptanalytic attacks on DES. With the increasing 
popularity of block ciphers with longer key lengths, including triple DES, brute- force 
attacks have become increasingly impractical. 
Confidentiality using Symmetric Encryption 
•


key: chroma:document
string_value:
• 
Placement of Encryption Function 
• 
What to encrypt and where to place the encrypted function 
• 
Encryption placement – end to end encryption and link encryption 
• 
There are number of locations where attacks can occur. 
• 
And, it is not under the physical control of the end user.


key: full_page_content
string_value:
44 
 
 
Fig. 26 Packet Switching Network 
Table 3 :  Difference Between Link Encryption and End-to-End Encryption 
 
Traffic Confidentiality 
• 
The following types of information can be derived from a traffic analysis attack. 
• 
Identities of partners 
• 
How frequently the partners are communicating 
• 
Message pattern, message length, quantity of messages that suggest important 
information being exchanged. 
• 
The events that correlate with special conversations between particular partners.


key: chroma:document
string_value:
44 
 
 
Fig. 26 Packet Switching Network 
Table 3 :  Difference Between Link Encryption and End-to-End Encryption 
 
Traffic Confidentiality 
• 
The following types of information can be derived from a traffic analysis attack. 
• 
Identities of partners 
• 
How frequently the partners are communicating 
•


key: chroma:document
string_value:
• 
Message pattern, message length, quantity of messages that suggest important 
information being exchanged. 
• 
The events that correlate with special conversations between particular partners.


key: full_page_content
string_value:
45 
 
Covert Channel 
• 
A covert channel is any communication channel that can be exploited by a process to transfer 
information in a manner that violates the systems security policy.  
• 
In short, covert channels transfer information using non-standard methods against the system 
design. 
• 
The main purpose of covert channels is to protect privacy or to increase security of critical 
communication. 
Symmetric Key Distribution 
• 
For symmetric encryption to work, the two parties to an exchange must share the same key, 
and that key must be protected from access by others.  
• 
Furthermore, frequent key changes are usually desirable to limit the amount of data 
compromised if an attacker learns the key.  
• 
Therefore, the strength of any cryptographic system rests with the key distribution technique, 
a term that refers to the means of delivering a key to two parties who wish to exchange data 
without allowing others to see the key. 
For two parties A and B, key distribution can be achieved in a number of ways, as follows: 
1. A can select a key and physically deliver it to B. 
2. A third party can select the key and physically deliver it to A and B. 
3. If A and B have previously and recently used a key, one party can transmit the new key to the 
other, encrypted using the old key. 
4. If A and B each has an encrypted connection to a third party C, C can deliver a key on the 
encrypted links to A and B. 
• Options 1 and 2 call for manual delivery of a key.  
• For link encryption, this is a reasonable requirement, because each link encryption device is going 
to be exchanging data only with its partner on the other end of the link.  
• However, for end-to-end encryption over a network, manual delivery is not good. In a distributed 
system, any given host or terminal may need to engage in exchanges with many other hosts and 
terminals over time. 
• Thus, each device needs a number of keys supplied dynamically. The problem is especially 
difficult in a wide-area distributed system.


key: chroma:document
string_value:
45 
 
Covert Channel 
• 
A covert channel is any communication channel that can be exploited by a process to transfer 
information in a manner that violates the systems security policy.  
• 
In short, covert channels transfer information using non-standard methods against the system 
design. 
•


key: chroma:document
string_value:
design. 
• 
The main purpose of covert channels is to protect privacy or to increase security of critical 
communication. 
Symmetric Key Distribution 
• 
For symmetric encryption to work, the two parties to an exchange must share the same key, 
and that key must be protected from access by others.  
•


key: chroma:document
string_value:
• 
Furthermore, frequent key changes are usually desirable to limit the amount of data 
compromised if an attacker learns the key.  
• 
Therefore, the strength of any cryptographic system rests with the key distribution technique, 
a term that refers to the means of delivering a key to two parties who wish to exchange data 
without allowing others to see the key.


key: chroma:document
string_value:
For two parties A and B, key distribution can be achieved in a number of ways, as follows: 
1. A can select a key and physically deliver it to B. 
2. A third party can select the key and physically deliver it to A and B. 
3. If A and B have previously and recently used a key, one party can transmit the new key to the 
other, encrypted using the old key.


key: chroma:document
string_value:
4. If A and B each has an encrypted connection to a third party C, C can deliver a key on the 
encrypted links to A and B. 
• Options 1 and 2 call for manual delivery of a key.  
• For link encryption, this is a reasonable requirement, because each link encryption device is going 
to be exchanging data only with its partner on the other end of the link.


key: chroma:document
string_value:
• However, for end-to-end encryption over a network, manual delivery is not good. In a distributed 
system, any given host or terminal may need to engage in exchanges with many other hosts and 
terminals over time. 
• Thus, each device needs a number of keys supplied dynamically. The problem is especially 
difficult in a wide-area distributed system.


key: full_page_content
string_value:
46 
 
• 
The scale of the problem depends on the number of communicating pairs that must be 
supported. If end-to-end encryption is done at a network or IP level, then a key is needed for 
each pair of hosts on the network that wish to communicate. 
• 
Thus, if there are N hosts, the number of required keys is [N(N-1)]/2. 
• 
If encryption is done at the application level, then a key is needed for every pair of users or 
processes that require communication. 
• 
The use of a key distribution center is based on the use of a hierarchy of keys. At a minimum, 
two levels of keys are used.  
• 
Communication between end systems is encrypted using a temporary key, often referred to 
as a session key. 
• 
Each session key is obtained from the key distribution center over the same networking 
facilities used for end-user communication. Accordingly, session keys are transmitted in 
encrypted form, using a master key that is shared by the key distribution center and an end 
system or user. 
 
Fig. 27 The Use of Key Hierarchy 
A Key Distribution Scenario 
• 
The key distribution concept can be deployed in a number of ways. 
• 
The scenario assumes that each user shares a unique master key with the key distribution 
center (KDC).


key: chroma:document
string_value:
46 
 
• 
The scale of the problem depends on the number of communicating pairs that must be 
supported. If end-to-end encryption is done at a network or IP level, then a key is needed for 
each pair of hosts on the network that wish to communicate. 
• 
Thus, if there are N hosts, the number of required keys is [N(N-1)]/2. 
•


key: chroma:document
string_value:
• 
If encryption is done at the application level, then a key is needed for every pair of users or 
processes that require communication. 
• 
The use of a key distribution center is based on the use of a hierarchy of keys. At a minimum, 
two levels of keys are used.  
• 
Communication between end systems is encrypted using a temporary key, often referred to 
as a session key. 
•


key: chroma:document
string_value:
• 
Each session key is obtained from the key distribution center over the same networking 
facilities used for end-user communication. Accordingly, session keys are transmitted in 
encrypted form, using a master key that is shared by the key distribution center and an end 
system or user. 
 
Fig. 27 The Use of Key Hierarchy 
A Key Distribution Scenario 
•


key: chroma:document
string_value:
• 
The key distribution concept can be deployed in a number of ways. 
• 
The scenario assumes that each user shares a unique master key with the key distribution 
center (KDC).


key: full_page_content
string_value:
47 
 
 
Fig.28 Symmetric Key Distribution Scenario 
• 
Let us assume that user A wishes to establish a logical connection with B and requires a one-
time session key to protect the data transmitted over the connection. 
• 
A has a master key, Ka  known only to itself and the KDC. similarly, B shares the master key 
Kb with the KDC. 
The following steps occur,


key: full_page_content
string_value:
48 
 
 
 
Hierarchical Key Control 
• 
It is not necessary to limit the key distribution function to a single KDC. Indeed, for very 
large networks, it may not be practical to do so. 
• 
As an alternative, a hierarchy of KDCs can be established. For example, there can be local 
KDCs, each responsible for a small domain of the overall internetwork, such as a single LAN 
or a single building. 
• 
For communication among entities within the same local domain, the local KDC is 
responsible for key distribution. If two entities in different domains desire a shared key, then 
the corresponding local KDCs can communicate through a global KDC. In this case, any one 
of the three KDCs involved can actually select the key. 
Session Key Lifetime 
• 
The more frequently session keys are exchanged, the more secure they are, because the 
opponent has less ciphertext to work with for any given session key.  
• 
On the other hand, the distribution of session keys delays the start of any exchange and places 
a burden on network capacity. 
• 
A security manager must try to balance these competing considerations in determining the 
lifetime of a particular session key.


key: chroma:document
string_value:
48 
 
 
 
Hierarchical Key Control 
• 
It is not necessary to limit the key distribution function to a single KDC. Indeed, for very 
large networks, it may not be practical to do so. 
• 
As an alternative, a hierarchy of KDCs can be established. For example, there can be local 
KDCs, each responsible for a small domain of the overall internetwork, such as a single LAN


key: chroma:document
string_value:
or a single building. 
• 
For communication among entities within the same local domain, the local KDC is 
responsible for key distribution. If two entities in different domains desire a shared key, then 
the corresponding local KDCs can communicate through a global KDC. In this case, any one 
of the three KDCs involved can actually select the key. 
Session Key Lifetime 
•


key: chroma:document
string_value:
• 
The more frequently session keys are exchanged, the more secure they are, because the 
opponent has less ciphertext to work with for any given session key.  
• 
On the other hand, the distribution of session keys delays the start of any exchange and places 
a burden on network capacity. 
•


key: chroma:document
string_value:
• 
A security manager must try to balance these competing considerations in determining the 
lifetime of a particular session key.


key: full_page_content
string_value:
49 
 
Decentralized Key Control 
• 
The use of a key distribution center imposes the requirement that the KDC be trusted and 
be protected from subversion.  
• 
This requirement can be avoided if key distribution is fully decentralized.  
• 
Although full decentralization is not practical for larger networks using symmetric 
encryption only, it may be useful within a local context. 
 
 
Fig. 29 Decentralized Key Control 
 
References: 
• William Stallings, “ Cryptography and Network Security”, 4th Edition, Pearson, 2009. 
• Behrouz A. Forouzan, “ Cryptography and Network Security”, Tata McGraw-Hill, 2008.


key: chroma:document
string_value:
49 
 
Decentralized Key Control 
• 
The use of a key distribution center imposes the requirement that the KDC be trusted and 
be protected from subversion.  
• 
This requirement can be avoided if key distribution is fully decentralized.  
• 
Although full decentralization is not practical for larger networks using symmetric


key: chroma:document
string_value:
encryption only, it may be useful within a local context. 
 
 
Fig. 29 Decentralized Key Control 
 
References: 
• William Stallings, “ Cryptography and Network Security”, 4th Edition, Pearson, 2009. 
• Behrouz A. Forouzan, “ Cryptography and Network Security”, Tata McGraw-Hill, 2008.


key: full_page_content
string_value:
50 
 
 
 
SCHOOL OF COMPUTING 
DEPARTMENT OF INFORMATION TECHNOLOGY 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
UNIT – II Public Key Cryptography  SITA1602


key: full_page_content
string_value:
51 
 
 
 
 
II. Public Key Cryptography 
Principles – RSA algorithm – Key Management – Diffie Hellman Key Exchange 
–Message Authentication Requirements, Functions, Message Authentication – 
Hash Functions – Digital Signatures – Authentication Protocols 
2.1 Introduction 
When the two parties communicate to each other to transfer the intelligible or sensible message, 
referred to as plaintext, is converted into apparently random nonsense for security purpose 
referred to as ciphertext.  
Encryption: 
The process of changing the plaintext into the ciphertext is referred to as encryption.  
The encryption process consists of an algorithm and a key. The key is a value independent of the 
plaintext.  
The security of conventional encryption depends on the major two factors:  
1. The Encryption algorithm 
2. Secrecy of the key 
Once the ciphertext is produced, it may be transmitted. The Encryption algorithm will produce a 
different output depending on the specific key being used at the time. Changing the key changes the 
output of the algorithm. Once the ciphertext is produced, it may be transmitted. Upon reception, the 
ciphertext can be transformed back to the original plaintext by using a decryption algorithm and the 
same key that was used for encryption.  
Decryption:  
The process of changing the ciphertext to the plaintext that process is known as decryption.  
Public Key Encryption :  Asymmetric is a form of Cryptosystem in which encryption and decryption 
are performed using different keys-Public key (known to everyone) and Private key (Secret key). 
This is known as Public Key Encryption. 
2.2 Principles of Public-Key Cryptosystems 
The concept of public-key cryptography evolved from an attempt to attack two of the most difficult 
problems associated with symmetric encryption. The first problem is that of key distribution, and the 
second one related to digital signature. key distribution under symmetric encryption requires either 
(1) that two communicants already share a key, which somehow has been distributed to them; or (2)


key: chroma:document
string_value:
51 
 
 
 
 
II. Public Key Cryptography 
Principles – RSA algorithm – Key Management – Diffie Hellman Key Exchange 
–Message Authentication Requirements, Functions, Message Authentication – 
Hash Functions – Digital Signatures – Authentication Protocols 
2.1 Introduction 
When the two parties communicate to each other to transfer the intelligible or sensible message,


key: chroma:document
string_value:
referred to as plaintext, is converted into apparently random nonsense for security purpose 
referred to as ciphertext.  
Encryption: 
The process of changing the plaintext into the ciphertext is referred to as encryption.  
The encryption process consists of an algorithm and a key. The key is a value independent of the 
plaintext.


key: chroma:document
string_value:
plaintext.  
The security of conventional encryption depends on the major two factors:  
1. The Encryption algorithm 
2. Secrecy of the key 
Once the ciphertext is produced, it may be transmitted. The Encryption algorithm will produce a 
different output depending on the specific key being used at the time. Changing the key changes the


key: chroma:document
string_value:
output of the algorithm. Once the ciphertext is produced, it may be transmitted. Upon reception, the 
ciphertext can be transformed back to the original plaintext by using a decryption algorithm and the 
same key that was used for encryption.  
Decryption:  
The process of changing the ciphertext to the plaintext that process is known as decryption.


key: chroma:document
string_value:
Public Key Encryption :  Asymmetric is a form of Cryptosystem in which encryption and decryption 
are performed using different keys-Public key (known to everyone) and Private key (Secret key). 
This is known as Public Key Encryption. 
2.2 Principles of Public-Key Cryptosystems 
The concept of public-key cryptography evolved from an attempt to attack two of the most difficult


key: chroma:document
string_value:
problems associated with symmetric encryption. The first problem is that of key distribution, and the 
second one related to digital signature. key distribution under symmetric encryption requires either 
(1) that two communicants already share a key, which somehow has been distributed to them; or (2)


key: full_page_content
string_value:
52 
 
the use of a key distribution center. The second problem is "digital signatures." If the use of 
cryptography was to become widespread, not just in military situations but for commercial and 
private purposes, then electronic messages and documents would need the equivalent of signatures 
used in paper documents.  
Asymmetric algorithms rely on one key for encryption and a different but related key for decryption.  
These algorithms have the following important characteristic: 
● It is computationally infeasible to determine the decryption key given only knowledge of the  
cryptographic algorithm and the encryption key. 
● Either of the two related keys can be used for encryption, with the other used for decryption. 
A public-key encryption scheme has six ingredients  
● Plaintext: This is the readable message or data that is fed into the algorithm as input. 
● Encryption algorithm: The encryption algorithm performs various transformations on the  
plaintext. 
● Public and private keys: This is a pair of keys that have been selected so that if one is used for  
encryption, the other is used for decryption. The exact transformations performed by the  
algorithm depend on the public or private key that is provided as input. 
● Ciphertext: This is the scrambled message produced as output. It depends on the plaintext and  
the key. For a given message, two different keys will produce two different ciphertexts. 
● Decryption algorithm: This algorithm accepts the ciphertext and the matching key and  
produces the original plaintext.


key: chroma:document
string_value:
52 
 
the use of a key distribution center. The second problem is "digital signatures." If the use of 
cryptography was to become widespread, not just in military situations but for commercial and 
private purposes, then electronic messages and documents would need the equivalent of signatures 
used in paper documents.


key: chroma:document
string_value:
Asymmetric algorithms rely on one key for encryption and a different but related key for decryption.  
These algorithms have the following important characteristic: 
● It is computationally infeasible to determine the decryption key given only knowledge of the  
cryptographic algorithm and the encryption key.


key: chroma:document
string_value:
● Either of the two related keys can be used for encryption, with the other used for decryption. 
A public-key encryption scheme has six ingredients  
● Plaintext: This is the readable message or data that is fed into the algorithm as input. 
● Encryption algorithm: The encryption algorithm performs various transformations on the  
plaintext.


key: chroma:document
string_value:
plaintext. 
● Public and private keys: This is a pair of keys that have been selected so that if one is used for  
encryption, the other is used for decryption. The exact transformations performed by the  
algorithm depend on the public or private key that is provided as input. 
● Ciphertext: This is the scrambled message produced as output. It depends on the plaintext and


key: chroma:document
string_value:
the key. For a given message, two different keys will produce two different ciphertexts. 
● Decryption algorithm: This algorithm accepts the ciphertext and the matching key and  
produces the original plaintext.


key: full_page_content
string_value:
53 
 
Fig: Process of Encryption and Decryption 
 
 
Difference between Symmetric encryption and Asymmetric encryption 
 
 
 
The essential steps are the following: 
1. Each user generates a pair of keys to be used for the encryption and decryption of messages.  
2. Each user places one of the two keys in a public register or other accessible file. This is the public  
key. The companion key is kept private. As Figure 9.1a suggests, each user maintains a collection  
of public keys obtained from others.  
S.No 
Symmetric encryption  
Asymmetric encryption  
1. 
It uses a single shared key (secret key) to 
encrypt and decrypt the message. 
It uses two different keys for encryption and 
decryption. 
2. 
The size of cipher text is same or smaller 
than the original plain text. 
The size of cipher text is same or larger than the 
original plain text. 
3. 
The encryption process is very fast. 
The encryption process is slow. 
4. 
It is used when a large amount of data is 
required to transfer. 
It is used to transfer small amount of data. 
5. 
It only provides confidentiality. 
It provides confidentiality, authenticity and non-
repudiation. 
6. 
It is less secured as there is a use of a 
single key for encryption. 
It is safer as there are two keys used for 
encryption and decryption. 
7. 
The algorithms used in symmetric 
encryption are 3DES, AES, DES, and 
RC4. 
RSA, DSA, Diffie-Hellman, ECC, ElGamal.


key: chroma:document
string_value:
53 
 
Fig: Process of Encryption and Decryption 
 
 
Difference between Symmetric encryption and Asymmetric encryption 
 
 
 
The essential steps are the following: 
1. Each user generates a pair of keys to be used for the encryption and decryption of messages.  
2. Each user places one of the two keys in a public register or other accessible file. This is the public


key: chroma:document
string_value:
key. The companion key is kept private. As Figure 9.1a suggests, each user maintains a collection  
of public keys obtained from others.  
S.No 
Symmetric encryption  
Asymmetric encryption  
1. 
It uses a single shared key (secret key) to 
encrypt and decrypt the message. 
It uses two different keys for encryption and 
decryption. 
2. 
The size of cipher text is same or smaller


key: chroma:document
string_value:
than the original plain text. 
The size of cipher text is same or larger than the 
original plain text. 
3. 
The encryption process is very fast. 
The encryption process is slow. 
4. 
It is used when a large amount of data is 
required to transfer. 
It is used to transfer small amount of data. 
5. 
It only provides confidentiality.


key: chroma:document
string_value:
It provides confidentiality, authenticity and non-
repudiation. 
6. 
It is less secured as there is a use of a 
single key for encryption. 
It is safer as there are two keys used for 
encryption and decryption. 
7. 
The algorithms used in symmetric 
encryption are 3DES, AES, DES, and 
RC4. 
RSA, DSA, Diffie-Hellman, ECC, ElGamal.


key: full_page_content
string_value:
54 
 
3. If Bob wishes to send a confidential message to Alice, Bob encrypts the message using Alice's  
public key.  
4. When Alice receives the message, she decrypts it using her private key. No other recipient can  
decrypt the message because only Alice knows Alice's private key. 
 
With this approach, all participants have access to public keys, and private keys are generated 
locally by each participant and therefore need never be distributed. As long as a user's private key 
remains protected and secret, incoming communication is secure. 
 
Table 1:  summarizes some of the important aspects of symmetric and public-key encryption. 
 
 
 
Requirements for Public-Key Cryptography 
 
1. It is computationally easy for a party B to generate a pair (public key PUb, private key PRb). 
2. It is computationally easy for a sender A, knowing the public key and the message to be  
encrypted, M, to generate the corresponding ciphertext:  
  
C = E(PUb, M) 
3. It is computationally easy for the receiver B to decrypt the resulting ciphertext using the private  
key to recover the original message: 
M = D(PRb, C) = D[PRb, E(PUb, M)] 
4. It is computationally infeasible for an adversary, knowing the public key, PUb, to determine the  
private key, PRb. 
5. It is computationally infeasible for an adversary, knowing the public key, PUb, and a ciphertext,  
C, to recover the original message, M. 
 
We can add a sixth requirement that, although useful, is not necessary for all public-key


key: chroma:document
string_value:
54 
 
3. If Bob wishes to send a confidential message to Alice, Bob encrypts the message using Alice's  
public key.  
4. When Alice receives the message, she decrypts it using her private key. No other recipient can  
decrypt the message because only Alice knows Alice's private key. 
 
With this approach, all participants have access to public keys, and private keys are generated


key: chroma:document
string_value:
locally by each participant and therefore need never be distributed. As long as a user's private key 
remains protected and secret, incoming communication is secure. 
 
Table 1:  summarizes some of the important aspects of symmetric and public-key encryption. 
 
 
 
Requirements for Public-Key Cryptography


key: chroma:document
string_value:
1. It is computationally easy for a party B to generate a pair (public key PUb, private key PRb). 
2. It is computationally easy for a sender A, knowing the public key and the message to be  
encrypted, M, to generate the corresponding ciphertext:  
  
C = E(PUb, M) 
3. It is computationally easy for the receiver B to decrypt the resulting ciphertext using the private


key: chroma:document
string_value:
key to recover the original message: 
M = D(PRb, C) = D[PRb, E(PUb, M)] 
4. It is computationally infeasible for an adversary, knowing the public key, PUb, to determine the  
private key, PRb. 
5. It is computationally infeasible for an adversary, knowing the public key, PUb, and a ciphertext,  
C, to recover the original message, M.


key: full_page_content
string_value:
55 
 
applications: 
6. The two keys can be applied in either order: 
M = D[PUb, E(PRb, M)] = D[PRb, E(PUb, M)] 
 
 
 
2.3 RSA Algorithm: 
Diffie and Hellman challenged cryptologists to come up with a cryptographic algorithm that 
met the requirements for public-key systems. 
One of the first successful responses to the challenge was developed in 1977 by Ron Rivest, Adi 
Shamir, and Len Adleman at MIT and first published in 1978. The Rivest-Shamir-Adleman (RSA) 
scheme has since that time reigned supreme as the most widely accepted and implemented general-
purpose approach to public-key encryption. 
The RSA scheme is a block cipher in which the plaintext and ciphertext are integers between 0 and 
n - 1 for some n. A typical size for n is 1024 bits, or 309 decimal digits. That is, n is less than 21024. 
We examine RSA in this section in some detail, beginning with an explanation of the algorithm. Then 
we examine some of the computational and cryptanalytical implications of RSA. 
2.3.1 Description of the Algorithm 
RSA makes use of an expression with exponentials. Plaintext is encrypted in blocks, with each block 
having a binary value less than some number n. That is, the block size must be less than or equal to 
log2(n) + 1; in practice, the block size is i bits, where 2i < n ≤ 2i+1. Encryption and decryption are 
of the following form, for some plaintext block M and ciphertext block C. 
 
Both sender and receiver must know the value of n. The sender knows the value of e, and 
only the receiver knows the value of d. Thus, this is a public-key encryption algorithm with a 
public key of PU = {e, n} and a private key of PR = {d, n}. 
For this algorithm to be satisfactory for public-key encryption, the following requirements 
must be met. 
1. It is possible to find values of e, d, n such that Med mod n = M for all M < n. 
2. It is relatively easy to calculate Me mod n and Cd mod n for all values of M < n. 
3. It is infeasible to determine d given e and n. 
We need to find a relationship of the form 
Med mod n = M 
The preceding relationship holds if e and d are multiplicative inverses modulo φ(n), where 
φ(n) is the Euler totient function. 
For p,q prime, φ (pq) = (p - 1)(q - 1). The relationship between e and d can be expressed as


key: chroma:document
string_value:
55 
 
applications: 
6. The two keys can be applied in either order: 
M = D[PUb, E(PRb, M)] = D[PRb, E(PUb, M)] 
 
 
 
2.3 RSA Algorithm: 
Diffie and Hellman challenged cryptologists to come up with a cryptographic algorithm that 
met the requirements for public-key systems. 
One of the first successful responses to the challenge was developed in 1977 by Ron Rivest, Adi


key: chroma:document
string_value:
Shamir, and Len Adleman at MIT and first published in 1978. The Rivest-Shamir-Adleman (RSA) 
scheme has since that time reigned supreme as the most widely accepted and implemented general-
purpose approach to public-key encryption. 
The RSA scheme is a block cipher in which the plaintext and ciphertext are integers between 0 and


key: chroma:document
string_value:
n - 1 for some n. A typical size for n is 1024 bits, or 309 decimal digits. That is, n is less than 21024. 
We examine RSA in this section in some detail, beginning with an explanation of the algorithm. Then 
we examine some of the computational and cryptanalytical implications of RSA. 
2.3.1 Description of the Algorithm


key: chroma:document
string_value:
RSA makes use of an expression with exponentials. Plaintext is encrypted in blocks, with each block 
having a binary value less than some number n. That is, the block size must be less than or equal to 
log2(n) + 1; in practice, the block size is i bits, where 2i < n ≤ 2i+1. Encryption and decryption are 
of the following form, for some plaintext block M and ciphertext block C.


key: chroma:document
string_value:
Both sender and receiver must know the value of n. The sender knows the value of e, and 
only the receiver knows the value of d. Thus, this is a public-key encryption algorithm with a 
public key of PU = {e, n} and a private key of PR = {d, n}. 
For this algorithm to be satisfactory for public-key encryption, the following requirements 
must be met.


key: chroma:document
string_value:
must be met. 
1. It is possible to find values of e, d, n such that Med mod n = M for all M < n. 
2. It is relatively easy to calculate Me mod n and Cd mod n for all values of M < n. 
3. It is infeasible to determine d given e and n. 
We need to find a relationship of the form 
Med mod n = M


key: chroma:document
string_value:
Med mod n = M 
The preceding relationship holds if e and d are multiplicative inverses modulo φ(n), where 
φ(n) is the Euler totient function. 
For p,q prime, φ (pq) = (p - 1)(q - 1). The relationship between e and d can be expressed as


key: full_page_content
string_value:
56 
 
ed mod φ(n) = 1 
 
This is equivalent to saying 
 
 
That is, e and d are multiplicative inverses mod  (n). Note that, according to the rules of modular 
arithmetic, this is true only if d (and therefore e) is relatively prime to  (n). Equivalently, gcd(  (n), 
d) = 1.  
The ingredients are the following:  
 
The private key consists of {d, n} and the public key consists of {e, n}. Suppose that user A has 
published its public key and that user B wishes to send the message M to A. Then B calculates C = 
Me mod n and transmits C. On receipt of this ciphertext, user A decrypts by calculating M = C d mod 
n. 
Table :2  RSA Algorithm


key: chroma:document
string_value:
56 
 
ed mod φ(n) = 1 
 
This is equivalent to saying 
 
 
That is, e and d are multiplicative inverses mod  (n). Note that, according to the rules of modular 
arithmetic, this is true only if d (and therefore e) is relatively prime to  (n). Equivalently, gcd(  (n), 
d) = 1.  
The ingredients are the following:


key: chroma:document
string_value:
The private key consists of {d, n} and the public key consists of {e, n}. Suppose that user A has 
published its public key and that user B wishes to send the message M to A. Then B calculates C = 
Me mod n and transmits C. On receipt of this ciphertext, user A decrypts by calculating M = C d mod 
n. 
Table :2  RSA Algorithm


key: full_page_content
string_value:
57 
 
 
Fig.1  Example of RSA Algorithm 
 
2.4 Key Management 
One of the major roles of public-key encryption has been to address the problem of key distribution. 
There are actually two distinct aspects to the use of public-key cryptography in this regard: 
● The distribution of public keys 
● The use of public-key encryption to distribute secret keys 
2.4.1 Distribution of Public Keys 
Several techniques have been proposed for the distribution of public keys. Virtually all these 
proposals can be grouped into the following general schemes: 
● Public announcement 
● Publicly available directory 
● Public-key authority 
● Public-key certificates 
2.4.1.1 Public Announcement of Public Key 
On the face of it, the point of public-key encryption is that the public key is public. Thus, if there is 
some broadly accepted public-key algorithm, such as RSA, any participant can send his or her public 
key to any other participant or broadcast the key to the community at large


key: chroma:document
string_value:
57 
 
 
Fig.1  Example of RSA Algorithm 
 
2.4 Key Management 
One of the major roles of public-key encryption has been to address the problem of key distribution. 
There are actually two distinct aspects to the use of public-key cryptography in this regard: 
● The distribution of public keys 
● The use of public-key encryption to distribute secret keys


key: chroma:document
string_value:
2.4.1 Distribution of Public Keys 
Several techniques have been proposed for the distribution of public keys. Virtually all these 
proposals can be grouped into the following general schemes: 
● Public announcement 
● Publicly available directory 
● Public-key authority 
● Public-key certificates 
2.4.1.1 Public Announcement of Public Key


key: chroma:document
string_value:
On the face of it, the point of public-key encryption is that the public key is public. Thus, if there is 
some broadly accepted public-key algorithm, such as RSA, any participant can send his or her public 
key to any other participant or broadcast the key to the community at large


key: full_page_content
string_value:
58 
 
 
 
 
 
Fig: 2 Uncontrolled Public-Key Distribution 
Publicly Available Directory 
A greater degree of security can be achieved by maintaining a publicly available dynamic directory 
of public keys. Maintenance and distribution of the public directory would have to be the 
responsibility of some trusted entity or organization (Figure 3). Such a scheme would include the 
following elements: 
 
Fig : 3 Public-Key Publication 
 
1. The authority maintains a directory with a {name, public key} entry for each participant. 
2. Each participant registers a public key with the directory authority. Registration would have to be  
in person or by some form of secure authenticated communication.


key: chroma:document
string_value:
58 
 
 
 
 
 
Fig: 2 Uncontrolled Public-Key Distribution 
Publicly Available Directory 
A greater degree of security can be achieved by maintaining a publicly available dynamic directory 
of public keys. Maintenance and distribution of the public directory would have to be the 
responsibility of some trusted entity or organization (Figure 3). Such a scheme would include the


key: chroma:document
string_value:
following elements: 
 
Fig : 3 Public-Key Publication 
 
1. The authority maintains a directory with a {name, public key} entry for each participant. 
2. Each participant registers a public key with the directory authority. Registration would have to be  
in person or by some form of secure authenticated communication.


key: full_page_content
string_value:
59 
 
3. A participant may replace the existing key with a new one at any time, either because of the  
desire to replace a public key that has already been used for a large amount of data, or because  
the corresponding private key has been compromised in some way. 
4. Participants could also access the directory electronically. For this purpose, secure, authenticated  
communication from the authority to the participant is mandatory. 
2.4.1.2 Public-Key Authority 
Stronger security for public-key distribution can be achieved by providing tighter control over the  
distribution of public keys from the directory. A typical scenario is illustrated in Figure 4 As before, 
the scenario assumes that a central authority maintains a dynamic directory of public keys of all 
participants. In addition, each participant reliably knows a public key for the authority, with only the 
authority knowing the corresponding private key.  
 
1. A sends a timestamped message to the public-key authority containing a request for the current  
public key of B.  
2. The authority responds with a message that is encrypted using the authority's private key, PRauth 
Thus, A is able to decrypt the message using the authority's public key. Therefore, A is assured  
that the message originated with the authority. The message includes the following:  
● B's public key, PUb  which A can use to encrypt messages destined for B 
● The original request, to enable A to match this response with the corresponding earlier  
request and to verify that the original request was not altered before reception by the  
authority


key: chroma:document
string_value:
59 
 
3. A participant may replace the existing key with a new one at any time, either because of the  
desire to replace a public key that has already been used for a large amount of data, or because  
the corresponding private key has been compromised in some way. 
4. Participants could also access the directory electronically. For this purpose, secure, authenticated


key: chroma:document
string_value:
communication from the authority to the participant is mandatory. 
2.4.1.2 Public-Key Authority 
Stronger security for public-key distribution can be achieved by providing tighter control over the  
distribution of public keys from the directory. A typical scenario is illustrated in Figure 4 As before,


key: chroma:document
string_value:
the scenario assumes that a central authority maintains a dynamic directory of public keys of all 
participants. In addition, each participant reliably knows a public key for the authority, with only the 
authority knowing the corresponding private key.  
 
1. A sends a timestamped message to the public-key authority containing a request for the current  
public key of B.


key: chroma:document
string_value:
public key of B.  
2. The authority responds with a message that is encrypted using the authority's private key, PRauth 
Thus, A is able to decrypt the message using the authority's public key. Therefore, A is assured  
that the message originated with the authority. The message includes the following:  
● B's public key, PUb  which A can use to encrypt messages destined for B


key: chroma:document
string_value:
● The original request, to enable A to match this response with the corresponding earlier  
request and to verify that the original request was not altered before reception by the  
authority


key: full_page_content
string_value:
60 
 
● The original timestamp, so A can determine that this is not an old message from the  
authority containing a key other than B's current public key 
3. A stores B's public key and also uses it to encrypt a message to B containing an identifier of A  
(IDA) and a nonce (N1), which is used to identify this transaction uniquely.  
4. B retrieves A's public key from the authority in the same manner as A retrieved B's public key.  
At this point, public keys have been securely delivered to A and B, and they may begin their  
protected exchange. However, two additional steps are desirable:  
6. B sends a message to A encrypted with PUa  and containing A's nonce (N1) as well as a new nonce  
generated by B (N2) Because only B could have decrypted message (3), the presence of N1 in  
message (6) assures A that the correspondent is B.  
7. A returns N2, encrypted using B's public key, to assure B that its correspondent is A.  
2.4.1.3 Public-Key Certificates 
The public key certificate relies on certificates that can be used by participants to exchange keys 
without contacting a public-key authority, in a way that is as reliable as if the keys were obtained 
directly from a public-key authority. In essence, a certificate consists of a public key, an identifier of 
the key owner, and the whole block signed by a trusted third party. 
 Typically, the third party is a certificate authority, such as a government agency or a financial 
institution that is trusted by the user community. A user can present his or her public key to the 
authority in a secure manner and obtain a certificate. The user can then publish the certificate. Anyone 
needing this user’s public key can obtain the certificate and verify that it is valid by way of the 
attached trusted signature. 
We can place the following requirements on this scheme: 
1.Any participant can read a certificate to determine the name and public key of the certificate’s 
owner. 
2.Any participant can verify that the certificate originated from the certificate authority and is not 
counterfeit. 
3.Only the certificate authority can create and update certificates. 
4.Any participant can verify the currency of the certificate. 
A certificate scheme is illustrated in Figure 5. Each participant applies to the certificate authority,  
supplying a public key and requesting a certificate.


key: chroma:document
string_value:
60 
 
● The original timestamp, so A can determine that this is not an old message from the  
authority containing a key other than B's current public key 
3. A stores B's public key and also uses it to encrypt a message to B containing an identifier of A  
(IDA) and a nonce (N1), which is used to identify this transaction uniquely.


key: chroma:document
string_value:
4. B retrieves A's public key from the authority in the same manner as A retrieved B's public key.  
At this point, public keys have been securely delivered to A and B, and they may begin their  
protected exchange. However, two additional steps are desirable:  
6. B sends a message to A encrypted with PUa  and containing A's nonce (N1) as well as a new nonce


key: chroma:document
string_value:
generated by B (N2) Because only B could have decrypted message (3), the presence of N1 in  
message (6) assures A that the correspondent is B.  
7. A returns N2, encrypted using B's public key, to assure B that its correspondent is A.  
2.4.1.3 Public-Key Certificates 
The public key certificate relies on certificates that can be used by participants to exchange keys


key: chroma:document
string_value:
without contacting a public-key authority, in a way that is as reliable as if the keys were obtained 
directly from a public-key authority. In essence, a certificate consists of a public key, an identifier of 
the key owner, and the whole block signed by a trusted third party. 
 Typically, the third party is a certificate authority, such as a government agency or a financial


key: chroma:document
string_value:
institution that is trusted by the user community. A user can present his or her public key to the 
authority in a secure manner and obtain a certificate. The user can then publish the certificate. Anyone 
needing this user’s public key can obtain the certificate and verify that it is valid by way of the 
attached trusted signature.


key: chroma:document
string_value:
We can place the following requirements on this scheme: 
1.Any participant can read a certificate to determine the name and public key of the certificate’s 
owner. 
2.Any participant can verify that the certificate originated from the certificate authority and is not 
counterfeit. 
3.Only the certificate authority can create and update certificates.


key: chroma:document
string_value:
4.Any participant can verify the currency of the certificate. 
A certificate scheme is illustrated in Figure 5. Each participant applies to the certificate authority,  
supplying a public key and requesting a certificate.


key: full_page_content
string_value:
61 
 
 
Fig: 6 Exchange of Public-Key Certificates 
 
Application must be in person or by some form of secure authenticated communication. For 
participant A, the authority provides a certificate of the form 
CA = E(PRauth, [T||IDA||PUa]) 
where PRauth is the private key used by the authority and T is a timestamp. A may then pass this  
certificate on to any other participant, who reads and verifies the certificate as follows: 
D(PUauth, CA) = D(PUauth, E(PRauth, [T||IDA||PUa])) = (T||IDA||PUa) 
The recipient uses the authority's public key, PUauth to decrypt the certificate. Because the certificate 
is readable only using the authority's public key, this verifies that the certificate came from the 
certificate authority. The elements IDA and PUa provide the recipient with the name and public key 
of the certificate's holder. The timestamp T validates the currency of the certificate. The timestamp 
counters the following scenario. A's private key is learned by an adversary. A generates a new 
private/public key pair and applies to the certificate authority for a new certificate. Meanwhile, the 
adversary replays the old certificate to B. If B then encrypts messages using the compromised old 
public key, the adversary can read those messages. 
2.4.2 Distribution of Secret Keys Using Public-Key Cryptography 
Once public keys have been distributed or have become accessible, secure communication that 
thwarts eavesdropping, tampering, or both is possible. However, few users will wish to make 
exclusive use of public-key encryption for communication because of the relatively slow data rates 
that can be achieved. Accordingly, public-key encryption provides for the distribution of secret keys 
to be used for conventional encryption. 
2.4.2.1 Simple Secret Key Distribution 
If A wishes to communicate with B, the following procedure is employed:


key: chroma:document
string_value:
61 
 
 
Fig: 6 Exchange of Public-Key Certificates 
 
Application must be in person or by some form of secure authenticated communication. For 
participant A, the authority provides a certificate of the form 
CA = E(PRauth, [T||IDA||PUa]) 
where PRauth is the private key used by the authority and T is a timestamp. A may then pass this


key: chroma:document
string_value:
certificate on to any other participant, who reads and verifies the certificate as follows: 
D(PUauth, CA) = D(PUauth, E(PRauth, [T||IDA||PUa])) = (T||IDA||PUa) 
The recipient uses the authority's public key, PUauth to decrypt the certificate. Because the certificate 
is readable only using the authority's public key, this verifies that the certificate came from the


key: chroma:document
string_value:
certificate authority. The elements IDA and PUa provide the recipient with the name and public key 
of the certificate's holder. The timestamp T validates the currency of the certificate. The timestamp 
counters the following scenario. A's private key is learned by an adversary. A generates a new


key: chroma:document
string_value:
private/public key pair and applies to the certificate authority for a new certificate. Meanwhile, the 
adversary replays the old certificate to B. If B then encrypts messages using the compromised old 
public key, the adversary can read those messages. 
2.4.2 Distribution of Secret Keys Using Public-Key Cryptography


key: chroma:document
string_value:
Once public keys have been distributed or have become accessible, secure communication that 
thwarts eavesdropping, tampering, or both is possible. However, few users will wish to make 
exclusive use of public-key encryption for communication because of the relatively slow data rates


key: chroma:document
string_value:
that can be achieved. Accordingly, public-key encryption provides for the distribution of secret keys 
to be used for conventional encryption. 
2.4.2.1 Simple Secret Key Distribution 
If A wishes to communicate with B, the following procedure is employed:


key: full_page_content
string_value:
62 
 
1. A generates a public/private key pair {PUa , PRa} and transmits a message to B consisting of PUa 
and an identifier of A, IDA.  
2. B generates a secret key, Ks, and transmits it to A, encrypted with A's public key.  
3. A computes D(PRa , E(PUa , Ks)) to recover the secret key. Because only A can decrypt the  
message, only A and B will know the identity of Ks.  
4. A discards PUa and PRa and B discards PUa. 
 
Fig: 6 Simple Use of Public-Key Encryption to Establish a Session Key 
A and B can now securely communicate using conventional encryption and the session key Ks. At 
the completion of the exchange, both A and B discard Ks. Despite its simplicity, this is an attractive 
protocol. No keys exist before the start of the communication and none exist after the completion of 
communication. Thus, the risk of compromise of the keys is minimal. At the same time, the 
communication is secure from eavesdropping. 
2.4.2.2 Secret Key Distribution with Confidentiality and Authentication 
It provides protection against both active and passive attacks. 
 
Fig: 7 Public-Key Distribution of Secret Keys 
 
Then the following steps occur:  
1. A uses B's public key to encrypt a message to B containing an identifier of A (IDA) and a nonce 
(N1), which is used to identify this transaction uniquely.


key: chroma:document
string_value:
62 
 
1. A generates a public/private key pair {PUa , PRa} and transmits a message to B consisting of PUa 
and an identifier of A, IDA.  
2. B generates a secret key, Ks, and transmits it to A, encrypted with A's public key.  
3. A computes D(PRa , E(PUa , Ks)) to recover the secret key. Because only A can decrypt the  
message, only A and B will know the identity of Ks.


key: chroma:document
string_value:
4. A discards PUa and PRa and B discards PUa. 
 
Fig: 6 Simple Use of Public-Key Encryption to Establish a Session Key 
A and B can now securely communicate using conventional encryption and the session key Ks. At 
the completion of the exchange, both A and B discard Ks. Despite its simplicity, this is an attractive


key: chroma:document
string_value:
protocol. No keys exist before the start of the communication and none exist after the completion of 
communication. Thus, the risk of compromise of the keys is minimal. At the same time, the 
communication is secure from eavesdropping. 
2.4.2.2 Secret Key Distribution with Confidentiality and Authentication 
It provides protection against both active and passive attacks.


key: chroma:document
string_value:
Fig: 7 Public-Key Distribution of Secret Keys 
 
Then the following steps occur:  
1. A uses B's public key to encrypt a message to B containing an identifier of A (IDA) and a nonce 
(N1), which is used to identify this transaction uniquely.


key: full_page_content
string_value:
63 
 
 2. B sends a message to A encrypted with PUa and containing A's nonce (N1) as well as a new nonce 
generated by B (N2) Because only B could have decrypted message (1), the presence of N1 in message 
(2) assures A that the correspondent is B. 
 3. A returns N2 encrypted using B's public key, to assure B that its correspondent is A.  
4. A selects a secret key Ks and sends M = E(PUb , E(PRa , Ks )) to B. Encryption of this message 
with B's public key ensures that only B can read it; encryption with A's private key ensures that only 
A could have sent it. 
 5. B computes D(PUa , D(PRb , M)) to recover the secret key. 
2.4.3 A Hybrid Scheme 
This scheme retains the use of a key distribution center (KDC) that shares a secret master key with 
each user and distributes secret session keys encrypted with the master key. A public key scheme is 
used to distribute the master keys. The following rationale is provided for using this three-level 
approach: 
● Performance: There are many applications, especially transaction-oriented applications, in which 
the session keys change frequently. Distribution of session keys by public-key encryption could 
degrade overall system performance because of the relatively high computational load of public-key 
encryption and decryption. With a three-level hierarchy, public-key encryption is used only 
occasionally to update the master key between a user and the KDC. 
● Backward compatibility: The hybrid scheme is easily overlaid on an existing KDC scheme, with  
minimal disruption or software changes 
2.5 Diffie-Hellman Key Exchange 
The purpose of the algorithm is to enable two users to exchange a key securely that can then be used 
for subsequent encryption of messages. The algorithm itself is limited to the exchange of secret 
values. 
The Diffie-Hellman algorithm depends for its effectiveness on the difficulty of computing discrete  
logarithms. we define a primitive root of a prime number p as one whose powers modulo p generate 
all the integers from 1 to p 1. That is, if a is a primitive root of the prime number p, then the numbers 
a mod p, a2 mod p,..., ap1 mod p 
are distinct and consist of the integers from 1 through p-1 in some permutation. 
For any integer b and a primitive root a of prime number p, we can find a unique exponent i such that 
 
The exponent i is referred to as the discrete logarithm of b for the base a, mod p. 
2.5.1 The Algorithm 
Figure 8 .summarizes the Diffie-Hellman key exchange algorithm. For this scheme, there are two


key: chroma:document
string_value:
63 
 
 2. B sends a message to A encrypted with PUa and containing A's nonce (N1) as well as a new nonce 
generated by B (N2) Because only B could have decrypted message (1), the presence of N1 in message 
(2) assures A that the correspondent is B. 
 3. A returns N2 encrypted using B's public key, to assure B that its correspondent is A.


key: chroma:document
string_value:
4. A selects a secret key Ks and sends M = E(PUb , E(PRa , Ks )) to B. Encryption of this message 
with B's public key ensures that only B can read it; encryption with A's private key ensures that only 
A could have sent it. 
 5. B computes D(PUa , D(PRb , M)) to recover the secret key. 
2.4.3 A Hybrid Scheme


key: chroma:document
string_value:
This scheme retains the use of a key distribution center (KDC) that shares a secret master key with 
each user and distributes secret session keys encrypted with the master key. A public key scheme is 
used to distribute the master keys. The following rationale is provided for using this three-level 
approach:


key: chroma:document
string_value:
approach: 
● Performance: There are many applications, especially transaction-oriented applications, in which 
the session keys change frequently. Distribution of session keys by public-key encryption could 
degrade overall system performance because of the relatively high computational load of public-key


key: chroma:document
string_value:
encryption and decryption. With a three-level hierarchy, public-key encryption is used only 
occasionally to update the master key between a user and the KDC. 
● Backward compatibility: The hybrid scheme is easily overlaid on an existing KDC scheme, with  
minimal disruption or software changes 
2.5 Diffie-Hellman Key Exchange


key: chroma:document
string_value:
The purpose of the algorithm is to enable two users to exchange a key securely that can then be used 
for subsequent encryption of messages. The algorithm itself is limited to the exchange of secret 
values. 
The Diffie-Hellman algorithm depends for its effectiveness on the difficulty of computing discrete


key: chroma:document
string_value:
logarithms. we define a primitive root of a prime number p as one whose powers modulo p generate 
all the integers from 1 to p 1. That is, if a is a primitive root of the prime number p, then the numbers 
a mod p, a2 mod p,..., ap1 mod p 
are distinct and consist of the integers from 1 through p-1 in some permutation.


key: chroma:document
string_value:
For any integer b and a primitive root a of prime number p, we can find a unique exponent i such that 
 
The exponent i is referred to as the discrete logarithm of b for the base a, mod p. 
2.5.1 The Algorithm 
Figure 8 .summarizes the Diffie-Hellman key exchange algorithm. For this scheme, there are two


key: full_page_content
string_value:
64 
 
publicly known numbers: a prime number q and an integer that is a primitive root of q. Suppose the  
users A and B wish to exchange a key. User A selects a random integer XA < q and computes YA = 
αXA mod q. Similarly, user B independently selects a random integer XA < q and computes YB = α 
XB mod q. Each side keeps the X value private and makes the Y value available publicly to the other 
side. User A computes the key as K = (YB) XA mod q and user B computes the key as K = (YA)XB  
mod q. These two calculations produce identical results 
Figure 8 The Diffie-Hellman Key Exchange Algorithm 
 
The security of the Diffie-Hellman key exchange lies in the fact that, while it is relatively easy to  
calculate exponentials modulo a prime, it is very difficult to calculate discrete logarithms. For large  
primes, the latter task is considered infeasible. 
2.5.2 Key Exchange Protocols 
Fig : 9 shows a simple protocol that makes use of the Diffie-Hellman calculation. Suppose that user  
A wishes to set up a connection with user B and use a secret key to encrypt messages on that


key: chroma:document
string_value:
64 
 
publicly known numbers: a prime number q and an integer that is a primitive root of q. Suppose the  
users A and B wish to exchange a key. User A selects a random integer XA < q and computes YA = 
αXA mod q. Similarly, user B independently selects a random integer XA < q and computes YB = α


key: chroma:document
string_value:
XB mod q. Each side keeps the X value private and makes the Y value available publicly to the other 
side. User A computes the key as K = (YB) XA mod q and user B computes the key as K = (YA)XB  
mod q. These two calculations produce identical results 
Figure 8 The Diffie-Hellman Key Exchange Algorithm


key: chroma:document
string_value:
The security of the Diffie-Hellman key exchange lies in the fact that, while it is relatively easy to  
calculate exponentials modulo a prime, it is very difficult to calculate discrete logarithms. For large  
primes, the latter task is considered infeasible. 
2.5.2 Key Exchange Protocols


key: chroma:document
string_value:
Fig : 9 shows a simple protocol that makes use of the Diffie-Hellman calculation. Suppose that user  
A wishes to set up a connection with user B and use a secret key to encrypt messages on that


key: full_page_content
string_value:
65 
 
connection. User A can generate a one-time private key XA, calculate YA, and send that to user B. 
User B responds by generating a private value XB calculating YB, and sending YB to user A. Both 
users can now calculate the key. The necessary public values q and α would need to be known ahead 
of time. Alternatively, user A could pick values for q and α and include those in the first message. 
 
 
Fig : 9 Diffie-Hellman Key Exchange 
2.6 Message Authentication 
Authentication Requirements 
Authentication Functions 
Message Encryption 
Message Authentication Code 
Hash Function 
2.6.1 Authentication Requirements 
In the context of communications across a network, the following attacks can be identified: 
1. Disclosure: Release of message contents to any person or process not possessing the  
appropriate cryptographic key. 
2. Traffic analysis: Discovery of the pattern of traffic between parties. In a connection-oriented  
application, the frequency and duration of connections could be determined. In either a  
connection-oriented or connectionless environment, the number and length of messages between  
parties could be determined. 
3. Masquerade: Insertion of messages into the network from a fraudulent source. This includes


key: chroma:document
string_value:
65 
 
connection. User A can generate a one-time private key XA, calculate YA, and send that to user B. 
User B responds by generating a private value XB calculating YB, and sending YB to user A. Both 
users can now calculate the key. The necessary public values q and α would need to be known ahead


key: chroma:document
string_value:
of time. Alternatively, user A could pick values for q and α and include those in the first message. 
 
 
Fig : 9 Diffie-Hellman Key Exchange 
2.6 Message Authentication 
Authentication Requirements 
Authentication Functions 
Message Encryption 
Message Authentication Code 
Hash Function 
2.6.1 Authentication Requirements


key: chroma:document
string_value:
In the context of communications across a network, the following attacks can be identified: 
1. Disclosure: Release of message contents to any person or process not possessing the  
appropriate cryptographic key. 
2. Traffic analysis: Discovery of the pattern of traffic between parties. In a connection-oriented


key: chroma:document
string_value:
application, the frequency and duration of connections could be determined. In either a  
connection-oriented or connectionless environment, the number and length of messages between  
parties could be determined. 
3. Masquerade: Insertion of messages into the network from a fraudulent source. This includes


key: full_page_content
string_value:
66 
 
the creation of messages by an opponent that are purported to come from an authorized entity.  
Also included are fraudulent acknowledgments of message receipt or nonreceipt by someone  
other than the message recipient. 
4. Content modification: Changes to the contents of a message, including insertion, deletion,  
transposition, and modification. 
5. Sequence modification: Any modification to a sequence of messages between parties,  
including insertion, deletion, and reordering. 
6. Timing modification: Delay or replay of messages. In a connection-oriented application, an  
entire session or sequence of messages could be a replay of some previous valid session, or  
individual messages in the sequence could be delayed or replayed. In a connectionless  
application, an individual message (e.g., datagram) could be delayed or replayed. 
7. Source repudiation: Denial of transmission of message by source. 
8. Destination repudiation: Denial of receipt of message by destination. 
2.6.2 Authentication Functions 
Any message authentication or digital signature mechanism has two levels of functionality. At the 
lower level, there must be some sort of function that produces an authenticator: a value to be used to 
authenticate a message. This lower-level function is then used as a primitive in a higher-level 
authentication protocol that enables a receiver to verify the authenticity of a message 
The types of functions that may be used to produce an authenticator that are grouped into three 
classes, as follows: 
● Message encryption: The ciphertext of the entire message serves as its authenticator 
● Message authentication code (MAC): A function of the message and a secret key that  
produces a fixed-length value that serves as the authenticator 
● Hash function: A function that maps a message of any length into a fixed-length hash 
value,  
which serves as the authenticator 
2.6.2.1 Message Encryption 
Message encryption by itself can provide a measure of authentication. The analysis differs for 
symmetric and public-key encryption schemes. 
2.6.2.1.1 Symmetric Encryption 
Consider the straightforward use of symmetric encryption (Figure 11.1a). A message M transmitted 
from source A to destination B is encrypted using a secret key K shared by A and B. If no other party


key: chroma:document
string_value:
66 
 
the creation of messages by an opponent that are purported to come from an authorized entity.  
Also included are fraudulent acknowledgments of message receipt or nonreceipt by someone  
other than the message recipient. 
4. Content modification: Changes to the contents of a message, including insertion, deletion,  
transposition, and modification.


key: chroma:document
string_value:
5. Sequence modification: Any modification to a sequence of messages between parties,  
including insertion, deletion, and reordering. 
6. Timing modification: Delay or replay of messages. In a connection-oriented application, an  
entire session or sequence of messages could be a replay of some previous valid session, or


key: chroma:document
string_value:
individual messages in the sequence could be delayed or replayed. In a connectionless  
application, an individual message (e.g., datagram) could be delayed or replayed. 
7. Source repudiation: Denial of transmission of message by source. 
8. Destination repudiation: Denial of receipt of message by destination. 
2.6.2 Authentication Functions


key: chroma:document
string_value:
Any message authentication or digital signature mechanism has two levels of functionality. At the 
lower level, there must be some sort of function that produces an authenticator: a value to be used to 
authenticate a message. This lower-level function is then used as a primitive in a higher-level


key: chroma:document
string_value:
authentication protocol that enables a receiver to verify the authenticity of a message 
The types of functions that may be used to produce an authenticator that are grouped into three 
classes, as follows: 
● Message encryption: The ciphertext of the entire message serves as its authenticator 
● Message authentication code (MAC): A function of the message and a secret key that


key: chroma:document
string_value:
produces a fixed-length value that serves as the authenticator 
● Hash function: A function that maps a message of any length into a fixed-length hash 
value,  
which serves as the authenticator 
2.6.2.1 Message Encryption 
Message encryption by itself can provide a measure of authentication. The analysis differs for 
symmetric and public-key encryption schemes.


key: chroma:document
string_value:
2.6.2.1.1 Symmetric Encryption 
Consider the straightforward use of symmetric encryption (Figure 11.1a). A message M transmitted 
from source A to destination B is encrypted using a secret key K shared by A and B. If no other party


key: full_page_content
string_value:
67 
 
knows the key, then confidentiality is provided: No other party can recover the plaintext of the 
message 
 
Figure 9 Basic Uses of Message Encryption 
 
In addition, we may say that B is assured that the message was generated by A. The message must 
have come from A because A is the only other party that possesses K and therefore the only other 
party with the information necessary to construct ciphertext that can be decrypted with K. 
Furthermore, if M is recovered, B knows that none of the bits of M have been altered, because an 
opponent that does not know K would not know how to alter bits in the ciphertext to produce desired 
changes in the plaintext. Symmetric encryption provides authentication as well as confidentiality.  
2.6.2.1.2 Public-Key Encryption 
The straightforward use of public-key encryption Figure 9b provides confidentiality but not 
authentication. The source (A) uses the public key PUb of the destination (B) to encrypt M. Because 
only B has the corresponding private key PRb , only B can decrypt the message. This scheme provides 
no authentication because any opponent could also use B's public key to encrypt a message, claiming 
to be A.


key: chroma:document
string_value:
67 
 
knows the key, then confidentiality is provided: No other party can recover the plaintext of the 
message 
 
Figure 9 Basic Uses of Message Encryption 
 
In addition, we may say that B is assured that the message was generated by A. The message must 
have come from A because A is the only other party that possesses K and therefore the only other


key: chroma:document
string_value:
party with the information necessary to construct ciphertext that can be decrypted with K. 
Furthermore, if M is recovered, B knows that none of the bits of M have been altered, because an 
opponent that does not know K would not know how to alter bits in the ciphertext to produce desired


key: chroma:document
string_value:
changes in the plaintext. Symmetric encryption provides authentication as well as confidentiality.  
2.6.2.1.2 Public-Key Encryption 
The straightforward use of public-key encryption Figure 9b provides confidentiality but not 
authentication. The source (A) uses the public key PUb of the destination (B) to encrypt M. Because


key: chroma:document
string_value:
only B has the corresponding private key PRb , only B can decrypt the message. This scheme provides 
no authentication because any opponent could also use B's public key to encrypt a message, claiming 
to be A.


key: full_page_content
string_value:
68 
 
To provide authentication, A uses its private key to encrypt the message, and B uses A's public key 
to decrypt (Figure 9c). This provides authentication using the same type of reasoning as in the 
symmetric encryption case: The message must have come from A because A is the only party that 
possesses PRa and therefore the only party with the information necessary to construct ciphertext that 
can be decrypted with PUa.  
2.6.2.2 Message Authentication Code 
An alternative authentication technique involves the use of a secret key to generate a small fixed-size  
block of data, known as a cryptographic checksum or MAC that is appended to the message. This  
technique assumes that two communicating parties, say A and B, share a common secret key K. When 
A has a message to send to B, it calculates the MAC as a function of the message and the key:MAC 
= C(K, M), where 
M = input message 
C = MAC function 
K = shared secret key 
MAC = message authentication code 
The message plus MAC is transmitted to the intended recipient. The recipient performs the same  
calculation on the received message, using the same secret key, to generate a new MAC. The received  
MAC is compared to the calculated MAC (Figure 10a). If we assume that only the receiver and the  
sender know the identity of the secret key, and if the received MAC matches the calculated MAC, 
then 
1. The receiver is assured that the message has not been altered. If an attacker alters the message  
but does not alter the MAC, then the receiver's calculation of the MAC will differ from the  
received MAC. Because the attacker is assumed not to know the secret key, the attacker cannot  
alter the MAC to correspond to the alterations in the message. 
2. The receiver is assured that the message is from the alleged sender. Because no one else knows  
the secret key, no one else could prepare a message with a proper MAC. 
3. If the message includes a sequence number, then the receiver can be assured of the proper sequence 
because an attacker cannot successfully alter the sequence number.


key: chroma:document
string_value:
68 
 
To provide authentication, A uses its private key to encrypt the message, and B uses A's public key 
to decrypt (Figure 9c). This provides authentication using the same type of reasoning as in the 
symmetric encryption case: The message must have come from A because A is the only party that


key: chroma:document
string_value:
possesses PRa and therefore the only party with the information necessary to construct ciphertext that 
can be decrypted with PUa.  
2.6.2.2 Message Authentication Code 
An alternative authentication technique involves the use of a secret key to generate a small fixed-size  
block of data, known as a cryptographic checksum or MAC that is appended to the message. This


key: chroma:document
string_value:
technique assumes that two communicating parties, say A and B, share a common secret key K. When 
A has a message to send to B, it calculates the MAC as a function of the message and the key:MAC 
= C(K, M), where 
M = input message 
C = MAC function 
K = shared secret key 
MAC = message authentication code


key: chroma:document
string_value:
The message plus MAC is transmitted to the intended recipient. The recipient performs the same  
calculation on the received message, using the same secret key, to generate a new MAC. The received  
MAC is compared to the calculated MAC (Figure 10a). If we assume that only the receiver and the


key: chroma:document
string_value:
sender know the identity of the secret key, and if the received MAC matches the calculated MAC, 
then 
1. The receiver is assured that the message has not been altered. If an attacker alters the message  
but does not alter the MAC, then the receiver's calculation of the MAC will differ from the


key: chroma:document
string_value:
received MAC. Because the attacker is assumed not to know the secret key, the attacker cannot  
alter the MAC to correspond to the alterations in the message. 
2. The receiver is assured that the message is from the alleged sender. Because no one else knows  
the secret key, no one else could prepare a message with a proper MAC.


key: chroma:document
string_value:
3. If the message includes a sequence number, then the receiver can be assured of the proper sequence 
because an attacker cannot successfully alter the sequence number.


key: full_page_content
string_value:
69 
 
 
Fig :10 Basic Uses of Message Authentication Code (MAC) 
2.6.2.3 Hash Function 
A variation on the message authentication code is the one-way hash function. As with the message 
authentication code, a hash function accepts a variable-size message M as input and produces a fixed 
-size output, referred to as a hash code H(M). Unlike a MAC, a hash code does not use a key but is a 
function only of the input message. The hash code is also referred to as a message digest or hash 
value. The hash code is a function of all the bits of the message and provides an error-detection 
capability: A change to any bit or bits in the message results in a change to the hash code. 
Fig 11 illustrates a variety of ways in which a hash code can be used to provide message 
authentication, as follows


key: chroma:document
string_value:
69 
 
 
Fig :10 Basic Uses of Message Authentication Code (MAC) 
2.6.2.3 Hash Function 
A variation on the message authentication code is the one-way hash function. As with the message 
authentication code, a hash function accepts a variable-size message M as input and produces a fixed


key: chroma:document
string_value:
-size output, referred to as a hash code H(M). Unlike a MAC, a hash code does not use a key but is a 
function only of the input message. The hash code is also referred to as a message digest or hash 
value. The hash code is a function of all the bits of the message and provides an error-detection


key: chroma:document
string_value:
capability: A change to any bit or bits in the message results in a change to the hash code. 
Fig 11 illustrates a variety of ways in which a hash code can be used to provide message 
authentication, as follows


key: full_page_content
string_value:
70 
 
 
Figure 11. Basic Uses of Hash Function 
a. The message plus concatenated hash code is encrypted using symmetric encryption. This is  
identical in structure to the internal error control strategy shown in Figure 11.2a. The same line  
of reasoning applies: Because only A and B share the secret key, the message must have come  
from A and has not been altered. The hash code provides the structure or redundancy required  
to achieve authentication. Because encryption is applied to the entire message plus hash code,  
confidentiality is also provided. 
b. Only the hash code is encrypted, using symmetric encryption. This reduces the processing  
burden for those applications that do not require confidentiality. 
c. Only the hash code is encrypted, using public-key encryption and using the sender's private key.  
As with (b), this provides authentication. It also provides a digital signature, because only the  
sender could have produced the encrypted hash code. In fact, this is the essence of the digital  
signature technique. 
d. If confidentiality as well as a digital signature is desired, then the message plus the private-key 
encrypted hash code can be encrypted using a symmetric secret key. This is a common technique.


key: chroma:document
string_value:
70 
 
 
Figure 11. Basic Uses of Hash Function 
a. The message plus concatenated hash code is encrypted using symmetric encryption. This is  
identical in structure to the internal error control strategy shown in Figure 11.2a. The same line  
of reasoning applies: Because only A and B share the secret key, the message must have come


key: chroma:document
string_value:
from A and has not been altered. The hash code provides the structure or redundancy required  
to achieve authentication. Because encryption is applied to the entire message plus hash code,  
confidentiality is also provided. 
b. Only the hash code is encrypted, using symmetric encryption. This reduces the processing


key: chroma:document
string_value:
burden for those applications that do not require confidentiality. 
c. Only the hash code is encrypted, using public-key encryption and using the sender's private key.  
As with (b), this provides authentication. It also provides a digital signature, because only the  
sender could have produced the encrypted hash code. In fact, this is the essence of the digital


key: chroma:document
string_value:
signature technique. 
d. If confidentiality as well as a digital signature is desired, then the message plus the private-key 
encrypted hash code can be encrypted using a symmetric secret key. This is a common technique.


key: full_page_content
string_value:
71 
 
e. It is possible to use a hash function but no encryption for message authentication. The technique  
assumes that the two communicating parties share a common secret value S. A compute the  
hash value over the concatenation of M and S and appends the resulting hash value to M.  
Because B possesses S, it can recompute the hash value to verify. Because the secret value itself  
is not sent, an opponent cannot modify an intercepted message and cannot generate a false  
message. 
f. Confidentiality can be added to the approach of (e) by encrypting the entire message plus the  
hash code. 
 
2.7 Hash Functions 
A hash value h is generated by a function H of the form 
h = H(M) 
where M is a variable-length message and H(M) is the fixed-length hash value. The hash value is  
appended to the message at the source at a time when the message is assumed or known to be correct.  
The receiver authenticates that message by recomputing the hash value. Because the hash function  
itself is not considered to be secret, some means is required to protect the hash value. 
Requirements for a Hash Function 
The purpose of a hash function is to produce a "fingerprint" of a file, message, or other block of data. 
To be useful for message authentication, a hash function H must have the following properties 
1. H can be applied to a block of data of any size. 
2. H produces a fixed-length output. 
3. H(x) is relatively easy to compute for any given x, making both hardware and software  
implementations practical. 
4. For any given value h, it is computationally infeasible to find x such that H(x) = h. This is  
sometimes referred to in the literature as the one-way property. 
5. For any given block x, it is computationally infeasible to find y≠x such that H(y) = H(x). This  
is sometimes referred to as weak collision resistance. 
6. It is computationally infeasible to find any pair (x, y) such that H(x) = H(y). This is sometimes  
referred to as strong collision resistance. 
The first three properties are requirements for the practical application of a hash function to message 
authentication. The fourth property, the one-way property, states that it is easy to generate a code given a


key: chroma:document
string_value:
71 
 
e. It is possible to use a hash function but no encryption for message authentication. The technique  
assumes that the two communicating parties share a common secret value S. A compute the  
hash value over the concatenation of M and S and appends the resulting hash value to M.


key: chroma:document
string_value:
Because B possesses S, it can recompute the hash value to verify. Because the secret value itself  
is not sent, an opponent cannot modify an intercepted message and cannot generate a false  
message. 
f. Confidentiality can be added to the approach of (e) by encrypting the entire message plus the  
hash code. 
 
2.7 Hash Functions


key: chroma:document
string_value:
2.7 Hash Functions 
A hash value h is generated by a function H of the form 
h = H(M) 
where M is a variable-length message and H(M) is the fixed-length hash value. The hash value is  
appended to the message at the source at a time when the message is assumed or known to be correct.


key: chroma:document
string_value:
The receiver authenticates that message by recomputing the hash value. Because the hash function  
itself is not considered to be secret, some means is required to protect the hash value. 
Requirements for a Hash Function 
The purpose of a hash function is to produce a "fingerprint" of a file, message, or other block of data.


key: chroma:document
string_value:
To be useful for message authentication, a hash function H must have the following properties 
1. H can be applied to a block of data of any size. 
2. H produces a fixed-length output. 
3. H(x) is relatively easy to compute for any given x, making both hardware and software  
implementations practical.


key: chroma:document
string_value:
4. For any given value h, it is computationally infeasible to find x such that H(x) = h. This is  
sometimes referred to in the literature as the one-way property. 
5. For any given block x, it is computationally infeasible to find y≠x such that H(y) = H(x). This  
is sometimes referred to as weak collision resistance.


key: chroma:document
string_value:
6. It is computationally infeasible to find any pair (x, y) such that H(x) = H(y). This is sometimes  
referred to as strong collision resistance. 
The first three properties are requirements for the practical application of a hash function to message 
authentication. The fourth property, the one-way property, states that it is easy to generate a code given a


key: full_page_content
string_value:
72 
 
message but virtually impossible to generate a message given a code. The fifth property guarantees that an 
alternative message hashing to the same value as a given message cannot be found. This prevents forgery 
when an encrypted hash code is used. The sixth property refers to how resistant the hash function is to a 
type of attack known as the birthday attack. 
2.7.1 Simple Hash Functions 
All hash functions operate using the following general principles. The input (message, file, etc.) is  
viewed as a sequence of n-bit blocks. The input is processed one block at a time in an iterative fashion  
to produce an n-bit hash function. 
One of the simplest hash functions is the bit-by-bit exclusive-OR (XOR) of every block. This can be  
expressed as follows: 
 
This operation produces a simple parity for each bit position and is known as a longitudinal 
redundancy check. It is reasonably effective for random data as a data integrity check. Each n-bit 
hash value is equally likely. Thus, the probability that a data error will result in an unchanged hash 
value is 2n. With more predictably formatted data, the function is less effective. For example, in most 
normal text files, the high-order bit of each octet is always zero. So if a 128-bit hash value is used, 
instead of an effectiveness of 2128, the hash function on this type of data has an effectiveness of 
2112. 
A simple way to improve matters is to perform a one-bit circular shift, or rotation, on the hash value  
after each block is processed. The procedure can be summarized as follows: 
1. Initially set the n-bit hash value to zero. 
2. Process each successive n-bit block of data as follows: 
a. Rotate the current hash value to the left by one bit. 
b. XOR the block into the hash value. 
Fig .12 illustrates these two types of hash functions for 16-bit hash values.


key: chroma:document
string_value:
72 
 
message but virtually impossible to generate a message given a code. The fifth property guarantees that an 
alternative message hashing to the same value as a given message cannot be found. This prevents forgery 
when an encrypted hash code is used. The sixth property refers to how resistant the hash function is to a 
type of attack known as the birthday attack.


key: chroma:document
string_value:
2.7.1 Simple Hash Functions 
All hash functions operate using the following general principles. The input (message, file, etc.) is  
viewed as a sequence of n-bit blocks. The input is processed one block at a time in an iterative fashion  
to produce an n-bit hash function. 
One of the simplest hash functions is the bit-by-bit exclusive-OR (XOR) of every block. This can be


key: chroma:document
string_value:
expressed as follows: 
 
This operation produces a simple parity for each bit position and is known as a longitudinal 
redundancy check. It is reasonably effective for random data as a data integrity check. Each n-bit 
hash value is equally likely. Thus, the probability that a data error will result in an unchanged hash


key: chroma:document
string_value:
value is 2n. With more predictably formatted data, the function is less effective. For example, in most 
normal text files, the high-order bit of each octet is always zero. So if a 128-bit hash value is used, 
instead of an effectiveness of 2128, the hash function on this type of data has an effectiveness of 
2112.


key: chroma:document
string_value:
2112. 
A simple way to improve matters is to perform a one-bit circular shift, or rotation, on the hash value  
after each block is processed. The procedure can be summarized as follows: 
1. Initially set the n-bit hash value to zero. 
2. Process each successive n-bit block of data as follows: 
a. Rotate the current hash value to the left by one bit.


key: chroma:document
string_value:
b. XOR the block into the hash value. 
Fig .12 illustrates these two types of hash functions for 16-bit hash values.


key: full_page_content
string_value:
73 
 
 
Fig. 12 Two types of hash Functions 
Although the second procedure provides a good measure of data integrity, it is virtually useless for 
data security when an encrypted hash code is used with a plaintext message, as in Figures 11.5b and 
c.  
Given a message, it is an easy matter to produce a new message that yields that hash code: Simply 
prepare the desired alternate message and then append an n-bit block that forces the new message 
plus block to yield the desired hash code. 
Although a simple XOR or rotated XOR (RXOR) is insufficient if only the hash code is encrypted, 
you may still feel that such a simple function could be useful when the message as well as the hash 
code are encrypted (Figure 11.5a). But you must be careful. A technique originally proposed by the 
National Bureau of Standards used the simple XOR applied to 64-bit blocks of the message and then 
an encryption of the entire message that used the cipher block chaining (CBC) mode. We can define 
the scheme as follows: Given a message consisting of a sequence of 64-bit blocks X1, X2,..., XN,


key: chroma:document
string_value:
73 
 
 
Fig. 12 Two types of hash Functions 
Although the second procedure provides a good measure of data integrity, it is virtually useless for 
data security when an encrypted hash code is used with a plaintext message, as in Figures 11.5b and 
c.  
Given a message, it is an easy matter to produce a new message that yields that hash code: Simply


key: chroma:document
string_value:
prepare the desired alternate message and then append an n-bit block that forces the new message 
plus block to yield the desired hash code. 
Although a simple XOR or rotated XOR (RXOR) is insufficient if only the hash code is encrypted, 
you may still feel that such a simple function could be useful when the message as well as the hash


key: chroma:document
string_value:
code are encrypted (Figure 11.5a). But you must be careful. A technique originally proposed by the 
National Bureau of Standards used the simple XOR applied to 64-bit blocks of the message and then 
an encryption of the entire message that used the cipher block chaining (CBC) mode. We can define


key: full_page_content
string_value:
74 
 
define the hash code C as the block-by-block XOR of all blocks and append the hash code as the final 
block: 
 
Next, encrypt the entire message plus hash code, using CBC mode to produce the encrypted message Y1, 
Y2,..., YN+1. 
 
 
Because the terms in the preceding equation can be XORed in any order, it follows that the hash code  
would not change if the ciphertext blocks were permuted. 
2.7.2 Birthday Attacks 
Suppose that a 64-bit hash code is used. One might think that this is quite secure. For example, if an 
encrypted hash code C is transmitted with the corresponding unencrypted message M, then an 
opponent would need to find an M' such that H(M') = H(M) to substitute another message and fool 
the receiver. A different sort of attack is possible, based on the birthday paradox. 
1. The source, A, is prepared to "sign" a message by appending the appropriate m-bit hash code  
and encrypting that hash code with A's private key. 
2. The opponent generates 2m/2 variations on the message, all of which convey essentially the  
same meaning. The opponent prepares an equal number of messages, all of which are variations  
on the fraudulent message to be substituted for the real one. 
3. The two sets of messages are compared to find a pair of messages that produces the same hash  
code. The probability of success, by the birthday paradox, is greater than 0.5. If no match is  
found, additional valid and fraudulent messages are generated until a match is made. 
4. The opponent offers the valid variation to A for signature. This signature can then be attached to


key: chroma:document
string_value:
74 
 
define the hash code C as the block-by-block XOR of all blocks and append the hash code as the final 
block: 
 
Next, encrypt the entire message plus hash code, using CBC mode to produce the encrypted message Y1, 
Y2,..., YN+1. 
 
 
Because the terms in the preceding equation can be XORed in any order, it follows that the hash code


key: chroma:document
string_value:
would not change if the ciphertext blocks were permuted. 
2.7.2 Birthday Attacks 
Suppose that a 64-bit hash code is used. One might think that this is quite secure. For example, if an 
encrypted hash code C is transmitted with the corresponding unencrypted message M, then an 
opponent would need to find an M' such that H(M') = H(M) to substitute another message and fool


key: chroma:document
string_value:
the receiver. A different sort of attack is possible, based on the birthday paradox. 
1. The source, A, is prepared to "sign" a message by appending the appropriate m-bit hash code  
and encrypting that hash code with A's private key. 
2. The opponent generates 2m/2 variations on the message, all of which convey essentially the


key: chroma:document
string_value:
same meaning. The opponent prepares an equal number of messages, all of which are variations  
on the fraudulent message to be substituted for the real one. 
3. The two sets of messages are compared to find a pair of messages that produces the same hash  
code. The probability of success, by the birthday paradox, is greater than 0.5. If no match is


key: chroma:document
string_value:
found, additional valid and fraudulent messages are generated until a match is made. 
4. The opponent offers the valid variation to A for signature. This signature can then be attached to


key: full_page_content
string_value:
75 
 
the fraudulent variation for transmission to the intended recipient. Because the two variations  
have the same hash code, they will produce the same signature; the opponent is assured of  
success even though the encryption key is not known. 
 
Thus, if a 64-bit hash code is used, the level of effort required is only on the order of 232. 
2.7.3 Block Chaining Techniques 
A number of proposals have been made for hash functions based on using a cipher block chaining 
technique, but without the secret key. 
Divide a message M into fixed-size blocks M1, M2,..., MN and use a symmetric encryption system to 
compute the hash code G as follows: 
 
There is no secret key in this scheme. As with any hash code, this scheme is subject to the birthday 
attack, and if the encryption algorithm is DES and only a 64-bit hash code is produced, then the 
system is vulnerable. the opponent intercepts a message with a signature in the form of an encrypted 
hash code and that the unencrypted hash code is m bits long: 
1. Use the algorithm defined at the beginning of this subsection to calculate the unencrypted hash  
code G. 
2. Construct any desired message in the form Q1, Q2,..., QN2. 
3. Compute for Hi = E(Qi, Hi1) for 1 i (N 2). 
4. Generate 2m/2 random blocks; for each block X, compute E(X, HN2). Generate an additional 2m/2 
random blocks; for each block Y, compute D(Y, G), where D is the decryption function  
corresponding to E. 
5. Based on the birthday paradox, with high probability there will be an X and Y such that E(X, HN2)  
= D(Y, G). 
6. Form the message Q1, Q2,..., QN2, X, Y. This message has the hash code G and therefore can be 
used with the intercepted encrypted signature. 
2.8 Digital Signatures 
Message authentication protects two parties who exchange messages from any third party. However, 
it does not protect the two parties against each other. Several forms of dispute between the two are 
possible.


key: chroma:document
string_value:
75 
 
the fraudulent variation for transmission to the intended recipient. Because the two variations  
have the same hash code, they will produce the same signature; the opponent is assured of  
success even though the encryption key is not known. 
 
Thus, if a 64-bit hash code is used, the level of effort required is only on the order of 232. 
2.7.3 Block Chaining Techniques


key: chroma:document
string_value:
A number of proposals have been made for hash functions based on using a cipher block chaining 
technique, but without the secret key. 
Divide a message M into fixed-size blocks M1, M2,..., MN and use a symmetric encryption system to 
compute the hash code G as follows: 
 
There is no secret key in this scheme. As with any hash code, this scheme is subject to the birthday


key: chroma:document
string_value:
attack, and if the encryption algorithm is DES and only a 64-bit hash code is produced, then the 
system is vulnerable. the opponent intercepts a message with a signature in the form of an encrypted 
hash code and that the unencrypted hash code is m bits long: 
1. Use the algorithm defined at the beginning of this subsection to calculate the unencrypted hash  
code G.


key: chroma:document
string_value:
code G. 
2. Construct any desired message in the form Q1, Q2,..., QN2. 
3. Compute for Hi = E(Qi, Hi1) for 1 i (N 2). 
4. Generate 2m/2 random blocks; for each block X, compute E(X, HN2). Generate an additional 2m/2 
random blocks; for each block Y, compute D(Y, G), where D is the decryption function  
corresponding to E.


key: chroma:document
string_value:
5. Based on the birthday paradox, with high probability there will be an X and Y such that E(X, HN2)  
= D(Y, G). 
6. Form the message Q1, Q2,..., QN2, X, Y. This message has the hash code G and therefore can be 
used with the intercepted encrypted signature. 
2.8 Digital Signatures 
Message authentication protects two parties who exchange messages from any third party. However,


key: chroma:document
string_value:
it does not protect the two parties against each other. Several forms of dispute between the two are 
possible.


key: full_page_content
string_value:
76 
 
2.8.1 Requirements 
For example, suppose that John sends an authenticated message to Mary. Consider the following 
disputes that could arise: 
1. Mary may forge a different message and claim that it came from John. Mary would simply have  
to create a message and append an authentication code using the key that John and Mary share. 
2. John can deny sending the message. Because it is possible for Mary to forge a message, there is  
no way to prove that John did in fact send the message. 
In situations where there is not complete trust between sender and receiver, something more than 
authentication is needed. The most attractive solution to this problem is the digital signature. The 
digital signature is analogous to the handwritten signature. It must have the following properties: 
● It must verify the author and the date and time of the signature. 
● It must to authenticate the contents at the time of the signature. 
● It must be verifiable by third parties, to resolve disputes. 
Thus, the digital signature function includes the authentication function. 
On the basis of these properties, we can formulate the following requirements for a digital signature: 
● The signature must be a bit pattern that depends on the message being signed. 
● The signature must use some information unique to the sender, to prevent both forgery and  
denial. 
● It must be relatively easy to produce the digital signature. 
● It must be relatively easy to recognize and verify the digital signature. 
● It must be computationally infeasible to forge a digital signature, either by constructing a new  
message for an existing digital signature or by constructing a fraudulent digital signature for a  
given message. 
● It must be practical to retain a copy of the digital signature in storage. 
A variety of approaches has been proposed for the digital signature function. These approaches fall 
into two categories: direct and arbitrated. 
1. Direct Digital Signature 
2. Arbitrated Digital Signature 
2.8.2 Direct Digital Signature 
The direct digital signature involves only the communicating parties (source, destination). It is 
assumed that the destination knows the public key of the source. A digital signature may be formed 
by encrypting the entire message with the sender's private key or by encrypting a hash code of the 
message with the sender's private key


key: chroma:document
string_value:
76 
 
2.8.1 Requirements 
For example, suppose that John sends an authenticated message to Mary. Consider the following 
disputes that could arise: 
1. Mary may forge a different message and claim that it came from John. Mary would simply have  
to create a message and append an authentication code using the key that John and Mary share.


key: chroma:document
string_value:
2. John can deny sending the message. Because it is possible for Mary to forge a message, there is  
no way to prove that John did in fact send the message. 
In situations where there is not complete trust between sender and receiver, something more than 
authentication is needed. The most attractive solution to this problem is the digital signature. The


key: chroma:document
string_value:
digital signature is analogous to the handwritten signature. It must have the following properties: 
● It must verify the author and the date and time of the signature. 
● It must to authenticate the contents at the time of the signature. 
● It must be verifiable by third parties, to resolve disputes. 
Thus, the digital signature function includes the authentication function.


key: chroma:document
string_value:
On the basis of these properties, we can formulate the following requirements for a digital signature: 
● The signature must be a bit pattern that depends on the message being signed. 
● The signature must use some information unique to the sender, to prevent both forgery and  
denial. 
● It must be relatively easy to produce the digital signature.


key: chroma:document
string_value:
● It must be relatively easy to recognize and verify the digital signature. 
● It must be computationally infeasible to forge a digital signature, either by constructing a new  
message for an existing digital signature or by constructing a fraudulent digital signature for a  
given message. 
● It must be practical to retain a copy of the digital signature in storage.


key: chroma:document
string_value:
A variety of approaches has been proposed for the digital signature function. These approaches fall 
into two categories: direct and arbitrated. 
1. Direct Digital Signature 
2. Arbitrated Digital Signature 
2.8.2 Direct Digital Signature 
The direct digital signature involves only the communicating parties (source, destination). It is


key: chroma:document
string_value:
assumed that the destination knows the public key of the source. A digital signature may be formed 
by encrypting the entire message with the sender's private key or by encrypting a hash code of the 
message with the sender's private key


key: full_page_content
string_value:
77 
 
Confidentiality can be provided by further encrypting the entire message plus signature with either 
the receiver's public key (public-key encryption) or a shared secret key (symmetric encryption). 
The validity of the scheme depends on the security of the sender's private key. If a sender later wishes 
to deny sending a particular message, the sender can claim that the private key was lost or stolen and 
that someone else forged his or her signature. Administrative controls relating to the security of 
private keys can be employed to thwart or at least weaken this ploy, but the threat is still there, at 
least to some degree. One example is to require every signed message to include a timestamp (date 
and time) and to require prompt reporting of compromised keys to a central authority. 
Another threat is that some private key might actually be stolen from X at time T. The opponent can 
then send a message signed with X's signature and stamped with a time before or equal to T. 
2.8.3 Arbitrated Digital Signature 
The problems associated with direct digital signatures can be addressed by using an arbiter. 
Every signed message from a sender X to a receiver Y goes first to an arbiter A, who subjects the 
message and its signature to a number of tests to check its origin and content. The message is then 
dated and sent to Y with an indication that it has been verified to the satisfaction of the arbiter. The 
presence of A solves the problem faced by direct signature schemes: that X might disown the 
message. 
In the first, symmetric encryption is used. It is assumed that the sender X and the arbiter A share a 
secret key Kxa and that A and Y share secret key Kay. X constructs a message M and computes its 
hash value H(M). Then X transmits the message plus a signature to A. The signature consists of an 
identifier IDX of X plus the hash value, all encrypted using Kxa. A decrypts the signature and checks 
the hash value to validate the message. Then A transmits a message to Y, encrypted with Kay. The 
message includes IDX, the original message from X, the signature, and a timestamp. Y can decrypt 
this to recover the message and the signature. The timestamp informs Y that this message is timely 
and not a replay. Y can store M and the signature. In case of dispute, Y, who claims to have received 
M from X, sends the following message to A:


key: chroma:document
string_value:
77 
 
Confidentiality can be provided by further encrypting the entire message plus signature with either 
the receiver's public key (public-key encryption) or a shared secret key (symmetric encryption). 
The validity of the scheme depends on the security of the sender's private key. If a sender later wishes


key: chroma:document
string_value:
to deny sending a particular message, the sender can claim that the private key was lost or stolen and 
that someone else forged his or her signature. Administrative controls relating to the security of 
private keys can be employed to thwart or at least weaken this ploy, but the threat is still there, at


key: chroma:document
string_value:
least to some degree. One example is to require every signed message to include a timestamp (date 
and time) and to require prompt reporting of compromised keys to a central authority. 
Another threat is that some private key might actually be stolen from X at time T. The opponent can 
then send a message signed with X's signature and stamped with a time before or equal to T.


key: chroma:document
string_value:
2.8.3 Arbitrated Digital Signature 
The problems associated with direct digital signatures can be addressed by using an arbiter. 
Every signed message from a sender X to a receiver Y goes first to an arbiter A, who subjects the 
message and its signature to a number of tests to check its origin and content. The message is then


key: chroma:document
string_value:
dated and sent to Y with an indication that it has been verified to the satisfaction of the arbiter. The 
presence of A solves the problem faced by direct signature schemes: that X might disown the 
message. 
In the first, symmetric encryption is used. It is assumed that the sender X and the arbiter A share a


key: chroma:document
string_value:
secret key Kxa and that A and Y share secret key Kay. X constructs a message M and computes its 
hash value H(M). Then X transmits the message plus a signature to A. The signature consists of an 
identifier IDX of X plus the hash value, all encrypted using Kxa. A decrypts the signature and checks


key: chroma:document
string_value:
the hash value to validate the message. Then A transmits a message to Y, encrypted with Kay. The 
message includes IDX, the original message from X, the signature, and a timestamp. Y can decrypt 
this to recover the message and the signature. The timestamp informs Y that this message is timely


key: chroma:document
string_value:
and not a replay. Y can store M and the signature. In case of dispute, Y, who claims to have received 
M from X, sends the following message to A:


key: full_page_content
string_value:
78 
 
 
Table 3. Arbitrated Digital Signature Techniques 
The arbiter uses Kay to recover IDX, M, and the signature, and then uses Kxa to decrypt the signature 
and verify the hash code. In this scheme, Y cannot directly check X's signature; the signature is there 
solely to settle disputes. Y considers the message from X authentic because it comes through A. In 
this scenario, both sides must have a high degree of trust in A: 
● X must trust A not to reveal Kxa and not to generate false signatures of the form E(Kxa, [IDX||H 
(M)]). 
● Y must trust A to send E(Kay, [IDX||M||E(Kxa, [IDX||H(M)])||T]) only if the hash value is correct  
and the signature was generated by X. 
● Both sides must trust A to resolve disputes fairly. 
If the arbiter does live up to this trust, then X is assured that no one can forge his signature and Y is  
assured that X cannot disavow his signature. 
2.9 Authentication Protocols 
Two general areas involved in authentication protocol. 
1. Mutual authentication 
2. One-way authentication 
2.9.1 Mutual Authentication 
This protocol enables communicating parties to satisfy themselves mutually about each other's 
identity and to exchange session keys.


key: chroma:document
string_value:
78 
 
 
Table 3. Arbitrated Digital Signature Techniques 
The arbiter uses Kay to recover IDX, M, and the signature, and then uses Kxa to decrypt the signature 
and verify the hash code. In this scheme, Y cannot directly check X's signature; the signature is there 
solely to settle disputes. Y considers the message from X authentic because it comes through A. In


key: chroma:document
string_value:
this scenario, both sides must have a high degree of trust in A: 
● X must trust A not to reveal Kxa and not to generate false signatures of the form E(Kxa, [IDX||H 
(M)]). 
● Y must trust A to send E(Kay, [IDX||M||E(Kxa, [IDX||H(M)])||T]) only if the hash value is correct  
and the signature was generated by X. 
● Both sides must trust A to resolve disputes fairly.


key: chroma:document
string_value:
If the arbiter does live up to this trust, then X is assured that no one can forge his signature and Y is  
assured that X cannot disavow his signature. 
2.9 Authentication Protocols 
Two general areas involved in authentication protocol. 
1. Mutual authentication 
2. One-way authentication 
2.9.1 Mutual Authentication


key: chroma:document
string_value:
This protocol enables communicating parties to satisfy themselves mutually about each other's 
identity and to exchange session keys.


key: full_page_content
string_value:
79 
 
Central to the problem of authenticated key exchange are two issues: confidentiality and timeliness. 
To prevent masquerade and to prevent compromise of session keys, essential identification and 
session key information must be communicated in encrypted form. This requires the prior existence 
of secret or public keys that can be used for this purpose. The second issue, timeliness, is important 
because of the threat of message replays. Such replays, at worst, could allow an opponent to 
compromise a session key or successfully impersonate another party. At minimum, a successful 
replay can disrupt operations by presenting parties with messages that appear genuine but are not. 
• Simple replay: The opponent simply copies a message and replays it later. 
● Repetition that can be logged: An opponent can replay a timestamped message within the  
valid time window. 
● Repetition that cannot be detected: This situation could arise because the original message  
could have been suppressed and thus did not arrive at its destination; only the replay message  
arrives. 
● Backward replay without modification: This is a replay back to the message sender. This  
attack is possible if symmetric encryption is used and the sender cannot easily recognize the  
difference between messages sent and messages received on the basis of content. 
One approach to coping with replay attacks is to attach a sequence number to each message used in 
an authentication exchange. A new message is accepted only if its sequence number is in the proper 
order. The difficulty with this approach is that it requires each party to keep track of the last sequence 
number for each claimant it has dealt with. Because of this overhead, sequence numbers are generally 
not used for authentication and key exchange. Instead, one of the following two general approaches 
is used: 
● Timestamps: Party A accepts a message as fresh only if the message contains a timestamp  
that, in A's judgment, is close enough to A's knowledge of current time. This approach requires  
that clocks among the various participants be synchronized. 
● Challenge/response: Party A, expecting a fresh message from B, first sends B a nonce  
(challenge) and requires that the subsequent message (response) received from B contain the correct 
nonce value. 
Two-level hierarchy of symmetric encryption keys can be used to provide confidentiality for 
communication in a distributed environment. In general, this strategy involves the use of a trusted 
key distribution center (KDC). Each party in the network shares a secret key, known as a master key, 
with the KDC. The KDC is responsible for generating keys to be used for a short time over a 
connection between two parties, known as session keys, and for distributing those keys using the 
master keys to protect the distribution.  
 Needham and Schroeder protocol can be summarized as follows:


key: chroma:document
string_value:
79 
 
Central to the problem of authenticated key exchange are two issues: confidentiality and timeliness. 
To prevent masquerade and to prevent compromise of session keys, essential identification and 
session key information must be communicated in encrypted form. This requires the prior existence


key: chroma:document
string_value:
of secret or public keys that can be used for this purpose. The second issue, timeliness, is important 
because of the threat of message replays. Such replays, at worst, could allow an opponent to 
compromise a session key or successfully impersonate another party. At minimum, a successful


key: chroma:document
string_value:
replay can disrupt operations by presenting parties with messages that appear genuine but are not. 
• Simple replay: The opponent simply copies a message and replays it later. 
● Repetition that can be logged: An opponent can replay a timestamped message within the  
valid time window. 
● Repetition that cannot be detected: This situation could arise because the original message


key: chroma:document
string_value:
could have been suppressed and thus did not arrive at its destination; only the replay message  
arrives. 
● Backward replay without modification: This is a replay back to the message sender. This  
attack is possible if symmetric encryption is used and the sender cannot easily recognize the  
difference between messages sent and messages received on the basis of content.


key: chroma:document
string_value:
One approach to coping with replay attacks is to attach a sequence number to each message used in 
an authentication exchange. A new message is accepted only if its sequence number is in the proper 
order. The difficulty with this approach is that it requires each party to keep track of the last sequence


key: chroma:document
string_value:
number for each claimant it has dealt with. Because of this overhead, sequence numbers are generally 
not used for authentication and key exchange. Instead, one of the following two general approaches 
is used: 
● Timestamps: Party A accepts a message as fresh only if the message contains a timestamp


key: chroma:document
string_value:
that, in A's judgment, is close enough to A's knowledge of current time. This approach requires  
that clocks among the various participants be synchronized. 
● Challenge/response: Party A, expecting a fresh message from B, first sends B a nonce  
(challenge) and requires that the subsequent message (response) received from B contain the correct 
nonce value.


key: chroma:document
string_value:
nonce value. 
Two-level hierarchy of symmetric encryption keys can be used to provide confidentiality for 
communication in a distributed environment. In general, this strategy involves the use of a trusted 
key distribution center (KDC). Each party in the network shares a secret key, known as a master key,


key: chroma:document
string_value:
with the KDC. The KDC is responsible for generating keys to be used for a short time over a 
connection between two parties, known as session keys, and for distributing those keys using the 
master keys to protect the distribution.  
 Needham and Schroeder protocol can be summarized as follows:


key: full_page_content
string_value:
80 
 
 
 
Secret keys Ka and Kb are shared between A and the KDC and B and the KDC, respectively. The 
purpose of the protocol is to distribute securely a session key Ks to A and B. A securely acquires a 
new session key in step 2. The message in step 3 can be decrypted, and hence understood, only by B. 
Step 4 reflects B's knowledge of Ks, and step 5 assures B of A's knowledge of Ks and assures B that 
this is a fresh message because of the use of the nonce N2. The purpose of steps 4 and 5 is to prevent 
a certain type of replay attack. 
Denning proposes to overcome this weakness by a modification to the Needham/Schroeder protocol 
that includes the addition of a timestamp to steps 2 and 3. Her proposal assumes that the master keys, 
Ka and Kb are secure, and it consists of the following steps:


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80 
 
 
 
Secret keys Ka and Kb are shared between A and the KDC and B and the KDC, respectively. The 
purpose of the protocol is to distribute securely a session key Ks to A and B. A securely acquires a 
new session key in step 2. The message in step 3 can be decrypted, and hence understood, only by B.


key: chroma:document
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Step 4 reflects B's knowledge of Ks, and step 5 assures B of A's knowledge of Ks and assures B that 
this is a fresh message because of the use of the nonce N2. The purpose of steps 4 and 5 is to prevent 
a certain type of replay attack. 
Denning proposes to overcome this weakness by a modification to the Needham/Schroeder protocol


key: chroma:document
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that includes the addition of a timestamp to steps 2 and 3. Her proposal assumes that the master keys, 
Ka and Kb are secure, and it consists of the following steps:


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81 
 
T is a timestamp that assures A and B that the session key has only just been generated. Thus, both 
A and B know that the key distribution is a fresh exchange. A and B can verify timeliness by checking 
that 
 
where ∆t1 is the estimated normal discrepancy between the KDC's clock and the local clock (at A or 
B) and ∆t2 is the expected network delay time. Each node can set its clock against some standard 
reference source. Because the timestamp T is encrypted using the secure master keys, an opponent, 
even with knowledge of an old session key, cannot succeed because a replay of step 3 will be detected 
by B as untimely. 
One way to counter suppress-replay attacks is to enforce the requirement that parties regularly check 
their clocks against the KDC's clock. The other alternative, which avoids the need for clock 
synchronization, is to rely on handshaking protocols using nonces. This latter alternative is not 
vulnerable to a suppress-replay attack because the nonces the recipient will choose in the future are 
unpredictable to the sender. 
an improved strategy was presented in 
 
Let us follow this exchange step by step. 
1. A initiates the authentication exchange by generating a nonce, Na, and sending that plus its  
identifier to B in plaintext. This nonce will be returned to A in an encrypted message that includes  
the session key, assuring A of its timeliness.  
2. B alerts the KDC that a session key is needed. Its message to the KDC includes its identifier and a  
nonce, Nb This nonce will be returned to B in an encrypted message that includes the session key,  
assuring B of its timeliness. B's message to the KDC also includes a block encrypted with the  
secret key shared by B and the KDC. This block is used to instruct the KDC to issue credentials to  
A; the block specifies the intended recipient of the credentials, a suggested expiration time for the  
credentials, and the nonce received from A. 
The KDC passes on to A B's nonce and a block encrypted with the secret key that B shares with  
the KDC. The block serves as a "ticket" that can be used by A for subsequent authentications, as  
will be seen. The KDC also sends to A a block encrypted with the secret key shared by A and the  
KDC. This block verifies that B has received A's initial message (IDB) and that this is a timely


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81 
 
T is a timestamp that assures A and B that the session key has only just been generated. Thus, both 
A and B know that the key distribution is a fresh exchange. A and B can verify timeliness by checking 
that 
 
where ∆t1 is the estimated normal discrepancy between the KDC's clock and the local clock (at A or


key: chroma:document
string_value:
B) and ∆t2 is the expected network delay time. Each node can set its clock against some standard 
reference source. Because the timestamp T is encrypted using the secure master keys, an opponent, 
even with knowledge of an old session key, cannot succeed because a replay of step 3 will be detected 
by B as untimely.


key: chroma:document
string_value:
by B as untimely. 
One way to counter suppress-replay attacks is to enforce the requirement that parties regularly check 
their clocks against the KDC's clock. The other alternative, which avoids the need for clock 
synchronization, is to rely on handshaking protocols using nonces. This latter alternative is not


key: chroma:document
string_value:
vulnerable to a suppress-replay attack because the nonces the recipient will choose in the future are 
unpredictable to the sender. 
an improved strategy was presented in 
 
Let us follow this exchange step by step. 
1. A initiates the authentication exchange by generating a nonce, Na, and sending that plus its


key: chroma:document
string_value:
identifier to B in plaintext. This nonce will be returned to A in an encrypted message that includes  
the session key, assuring A of its timeliness.  
2. B alerts the KDC that a session key is needed. Its message to the KDC includes its identifier and a  
nonce, Nb This nonce will be returned to B in an encrypted message that includes the session key,


key: chroma:document
string_value:
assuring B of its timeliness. B's message to the KDC also includes a block encrypted with the  
secret key shared by B and the KDC. This block is used to instruct the KDC to issue credentials to  
A; the block specifies the intended recipient of the credentials, a suggested expiration time for the  
credentials, and the nonce received from A.


key: chroma:document
string_value:
The KDC passes on to A B's nonce and a block encrypted with the secret key that B shares with  
the KDC. The block serves as a "ticket" that can be used by A for subsequent authentications, as  
will be seen. The KDC also sends to A a block encrypted with the secret key shared by A and the


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82 
 
message and not a replay (Na) and it provides A with a session key (Ks) and the time limit on its  
use (Tb).  
4. A transmits the ticket to B, together with the B's nonce, the latter encrypted with the session key.  
The ticket provides B with the secret key that is used to decrypt E(Ks, Nb) to recover the nonce.  
The fact that B's nonce is encrypted with the session key authenticates that the message came  
from A and is not a replay. 
This protocol provides an effective, secure means for A and B to establish a session with a secure  
session key.  Furthermore, the protocol leaves A in possession of a key that can be used for subsequent 
authentication to B, avoiding the need to contact the authentication server repeatedly. Suppose that 
A and B establish a session using the aforementioned protocol and then conclude that session. 
Subsequently, but within the time limit established by the protocol, A desires a new session with B. 
The following protocol ensues: 
 
When B receives the message in step 1, it verifies that the ticket has not expired. The newly generated  
nonces N'a and N'b assure each party that there is no replay attack. 
2.9.2 One-Way Authentication 
With some refinement, the KDC strategy is a candidate for encrypted electronic mail.  
Because we wish to avoid requiring that the recipient (B) be on line at the same time as the sender 
(A), steps 4 and 5 must be eliminated. 
 For a message with content M, the sequence is as follows: 
 
This approach guarantees that only the intended recipient of a message will be able to read it. 
 It also provides a level of authentication that the sender is A. 
References: 
• William Stallings, “ Cryptography and Network Security”, 4th Edition, Pearson, 2009. 
• Behrouz A. Forouzan, “ Cryptography and Network Security”, Tata McGraw-Hill, 2008.


key: chroma:document
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82 
 
message and not a replay (Na) and it provides A with a session key (Ks) and the time limit on its  
use (Tb).  
4. A transmits the ticket to B, together with the B's nonce, the latter encrypted with the session key.  
The ticket provides B with the secret key that is used to decrypt E(Ks, Nb) to recover the nonce.


key: chroma:document
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The fact that B's nonce is encrypted with the session key authenticates that the message came  
from A and is not a replay. 
This protocol provides an effective, secure means for A and B to establish a session with a secure  
session key.  Furthermore, the protocol leaves A in possession of a key that can be used for subsequent


key: chroma:document
string_value:
authentication to B, avoiding the need to contact the authentication server repeatedly. Suppose that 
A and B establish a session using the aforementioned protocol and then conclude that session. 
Subsequently, but within the time limit established by the protocol, A desires a new session with B. 
The following protocol ensues:


key: chroma:document
string_value:
When B receives the message in step 1, it verifies that the ticket has not expired. The newly generated  
nonces N'a and N'b assure each party that there is no replay attack. 
2.9.2 One-Way Authentication 
With some refinement, the KDC strategy is a candidate for encrypted electronic mail.


key: chroma:document
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Because we wish to avoid requiring that the recipient (B) be on line at the same time as the sender 
(A), steps 4 and 5 must be eliminated. 
 For a message with content M, the sequence is as follows: 
 
This approach guarantees that only the intended recipient of a message will be able to read it. 
 It also provides a level of authentication that the sender is A. 
References:


key: chroma:document
string_value:
References: 
• William Stallings, “ Cryptography and Network Security”, 4th Edition, Pearson, 2009. 
• Behrouz A. Forouzan, “ Cryptography and Network Security”, Tata McGraw-Hill, 2008.


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83 
 
 
 
SCHOOL OF COMPUTING 
DEPARTMENT OF INFORMATION TECHNOLOGY 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
UNIT – III SYSTEM SECURITY – SITA1602


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84 
 
 
UNIT 3 SYSTEM SECURITY  
Intruders – Detection – Password Management – Malicious Software – Virus – 
Countermeasures – Distributed Denial of Service Attacks – Firewalls – Design 
Principles – Trusted Systems 
3.1 Intruders 
One of the two most publicized threats to security is the intruder (the other is 
viruses), generally referred to as a hacker or cracker. There are three classes of intruders: 
• Masquerader: An individual who is not authorized to use the computer and who 
penetrates a system's access controls to exploit a legitimate user's account 
● Misfeasor: A legitimate user who accesses data, programs, or resources for which such 
access is not authorized, or who is authorized for such access but misuses his or her 
privileges 
● Clandestine user: An individual who seizes supervisory control of the system and uses 
this control to evade auditing and access controls or to suppress audit collection 
An analysis of previous attack revealed that there were two levels of hackers: 
·       The high levels were sophisticated users with a thorough knowledge of the 
technology.   
·       The low levels were the „foot soldiers‟ who merely use the supplied cracking 
programs with little understanding of how they work.   
In addition to running password cracking programs, the intruders attempted to 
modify login software to enable them to capture passwords of users logging onto the 
systems. 
3.1.1 Intrusion techniques 
The objective of the intruders is to gain access to a system or to increase the range of 
privileges accessible on a system. Generally, this requires the intruders to acquire 
information that should be protected.  
The password files can be protected in one of the two ways: 
One way encryption – the system stores only an encrypted form of user‟s password. 
In practice, the system usually performs a one way transformation (not reversible) in 
which the password is used to generate a key for the encryption function and in which a 
fixed length output is produced.   
 Access control – access to the password file is limited to one or a very few accounts.   
The following techniques are used for learning passwords. 
1.  Try default passwords used with standard accounts that are shipped with the 
system.  Many administrators do not bother to change these defaults.


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84 
 
 
UNIT 3 SYSTEM SECURITY  
Intruders – Detection – Password Management – Malicious Software – Virus – 
Countermeasures – Distributed Denial of Service Attacks – Firewalls – Design 
Principles – Trusted Systems 
3.1 Intruders 
One of the two most publicized threats to security is the intruder (the other is


key: chroma:document
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viruses), generally referred to as a hacker or cracker. There are three classes of intruders: 
• Masquerader: An individual who is not authorized to use the computer and who 
penetrates a system's access controls to exploit a legitimate user's account 
● Misfeasor: A legitimate user who accesses data, programs, or resources for which such


key: chroma:document
string_value:
access is not authorized, or who is authorized for such access but misuses his or her 
privileges 
● Clandestine user: An individual who seizes supervisory control of the system and uses 
this control to evade auditing and access controls or to suppress audit collection 
An analysis of previous attack revealed that there were two levels of hackers:


key: chroma:document
string_value:
·       The high levels were sophisticated users with a thorough knowledge of the 
technology.   
·       The low levels were the „foot soldiers‟ who merely use the supplied cracking 
programs with little understanding of how they work.   
In addition to running password cracking programs, the intruders attempted to


key: chroma:document
string_value:
modify login software to enable them to capture passwords of users logging onto the 
systems. 
3.1.1 Intrusion techniques 
The objective of the intruders is to gain access to a system or to increase the range of 
privileges accessible on a system. Generally, this requires the intruders to acquire 
information that should be protected.


key: chroma:document
string_value:
The password files can be protected in one of the two ways: 
One way encryption – the system stores only an encrypted form of user‟s password. 
In practice, the system usually performs a one way transformation (not reversible) in 
which the password is used to generate a key for the encryption function and in which a 
fixed length output is produced.


key: chroma:document
string_value:
Access control – access to the password file is limited to one or a very few accounts.   
The following techniques are used for learning passwords. 
1.  Try default passwords used with standard accounts that are shipped with the 
system.  Many administrators do not bother to change these defaults.


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85 
 
2.  Exhaustively try all short passwords. 
3. Try words in the system’s online dictionary or a list of likely passwords. 
4.  Collect information about users such as their full names, the name of their spouse and 
children, pictures in their office and books in their office that are related to hobbies. 
5. Try user’s phone number, social security numbers and room numbers. 
6. Try all legitimate license plate numbers. 
7. Use a torjan horse to bypass restriction on access. 
8. Tap the line between a remote user and the host system. 
 3.2 Intrusion Detection 
Inevitably, the best intrusion prevention system will fail. A system's second line of 
defense is intrusion detection, and this has been the focus of much research in recent 
years. This interest is motivated by a number of considerations, including the following: 
 ·    If an intrusion is detected quickly enough, the intruder can be identified and ejected 
from the system     before any damage is done or any data are compromised 
·    An effective intrusion detection system can serve as a deterrent, so acting to prevent 
intrusions. 
.    Intrusion detection enables the collection of information about intrusion techniques 
that can be used to strengthen the intrusion prevention facility. 
Intrusion detection is based on the assumption that the behavior of the intruder differs 
from that of a legitimate user in ways that can be quantified. 
Figure 3.1 suggests, in very abstract terms, the nature of the task confronting the 
designer of an intrusion detection system. Although the typical behavior of an intruder 
differs from the typical behavior of an authorized user, there is an overlap in these 
behaviors. Thus, a loose interpretation of intruder behavior, which will catch more 
intruders, will also lead to a number of "false positives," or authorized users identified as 
intruders.


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85 
 
2.  Exhaustively try all short passwords. 
3. Try words in the system’s online dictionary or a list of likely passwords. 
4.  Collect information about users such as their full names, the name of their spouse and 
children, pictures in their office and books in their office that are related to hobbies. 
5. Try user’s phone number, social security numbers and room numbers.


key: chroma:document
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6. Try all legitimate license plate numbers. 
7. Use a torjan horse to bypass restriction on access. 
8. Tap the line between a remote user and the host system. 
 3.2 Intrusion Detection 
Inevitably, the best intrusion prevention system will fail. A system's second line of 
defense is intrusion detection, and this has been the focus of much research in recent


key: chroma:document
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years. This interest is motivated by a number of considerations, including the following: 
 ·    If an intrusion is detected quickly enough, the intruder can be identified and ejected 
from the system     before any damage is done or any data are compromised 
·    An effective intrusion detection system can serve as a deterrent, so acting to prevent 
intrusions.


key: chroma:document
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intrusions. 
.    Intrusion detection enables the collection of information about intrusion techniques 
that can be used to strengthen the intrusion prevention facility. 
Intrusion detection is based on the assumption that the behavior of the intruder differs 
from that of a legitimate user in ways that can be quantified.


key: chroma:document
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Figure 3.1 suggests, in very abstract terms, the nature of the task confronting the 
designer of an intrusion detection system. Although the typical behavior of an intruder 
differs from the typical behavior of an authorized user, there is an overlap in these 
behaviors. Thus, a loose interpretation of intruder behavior, which will catch more


key: chroma:document
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intruders, will also lead to a number of "false positives," or authorized users identified as 
intruders.


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86 
 
 
Fig :3.1 Profiles of Behavior of Intruders and Authorized Users 
The approaches to intrusion detection: 
1. Statistical anomaly detection: Involves the collection of data relating to the behavior of 
legitimate users over a period of time. Then statistical tests are applied to observed 
behavior to determine with a high level of confidence whether that behavior is not 
legitimate user behavior.  
Threshold detection: This approach involves defining thresholds, independent of user, 
for the frequency of occurrence of various events.  
Profile based: A profile of the activity of each user is developed and used to 
detect changes in the behavior of individual accounts. 
2. Rule-based detection: Involves an attempt to define a set of rules that can be used to 
decide that a given behavior is that of an intruder.  
Anomaly detection: Rules are developed to detect deviation from previous 
usage patterns.  
Penetration identification: An expert system approach that searches for suspicious 
behavior.  
3.2.1 Audit Records 
 A fundamental tool for intrusion detection is the audit record. Some record of ongoing 
activity by users must be maintained as input to an intrusion detection system. Basically, 
two plans are used: 
• Native audit records: Virtually all multiuser operating systems include accounting 
software that collects information on user activity. The advantage of using this 
information is that no additional collection software is needed. The disadvantage is that 
the native audit records may not contain the needed information or may not contain it in 
a convenient form.


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86 
 
 
Fig :3.1 Profiles of Behavior of Intruders and Authorized Users 
The approaches to intrusion detection: 
1. Statistical anomaly detection: Involves the collection of data relating to the behavior of 
legitimate users over a period of time. Then statistical tests are applied to observed 
behavior to determine with a high level of confidence whether that behavior is not


key: chroma:document
string_value:
legitimate user behavior.  
Threshold detection: This approach involves defining thresholds, independent of user, 
for the frequency of occurrence of various events.  
Profile based: A profile of the activity of each user is developed and used to 
detect changes in the behavior of individual accounts.


key: chroma:document
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2. Rule-based detection: Involves an attempt to define a set of rules that can be used to 
decide that a given behavior is that of an intruder.  
Anomaly detection: Rules are developed to detect deviation from previous 
usage patterns.  
Penetration identification: An expert system approach that searches for suspicious 
behavior.  
3.2.1 Audit Records


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A fundamental tool for intrusion detection is the audit record. Some record of ongoing 
activity by users must be maintained as input to an intrusion detection system. Basically, 
two plans are used: 
• Native audit records: Virtually all multiuser operating systems include accounting 
software that collects information on user activity. The advantage of using this


key: chroma:document
string_value:
information is that no additional collection software is needed. The disadvantage is that 
the native audit records may not contain the needed information or may not contain it in 
a convenient form.


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87 
 
● Detection-specific audit records: A collection facility can be implemented that 
generates audit records containing only that information required by the intrusion 
detection system. One advantage of such an approach is that it could be made vendor 
independent and ported to a variety of systems. The disadvantage is the extra overhead 
involved in having, in effect, two accounting packages running on a machine. 
Each audit record contains the following fields: 
• Subject: Initiators of actions. A subject is typically a terminal user but might also be a 
process  
acting on behalf of users or groups of users. All activity arises through commands issued 
by  
subjects. Subjects may be grouped into different access classes, and these classes may 
overlap. 
● Action: Operation performed by the subject on or with an object; for example, login, 
read,  
perform I/O, execute. 
● Object: Receptors of actions. Examples include files, programs, messages, records, 
terminals, printers, and user- or program-created structures. When a subject is the 
recipient of an action, such as electronic mail, then that subject is considered an object. 
Objects may be grouped by type. Object granularity may vary by object type and by 
environment. For example, database actions may be audited for the database as a whole 
or at the record level. 
● Exception-Condition: Denotes which, if any, exception condition is raised on return. 
● Resource-Usage: A list of quantitative elements in which each element gives the 
amount used of some resource (e.g., number of lines printed or displayed, number of 
records read or written, processor time, I/O units used, session elapsed time). 
● Time-Stamp: Unique time-and-date stamp identifying when the action took placeMost 
user operations are made up of a number of elementary actions. For example, a file copy 
involves the execution of the user command, which includes doing access validation and 
setting up the copy, plus the read from one file, plus the write to another file. Consider 
the command 
COPY GAME.EXE TO <Library>GAME.EXE 
issued by Smith to copy an executable file GAME from the current directory to the 
<Library> directory. The following audit records may be generated:


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87 
 
● Detection-specific audit records: A collection facility can be implemented that 
generates audit records containing only that information required by the intrusion 
detection system. One advantage of such an approach is that it could be made vendor 
independent and ported to a variety of systems. The disadvantage is the extra overhead


key: chroma:document
string_value:
involved in having, in effect, two accounting packages running on a machine. 
Each audit record contains the following fields: 
• Subject: Initiators of actions. A subject is typically a terminal user but might also be a 
process  
acting on behalf of users or groups of users. All activity arises through commands issued 
by


key: chroma:document
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by  
subjects. Subjects may be grouped into different access classes, and these classes may 
overlap. 
● Action: Operation performed by the subject on or with an object; for example, login, 
read,  
perform I/O, execute. 
● Object: Receptors of actions. Examples include files, programs, messages, records,


key: chroma:document
string_value:
terminals, printers, and user- or program-created structures. When a subject is the 
recipient of an action, such as electronic mail, then that subject is considered an object. 
Objects may be grouped by type. Object granularity may vary by object type and by 
environment. For example, database actions may be audited for the database as a whole 
or at the record level.


key: chroma:document
string_value:
● Exception-Condition: Denotes which, if any, exception condition is raised on return. 
● Resource-Usage: A list of quantitative elements in which each element gives the 
amount used of some resource (e.g., number of lines printed or displayed, number of 
records read or written, processor time, I/O units used, session elapsed time).


key: chroma:document
string_value:
● Time-Stamp: Unique time-and-date stamp identifying when the action took placeMost 
user operations are made up of a number of elementary actions. For example, a file copy 
involves the execution of the user command, which includes doing access validation and 
setting up the copy, plus the read from one file, plus the write to another file. Consider 
the command


key: chroma:document
string_value:
the command 
COPY GAME.EXE TO <Library>GAME.EXE 
issued by Smith to copy an executable file GAME from the current directory to the 
<Library> directory. The following audit records may be generated:


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88 
 
 
In this case, the copy is aborted because Smith does not have write permission to 
<Library> The decomposition of a user operation into elementary actions has three 
advantages: 
1. Because objects are the protectable entities in a system, the use of elementary actions 
enables an audit of all behavior affecting an object.  
2. Single-object, single-action audit records simplify the model and the implementation. 
3. Because of the simple, uniform structure of the detection-specific audit records, it may be 
relatively easy to obtain this information or at least part of it by a straightforward mapping 
from existing native audit records to the detection-specific audit records. 
3.2.2 Statistical Anomaly Detection 
As was mentioned, statistical anomaly detection techniques fall into two broad categories:  
Threshold detection involves counting the number of occurrences of a specific event type 
over an interval of time. If the count surpasses what is considered a reasonable number 
that one might expect to occur, then intrusion is assumed. 
Profile-based anomaly detection focuses on characterizing the past behavior of individual 
users or related groups of users and then detecting significant deviations. A profile may 
consist of a set of parameters, so that deviation on just a single parameter may not be 
sufficient in itself to signal an alert.Examples of metrics that are useful for profile-based 
intrusion detection are the following: 
• Counter: A nonnegative integer that may be incremented but not decremented until it is 
reset by management action. Examples include the number of logins by a single user 
during an hour, the number of times a given command is executed during a single user 
session, and the number of password failures during a minute. 
● Gauge: A nonnegative integer that may be incremented or decremented. Typically, a 
gauge is used to measure the current value of some entity. Examples include the number 
of logical connections assigned to a user application and the number of outgoing 
messages queued for a user process. 
● Interval timer: The length of time between two related events. An example is the 
length of time between successive logins to an account. 
● Resource utilization: Quantity of resources consumed during a specified period. 
Examples include the number of pages printed during a user session and total time


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88 
 
 
In this case, the copy is aborted because Smith does not have write permission to 
<Library> The decomposition of a user operation into elementary actions has three 
advantages: 
1. Because objects are the protectable entities in a system, the use of elementary actions 
enables an audit of all behavior affecting an object.


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2. Single-object, single-action audit records simplify the model and the implementation. 
3. Because of the simple, uniform structure of the detection-specific audit records, it may be 
relatively easy to obtain this information or at least part of it by a straightforward mapping 
from existing native audit records to the detection-specific audit records.


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3.2.2 Statistical Anomaly Detection 
As was mentioned, statistical anomaly detection techniques fall into two broad categories:  
Threshold detection involves counting the number of occurrences of a specific event type 
over an interval of time. If the count surpasses what is considered a reasonable number 
that one might expect to occur, then intrusion is assumed.


key: chroma:document
string_value:
Profile-based anomaly detection focuses on characterizing the past behavior of individual 
users or related groups of users and then detecting significant deviations. A profile may 
consist of a set of parameters, so that deviation on just a single parameter may not be 
sufficient in itself to signal an alert.Examples of metrics that are useful for profile-based


key: chroma:document
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intrusion detection are the following: 
• Counter: A nonnegative integer that may be incremented but not decremented until it is 
reset by management action. Examples include the number of logins by a single user 
during an hour, the number of times a given command is executed during a single user 
session, and the number of password failures during a minute.


key: chroma:document
string_value:
● Gauge: A nonnegative integer that may be incremented or decremented. Typically, a 
gauge is used to measure the current value of some entity. Examples include the number 
of logical connections assigned to a user application and the number of outgoing 
messages queued for a user process. 
● Interval timer: The length of time between two related events. An example is the


key: chroma:document
string_value:
length of time between successive logins to an account. 
● Resource utilization: Quantity of resources consumed during a specified period. 
Examples include the number of pages printed during a user session and total time


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89 
 
consumed by a program execution. Given these general metrics, various tests can be 
performed to determine whether current activity fits within acceptable limits. 
● Mean and standard deviation 
● Multivariate 
● Markov process 
● Time 
● Operational 
The simplest statistical test is to measure the mean and standard deviation of a parameter 
over some historical period. This gives a reflection of the average behavior and its 
variability. 
A multivariate model is based on correlations between two or more variables. Intruder 
behavior may be characterized with greater confidence by considering such correlations 
(for example, processor time and resource usage, or login frequency and session elapsed 
time). 
 A Markov process model is used to establish transition probabilities among various 
states. As an example, this model might be used to look at transitions between certain 
commands. 
 A time series model focuses on time intervals, looking for sequences of events that 
happen too rapidly or too slowly. A variety of statistical tests can be applied to 
characterize abnormal timing. Finally, an operational model is based on a judgment of 
what is considered abnormal, rather than an automated analysis of past audit records.  
3.2.3 Rule-Based Intrusion Detection 
Rule-based techniques detect intrusion by observing events in the system and applying a 
set of rules that lead to a decision regarding whether a given pattern of activity is or is 
not suspicious.  
Rule-based anomaly detection is similar in terms of its approach and strengths to 
statistical anomaly detection. With the rule-based approach, historical audit records are 
analyzed to identify usage patterns and to generate automatically rules that describe those 
patterns. Rules may represent past behavior patterns of users, programs, privileges, time 
slots, terminals, and so on. Current behavior is then observed, and each transaction is 
matched against the set of rules to determine if it conforms to any historically observed 
pattern of behavior. 
Rule-based penetration identification takes a very different approach to intrusion 
detection, one based on expert system technology. The key feature of such systems is the 
use of rules for identifying known penetrations or penetrations that would exploit known 
weaknesses 
 Example heuristics are the following:


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89 
 
consumed by a program execution. Given these general metrics, various tests can be 
performed to determine whether current activity fits within acceptable limits. 
● Mean and standard deviation 
● Multivariate 
● Markov process 
● Time 
● Operational 
The simplest statistical test is to measure the mean and standard deviation of a parameter


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over some historical period. This gives a reflection of the average behavior and its 
variability. 
A multivariate model is based on correlations between two or more variables. Intruder 
behavior may be characterized with greater confidence by considering such correlations 
(for example, processor time and resource usage, or login frequency and session elapsed 
time).


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time). 
 A Markov process model is used to establish transition probabilities among various 
states. As an example, this model might be used to look at transitions between certain 
commands. 
 A time series model focuses on time intervals, looking for sequences of events that 
happen too rapidly or too slowly. A variety of statistical tests can be applied to


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characterize abnormal timing. Finally, an operational model is based on a judgment of 
what is considered abnormal, rather than an automated analysis of past audit records.  
3.2.3 Rule-Based Intrusion Detection 
Rule-based techniques detect intrusion by observing events in the system and applying a


key: chroma:document
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set of rules that lead to a decision regarding whether a given pattern of activity is or is 
not suspicious.  
Rule-based anomaly detection is similar in terms of its approach and strengths to 
statistical anomaly detection. With the rule-based approach, historical audit records are 
analyzed to identify usage patterns and to generate automatically rules that describe those


key: chroma:document
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patterns. Rules may represent past behavior patterns of users, programs, privileges, time 
slots, terminals, and so on. Current behavior is then observed, and each transaction is 
matched against the set of rules to determine if it conforms to any historically observed 
pattern of behavior. 
Rule-based penetration identification takes a very different approach to intrusion


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detection, one based on expert system technology. The key feature of such systems is the 
use of rules for identifying known penetrations or penetrations that would exploit known 
weaknesses 
 Example heuristics are the following:


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1. Users should not read files in other users' personal directories. 
2. Users must not write other users' files. 
3. Users who log in after hours often access the same files they used earlier. 
4. Users do not generally open disk devices directly but rely on higher-level operating 
system  
utilities. 
5. Users should not be logged in more than once to the same system. 
 3.2.4 The Base-Rate Fallacy 
      To be of practical use, an intrusion detection system should detect a substantial 
percentage of intrusions while keeping the false alarm rate at an acceptable level. If only 
a modest percentage of actual intrusions are detected, the system provides a false sense 
of security. On the other hand, if the system frequently triggers an alert when there is no 
intrusion (a false alarm), then either system managers will begin to ignore the alarms, or 
much time will be wasted analyzing the false alarms. 
     3.2.5 Distributed Intrusion Detection 
Until recently, work on intrusion detection systems focused on single-system stand-
alone facilities. The typical organization, however, needs to defend a distributed 
collection of hosts supported by a LAN Porras points out the following major issues in 
the design of a distributed intrusion detection system 
✓ 
A distributed intrusion detection system may need to deal with different audit 
record formats. In a heterogeneous environment, different systems will employ different 
native audit collection systems and, if using intrusion detection, may employ different 
formats for security-related audit records.   
✓ 
One or more nodes in the network will serve as collection and analysis points 
for the data from the systems on the network. Thus, either raw audit data or summary 
data must be transmitted across the network. Therefore, there is a requirement to assure 
the integrity and confidentiality of these data.  
✓ 
Either a centralized or decentralized architecture can be used. With a centralized 
architecture, there is a single central point of collection and analysis of all audit data. This 
eases the task of correlating incoming reports but creates a potential bottleneck and single 
point of failure. With a decentralized architecture, there are more than one analysis 
centers, but these must coordinate their activities and exchange information. 
A good example of a distributed intrusion detection system is developed. Figure 3.2  
shows the overall architecture, which consists of three main components:  
● Host agent module: An audit collection module operating as a background process on 
a monitored system. Its purpose is to collect data on security-related events on the host 
and transmit these to the central manager.


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90 
 
1. Users should not read files in other users' personal directories. 
2. Users must not write other users' files. 
3. Users who log in after hours often access the same files they used earlier. 
4. Users do not generally open disk devices directly but rely on higher-level operating 
system  
utilities. 
5. Users should not be logged in more than once to the same system.


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3.2.4 The Base-Rate Fallacy 
      To be of practical use, an intrusion detection system should detect a substantial 
percentage of intrusions while keeping the false alarm rate at an acceptable level. If only 
a modest percentage of actual intrusions are detected, the system provides a false sense


key: chroma:document
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of security. On the other hand, if the system frequently triggers an alert when there is no 
intrusion (a false alarm), then either system managers will begin to ignore the alarms, or 
much time will be wasted analyzing the false alarms. 
     3.2.5 Distributed Intrusion Detection 
Until recently, work on intrusion detection systems focused on single-system stand-


key: chroma:document
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alone facilities. The typical organization, however, needs to defend a distributed 
collection of hosts supported by a LAN Porras points out the following major issues in 
the design of a distributed intrusion detection system 
✓ 
A distributed intrusion detection system may need to deal with different audit


key: chroma:document
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record formats. In a heterogeneous environment, different systems will employ different 
native audit collection systems and, if using intrusion detection, may employ different 
formats for security-related audit records.   
✓ 
One or more nodes in the network will serve as collection and analysis points


key: chroma:document
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for the data from the systems on the network. Thus, either raw audit data or summary 
data must be transmitted across the network. Therefore, there is a requirement to assure 
the integrity and confidentiality of these data.  
✓ 
Either a centralized or decentralized architecture can be used. With a centralized


key: chroma:document
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architecture, there is a single central point of collection and analysis of all audit data. This 
eases the task of correlating incoming reports but creates a potential bottleneck and single 
point of failure. With a decentralized architecture, there are more than one analysis 
centers, but these must coordinate their activities and exchange information.


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A good example of a distributed intrusion detection system is developed. Figure 3.2  
shows the overall architecture, which consists of three main components:  
● Host agent module: An audit collection module operating as a background process on 
a monitored system. Its purpose is to collect data on security-related events on the host 
and transmit these to the central manager.


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91 
 
● LAN monitor agent module: Operates in the same fashion as a host agent module 
except that it analyzes LAN traffic and reports the results to the central manager. 
 ● Central manager module: Receives reports from LAN monitor and host agents and 
processes and correlates these reports to detect intrusion 
 
Fig 3.2. Architecture for Distributed Intrusion Detection 
The scheme is designed to be independent of any operating system or system auditing 
implementation.


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91 
 
● LAN monitor agent module: Operates in the same fashion as a host agent module 
except that it analyzes LAN traffic and reports the results to the central manager. 
 ● Central manager module: Receives reports from LAN monitor and host agents and 
processes and correlates these reports to detect intrusion 
 
Fig 3.2. Architecture for Distributed Intrusion Detection


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The scheme is designed to be independent of any operating system or system auditing 
implementation.


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92 
 
 
 
•        The agent captures each audit record produced by the native audit collection system. 
•        A filter is applied that retains only those records that are of security interest. 
•        These records are then reformatted into a standardized format referred to as the host 
audit record (HAR). 
•        Next, a template-driven logic module analyzes the records for suspicious activity. 
•        At the lowestlevel, the agent scans for notable events that are of interest independent 
of any past events. 
•        Examples include failed file accesses, accessing system files, and changing a file's 
access control. 
•        At the next higher level, the agent looks for sequences of events, such as known attack 
atterns (signatures). 
•        Finally, the agent looks for anomalous behavior of an individual user based on a 
historical profile of that user, such as number of programs executed, number of files 
accessed, and the like. 
•        When suspicious activity is detected, an alert is sent to the central manager.


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92 
 
 
 
•        The agent captures each audit record produced by the native audit collection system. 
•        A filter is applied that retains only those records that are of security interest. 
•        These records are then reformatted into a standardized format referred to as the host 
audit record (HAR).


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•        Next, a template-driven logic module analyzes the records for suspicious activity. 
•        At the lowestlevel, the agent scans for notable events that are of interest independent 
of any past events. 
•        Examples include failed file accesses, accessing system files, and changing a file's 
access control.


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access control. 
•        At the next higher level, the agent looks for sequences of events, such as known attack 
atterns (signatures). 
•        Finally, the agent looks for anomalous behavior of an individual user based on a 
historical profile of that user, such as number of programs executed, number of files 
accessed, and the like.


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93 
 
•        The central manager includes an expert system that can draw inferences from received 
data. 
•        The manager may also query individual systems for copies of HARs to correlate with 
those from other agents. 
•        The LAN monitor agent also supplies information to the central manager. 
•        The LAN monitor agent audits host-host connections, services used, and volume of 
traffic. 
•        It  
searches  
for  
significant  
events,  
such  
as  
sudden  
changes 
in 
network  load,  the  use  of 
•        security-related services, and network activities such as rlogin. 
 3.2.6 Honeypots 
A relatively recent innovation in intrusion detection technology is the honeypot. 
Honeypots are designed to 
● divert an attacker from accessing critical systems 
● collect information about the attacker's activity 
● encourage the attacker to stay on the system long enough for administrators to respond 
3.2.7 Intrusion Detection Exchange Format 
To facilitate the development of distributed intrusion detection systems that can 
function across a wide range of platforms and environments, standards are needed to 
support interoperability.  
 The outputs of this working group include the following: 
1. A requirements document, which describes the high-level functional requirements for 
communication 
between 
intrusion 
detection 
systems 
and 
requirements 
for 
communication between intrusion detection systems and with management systems, 
including the rationale for those requirements. Scenarios will be used to illustrate the 
requirements. 
2. A common intrusion language specification, which describes data formats that satisfy 
the requirements. 
3. A framework document, which identifies existing protocols best used for 
communication between intrusion detection systems, and describes how the devised data 
formats relate to them. 
3.3 Password Management 
3.3.1 Password Protection 
The front line of defense against intruders is the password system. Virtually all 
multiuser systems require that a user provide not only a name or identifier (ID) but also


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93 
 
•        The central manager includes an expert system that can draw inferences from received 
data. 
•        The manager may also query individual systems for copies of HARs to correlate with 
those from other agents. 
•        The LAN monitor agent also supplies information to the central manager.


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•        The LAN monitor agent audits host-host connections, services used, and volume of 
traffic. 
•        It  
searches  
for  
significant  
events,  
such  
as  
sudden  
changes 
in 
network  load,  the  use  of 
•        security-related services, and network activities such as rlogin. 
 3.2.6 Honeypots


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3.2.6 Honeypots 
A relatively recent innovation in intrusion detection technology is the honeypot. 
Honeypots are designed to 
● divert an attacker from accessing critical systems 
● collect information about the attacker's activity 
● encourage the attacker to stay on the system long enough for administrators to respond 
3.2.7 Intrusion Detection Exchange Format


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To facilitate the development of distributed intrusion detection systems that can 
function across a wide range of platforms and environments, standards are needed to 
support interoperability.  
 The outputs of this working group include the following: 
1. A requirements document, which describes the high-level functional requirements for 
communication 
between 
intrusion


key: chroma:document
string_value:
between 
intrusion 
detection 
systems 
and 
requirements 
for 
communication between intrusion detection systems and with management systems, 
including the rationale for those requirements. Scenarios will be used to illustrate the 
requirements. 
2. A common intrusion language specification, which describes data formats that satisfy 
the requirements.


key: chroma:document
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the requirements. 
3. A framework document, which identifies existing protocols best used for 
communication between intrusion detection systems, and describes how the devised data 
formats relate to them. 
3.3 Password Management 
3.3.1 Password Protection 
The front line of defense against intruders is the password system. Virtually all


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a password. The password serves to authenticate the ID of the individual logging on to 
the system. In turn, the ID provides security in the following ways: 
• The ID determines whether the user is authorized to gain access to a system.  
•  The ID determines the privileges accorded to the user.  
•  The ID is used in what is referred to as discretionary access control. For example, by 
listing the IDs of the other users, a user may grant permission to them to read files owned 
by that user. 
3.3.1.1 The Vulnerability of Passwords 
To understand the nature of the threat to password-based systems, let us consider a 
scheme that is widely used on UNIX, the following procedure is employed in figure 3.3. 
• Each user selects a password of up to eight printable characters in length.  
• This is converted into a 56-bit value (using 7-bit ASCII) that serves as the key input to 
an encryption routine.  
• The encryption routine, known as crypt(3), is based on DES. The DES algorithm is 
modified using a 12-bit "salt" value. 
• Typically, this value is related to the time at which the password is assigned to the user.  
• The modified DES algorithm is exercised with a data input consisting of a 64-bit block 
of zeros.  
• The output of the algorithm then serves as input for a second encryption.  
• This process is repeated for a total of 25 encryptions. 
• The resulting 64-bit output is then translated into an 11-character sequence.  
• The hashed password is then stored, together with a plaintext copy of the salt, in the 
password file for the corresponding user ID.  
• This method has been shown to be secure against a variety of cryptanalytic attack 
The salt serves three purposes: 
● It prevents duplicate passwords from being visible in the password file. Even if two 
users choose the same password, those passwords will be assigned at different times. 
Hence, the "extended" passwords of the two users will differ. 
● It effectively increases the length of the password without requiring the user to 
remember two additional characters.  
● It prevents the use of a hardware implementation of DES, which would ease the 
difficulty of a brute-force guessing attack. 
When a user attempts to log on to a UNIX system, the user provides an ID and a password. 
The operating system uses the ID to index into the password file and retrieve the plaintext


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a password. The password serves to authenticate the ID of the individual logging on to 
the system. In turn, the ID provides security in the following ways: 
• The ID determines whether the user is authorized to gain access to a system.  
•  The ID determines the privileges accorded to the user.


key: chroma:document
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•  The ID is used in what is referred to as discretionary access control. For example, by 
listing the IDs of the other users, a user may grant permission to them to read files owned 
by that user. 
3.3.1.1 The Vulnerability of Passwords 
To understand the nature of the threat to password-based systems, let us consider a


key: chroma:document
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scheme that is widely used on UNIX, the following procedure is employed in figure 3.3. 
• Each user selects a password of up to eight printable characters in length.  
• This is converted into a 56-bit value (using 7-bit ASCII) that serves as the key input to 
an encryption routine.  
• The encryption routine, known as crypt(3), is based on DES. The DES algorithm is


key: chroma:document
string_value:
modified using a 12-bit "salt" value. 
• Typically, this value is related to the time at which the password is assigned to the user.  
• The modified DES algorithm is exercised with a data input consisting of a 64-bit block 
of zeros.  
• The output of the algorithm then serves as input for a second encryption.  
• This process is repeated for a total of 25 encryptions.


key: chroma:document
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• The resulting 64-bit output is then translated into an 11-character sequence.  
• The hashed password is then stored, together with a plaintext copy of the salt, in the 
password file for the corresponding user ID.  
• This method has been shown to be secure against a variety of cryptanalytic attack 
The salt serves three purposes:


key: chroma:document
string_value:
● It prevents duplicate passwords from being visible in the password file. Even if two 
users choose the same password, those passwords will be assigned at different times. 
Hence, the "extended" passwords of the two users will differ. 
● It effectively increases the length of the password without requiring the user to 
remember two additional characters.


key: chroma:document
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● It prevents the use of a hardware implementation of DES, which would ease the 
difficulty of a brute-force guessing attack. 
When a user attempts to log on to a UNIX system, the user provides an ID and a password. 
The operating system uses the ID to index into the password file and retrieve the plaintext


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95 
 
salt and the encrypted password. The salt and user-supplied password are used as input 
to the encryption routine. If the result matches the stored value, the password is accepted. 
The encryption routine is designed to discourage guessing attacks. Software 
implementations of DES are slow compared to hardware versions, and the use of 25 
iterations multiplies the time required by 25. 
 
 
Fig 3.3 : UNIX Password Scheme 
 
 
Thus, there are two threats to the UNIX password scheme. First, a user can gain 
access on a machine using a guest account or by some other means and then run a 
password guessing program, called a password cracker, on that machine.


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salt and the encrypted password. The salt and user-supplied password are used as input 
to the encryption routine. If the result matches the stored value, the password is accepted. 
The encryption routine is designed to discourage guessing attacks. Software 
implementations of DES are slow compared to hardware versions, and the use of 25


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iterations multiplies the time required by 25. 
 
 
Fig 3.3 : UNIX Password Scheme 
 
 
Thus, there are two threats to the UNIX password scheme. First, a user can gain 
access on a machine using a guest account or by some other means and then run a 
password guessing program, called a password cracker, on that machine.


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96 
 
As an example, a password cracker was reported on the Internet in August 1993. 
Using a Thinking Machines Corporation parallel computer, a performance of 1560 
encryptions per second per vector unit was achieved. With four vector units per 
processing node (a standard configuration), this works out to 800,000 encryptions per 
second on a 128-node machine (which is a modest size) and 6.4 million encryptions per 
second on a 1024-node machine. 
Password length is only part of the problem. Many people, when permitted to 
choose their own password, pick a password that is guessable, such as their own name, 
their street name, a common dictionary word, and so forth. This makes the job of 
password cracking straightforward. 
The following strategy was used: 
1. Try the user's name, initials, account name, and other relevant personal information. In 
all, 130 different permutations for each user were tried. 
2. Try words from various dictionaries.  
3. Try various permutations on the words from step 2.  
4. Try various capitalization permutations on the words from step 2 that were not 
considered in step 3. This added almost 2 million additional words to the list. 
3.3.1.2 Access Control 
One way to thwart a password attack is to deny the opponent access to the password file. 
If the encrypted password portion of the file is accessible only by a privileged user, then 
the opponent cannot read it without already knowing the password of a privileged user. 
 3.3.2 Password Selection Strategies 
Four basic techniques are in use: 
• User education  
• Computer-generated passwords 
• Reactive password checking  
• Proactive password checking 
 
Users can be told the importance of using hard-to-guess passwords and can be 
provided with guidelines for selecting strong passwords. This user education strategy 
is unlikely to succeed at most installations, particularly where there is a large user 
population or a lot of turnover. Many users will simply ignore the guidelines 
Computer-generated passwords also have problems. If the passwords are quite random 
in nature, users will not be able to remember them. Even if the password is 
pronounceable, the user may have difficulty remembering it and so be tempted to write it 
down.


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96 
 
As an example, a password cracker was reported on the Internet in August 1993. 
Using a Thinking Machines Corporation parallel computer, a performance of 1560 
encryptions per second per vector unit was achieved. With four vector units per 
processing node (a standard configuration), this works out to 800,000 encryptions per


key: chroma:document
string_value:
second on a 128-node machine (which is a modest size) and 6.4 million encryptions per 
second on a 1024-node machine. 
Password length is only part of the problem. Many people, when permitted to 
choose their own password, pick a password that is guessable, such as their own name, 
their street name, a common dictionary word, and so forth. This makes the job of


key: chroma:document
string_value:
password cracking straightforward. 
The following strategy was used: 
1. Try the user's name, initials, account name, and other relevant personal information. In 
all, 130 different permutations for each user were tried. 
2. Try words from various dictionaries.  
3. Try various permutations on the words from step 2.


key: chroma:document
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4. Try various capitalization permutations on the words from step 2 that were not 
considered in step 3. This added almost 2 million additional words to the list. 
3.3.1.2 Access Control 
One way to thwart a password attack is to deny the opponent access to the password file. 
If the encrypted password portion of the file is accessible only by a privileged user, then


key: chroma:document
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the opponent cannot read it without already knowing the password of a privileged user. 
 3.3.2 Password Selection Strategies 
Four basic techniques are in use: 
• User education  
• Computer-generated passwords 
• Reactive password checking  
• Proactive password checking 
 
Users can be told the importance of using hard-to-guess passwords and can be


key: chroma:document
string_value:
provided with guidelines for selecting strong passwords. This user education strategy 
is unlikely to succeed at most installations, particularly where there is a large user 
population or a lot of turnover. Many users will simply ignore the guidelines 
Computer-generated passwords also have problems. If the passwords are quite random


key: chroma:document
string_value:
in nature, users will not be able to remember them. Even if the password is 
pronounceable, the user may have difficulty remembering it and so be tempted to write it 
down.


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string_value:
97 
 
A reactive password checking strategy is one in which the system periodically runs its 
own password   cracker to find guessable passwords. 
 
The most promising approach to improved password security is a proactive 
password checker. In this scheme, a user is allowed to select his or her own password. 
However, at the time of selection, the system checks to see if the password is allowable 
and, if not, rejects it. Such checkers are based on the philosophy that, with sufficient 
guidance from the system, users can select memorable passwords from a fairly large 
password space that are not likely to be guessed in a dictionary attack. 
The first approach is a simple system for rule enforcement. For example, the following 
rules could be enforced: 
● All passwords must be at least eight characters long. 
● In the first eight characters, the passwords must include at least one each of uppercase, 
lowercase, numeric digits, and punctuation marks. 
There are two problems with this approach: 
● Space: The dictionary must be very large to be effective.  
● Time: The time required to search a large dictionary may itself be large.  
Two techniques for developing an effective and efficient proactive password checker that 
is based on rejecting words on a list show promise. One of these develops a Markov 
model for the generation of guessable passwords. Figure 3.4 shows a simplified version 
of such a model. This model shows a language consisting of an alphabet of three 
characters. The state of the system at any time is the identity of the most recent letter. The 
value on the transition from one state to another represents the probability that one letter 
follows another. Thus, the probability that the next letter is b, given that the current letter 
is a, is 0.5.


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string_value:
97 
 
A reactive password checking strategy is one in which the system periodically runs its 
own password   cracker to find guessable passwords. 
 
The most promising approach to improved password security is a proactive 
password checker. In this scheme, a user is allowed to select his or her own password.


key: chroma:document
string_value:
However, at the time of selection, the system checks to see if the password is allowable 
and, if not, rejects it. Such checkers are based on the philosophy that, with sufficient 
guidance from the system, users can select memorable passwords from a fairly large 
password space that are not likely to be guessed in a dictionary attack.


key: chroma:document
string_value:
The first approach is a simple system for rule enforcement. For example, the following 
rules could be enforced: 
● All passwords must be at least eight characters long. 
● In the first eight characters, the passwords must include at least one each of uppercase, 
lowercase, numeric digits, and punctuation marks. 
There are two problems with this approach:


key: chroma:document
string_value:
● Space: The dictionary must be very large to be effective.  
● Time: The time required to search a large dictionary may itself be large.  
Two techniques for developing an effective and efficient proactive password checker that 
is based on rejecting words on a list show promise. One of these develops a Markov


key: chroma:document
string_value:
model for the generation of guessable passwords. Figure 3.4 shows a simplified version 
of such a model. This model shows a language consisting of an alphabet of three 
characters. The state of the system at any time is the identity of the most recent letter. The 
value on the transition from one state to another represents the probability that one letter


key: chroma:document
string_value:
follows another. Thus, the probability that the next letter is b, given that the current letter 
is a, is 0.5.


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98 
 
 
Figure 3.4. An Example Markov Model 
 
In general, a Markov model is a quadruple [m, A, T, k], where m is the number of 
states in the model, A is the state space, T is the matrix of transition probabilities, and k 
is the order of the model. For a kth-order model, the probability of making a transition to 
a particular letter depends on the previous k letters that have been generated. 
 
The authors report on the development and use of a second-order model. To begin, a 
dictionary of guessable passwords is constructed. Then the transition matrix is calculated 
as follows: 
1. Determine the frequency matrix f, where f(i, j, k) is the number of occurrences of the 
trigram  
consisting of the ith, jth, and kth character. For example, the password parsnips yields the  
trigrams par, ars, rsn, sni, nip, and ips. 
2. For each bigram ij, calculate f(i, j, ) as the total number of trigrams beginning with ij. 
For  
example, f(a, b, ) would be the total number of trigrams of the form aba, abb, abc, and so 
on. 
3. Compute the entries of T as follows:


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98 
 
 
Figure 3.4. An Example Markov Model 
 
In general, a Markov model is a quadruple [m, A, T, k], where m is the number of 
states in the model, A is the state space, T is the matrix of transition probabilities, and k 
is the order of the model. For a kth-order model, the probability of making a transition to


key: chroma:document
string_value:
a particular letter depends on the previous k letters that have been generated. 
 
The authors report on the development and use of a second-order model. To begin, a 
dictionary of guessable passwords is constructed. Then the transition matrix is calculated 
as follows: 
1. Determine the frequency matrix f, where f(i, j, k) is the number of occurrences of the 
trigram


key: chroma:document
string_value:
trigram  
consisting of the ith, jth, and kth character. For example, the password parsnips yields the  
trigrams par, ars, rsn, sni, nip, and ips. 
2. For each bigram ij, calculate f(i, j, ) as the total number of trigrams beginning with ij. 
For  
example, f(a, b, ) would be the total number of trigrams of the form aba, abb, abc, and so 
on.


key: full_page_content
string_value:
99 
 
The result is a model that reflects the structure of the words in the dictionary. 
A quite different approach has been reported by Spafford [SPAF92a, SPAF92b]. It is 
based on the use of a Bloom filter [BLOO70]. To begin, we explain the operation of the 
Bloom filter. A Bloom filter of order k consists of a set of k independent hash functions 
H1(x), H2(x),..., Hk (x), where each function maps a password into a hash value in the 
range 0 to N - 1 That is, 
 
D = number of words in password dictionary 
The following procedure is then applied to the dictionary: 
1. A hash table of N bits is defined, with all bits initially set to 0. 
2. For each password, its k hash values are calculated, and the corresponding bits in the 
hash table are set to 1. Thus, if Hi(Xj) = 67 for some (i, j), then the sixty-seventh bit of 
the hash table is set to 1; if the bit already has the value 1, it remains at 1.When a new 
password is presented to the checker, its k hash values are calculated. If all the 
corresponding bits of the hash table are equal to 1, then the password is rejected. 
 
3.4 Malcious Software 
3.4.1 Malicious Programs 
 
Malicious software can be divided into two categories: those that need a host 
program, and those that are independent. The former, referred to as parasitic, are 
essentially fragments of programs that cannot exist independently of some actual 
application program, utility, or system program. Viruses, logic  bombs,


key: chroma:document
string_value:
99 
 
The result is a model that reflects the structure of the words in the dictionary. 
A quite different approach has been reported by Spafford [SPAF92a, SPAF92b]. It is 
based on the use of a Bloom filter [BLOO70]. To begin, we explain the operation of the 
Bloom filter. A Bloom filter of order k consists of a set of k independent hash functions


key: chroma:document
string_value:
H1(x), H2(x),..., Hk (x), where each function maps a password into a hash value in the 
range 0 to N - 1 That is, 
 
D = number of words in password dictionary 
The following procedure is then applied to the dictionary: 
1. A hash table of N bits is defined, with all bits initially set to 0.


key: chroma:document
string_value:
2. For each password, its k hash values are calculated, and the corresponding bits in the 
hash table are set to 1. Thus, if Hi(Xj) = 67 for some (i, j), then the sixty-seventh bit of 
the hash table is set to 1; if the bit already has the value 1, it remains at 1.When a new 
password is presented to the checker, its k hash values are calculated. If all the


key: chroma:document
string_value:
corresponding bits of the hash table are equal to 1, then the password is rejected. 
 
3.4 Malcious Software 
3.4.1 Malicious Programs 
 
Malicious software can be divided into two categories: those that need a host 
program, and those that are independent. The former, referred to as parasitic, are 
essentially fragments of programs that cannot exist independently of some actual


key: full_page_content
string_value:
100 
 
 
 
Table 3.1 Terminology of Malicious Programs 
and backdoors are examples. Independent malware is a self-contained program that can 
be scheduled and run by the operating system. Worms and bot programs are examples. 
 
3.4.1.1 Backdoor 
 
A backdoor, also known as a trapdoor, is a secret entry point into a program that 
allows someone who is aware of the backdoor to gain access without going through the 
usual security access procedures. Programmers have used backdoors legiti- mately for 
many years to debug and test programs; such a backdoor is called a maintenance hook. 
This usually is done when the programmer is developing an application that has an 
authentication procedure, or a long setup, requiring the user to enter many different values 
to run the application. To debug the program, the developer may wish to gain special 
privileges or to avoid all the necessary setup and authentication.  
 
Backdoors become threats when unscrupulous programmers use them to gain 
unauthorized access. The backdoor was the basic idea for the vulnerability portrayed in 
the movie War Games. Another example is that during the develop- ment of Multics, 
penetration tests were conducted by an Air Force “tiger team” (simulating adversaries).


key: chroma:document
string_value:
100 
 
 
 
Table 3.1 Terminology of Malicious Programs 
and backdoors are examples. Independent malware is a self-contained program that can 
be scheduled and run by the operating system. Worms and bot programs are examples. 
 
3.4.1.1 Backdoor 
 
A backdoor, also known as a trapdoor, is a secret entry point into a program that


key: chroma:document
string_value:
allows someone who is aware of the backdoor to gain access without going through the 
usual security access procedures. Programmers have used backdoors legiti- mately for 
many years to debug and test programs; such a backdoor is called a maintenance hook. 
This usually is done when the programmer is developing an application that has an


key: chroma:document
string_value:
authentication procedure, or a long setup, requiring the user to enter many different values 
to run the application. To debug the program, the developer may wish to gain special 
privileges or to avoid all the necessary setup and authentication.  
 
Backdoors become threats when unscrupulous programmers use them to gain


key: chroma:document
string_value:
unauthorized access. The backdoor was the basic idea for the vulnerability portrayed in 
the movie War Games. Another example is that during the develop- ment of Multics, 
penetration tests were conducted by an Air Force “tiger team” (simulating adversaries).


key: full_page_content
string_value:
101 
 
 
It is difficult to implement operating system controls for backdoors. Security 
measures must focus on the program development and software update activities. 
 
3.4.1.2 Logic Bomb 
 
 
The logic bomb is code embedded in some legitimate program that is set to “explode” 
when certain conditions are met. Examples of conditions that can be used as triggers for 
a logic bomb are the presence or absence of certain files, a particular day of the week or 
date, or a particular user running the appli- cation. Once triggered, a bomb may alter or 
delete data or entire files, cause a machine halt, or do some other damage.  
 
3.4.1.3 Trojan Horses 
 
A Trojan horse1 is a useful, or apparently useful, program or command procedure 
containing hidden code that, when invoked, performs some unwanted or harmful 
function. 
 
Trojan horse programs can be used to accomplish functions indirectly that an 
unauthorized user could not accomplish directly. For example, to gain access to the files 
of another user on a shared system, a user could create a Trojan horse program that, when 
executed, changes the invoking user’s file permissions so that the files are readable by 
any user. The author could then induce users to run the program by placing it in a common 
directory and naming it such that it appears to be a useful utility program or application. 
An example is a program that ostensibly produces a listing of the user’s files in a desirable 
format. After another user has run the program, the author of the program can then access 
the information in the user’s files. An example of a Trojan horse program that would be 
difficult to detect is a compiler that has been modified to insert additional code into 
certain programs as they are compiled, such as a system login program. The code creates 
a backdoor in the login program that permits the author to log on to the system using a 
special password. This Trojan horse can never be discovered by reading the source code 
of the login program. 
 
3.4.1.4 Zombie  
A zombie is a program that secretly takes over another Internet-attached computer and 
then uses that computer to launch attacks that are difficult to trace to the zombie's creator. 
Zombies are used in denial-of-service attacks, typically against targeted Web sites. The 
zombie is planted on hundreds of computers belonging to unsuspecting third parties, and 
then used to overwhelm the target Web site by launching an overwhelming onslaught of 
Internet traffic.  
 
3.4.2 The nature of viruses 
A computer virus is a piece of software that can “infect” other programs by modifying  
hem; the modification includes injecting the original program with a routine to make 
copies of the virus program, which can then go on to infect other programs. The typical


key: chroma:document
string_value:
101 
 
 
It is difficult to implement operating system controls for backdoors. Security 
measures must focus on the program development and software update activities. 
 
3.4.1.2 Logic Bomb 
 
 
The logic bomb is code embedded in some legitimate program that is set to “explode” 
when certain conditions are met. Examples of conditions that can be used as triggers for


key: chroma:document
string_value:
a logic bomb are the presence or absence of certain files, a particular day of the week or 
date, or a particular user running the appli- cation. Once triggered, a bomb may alter or 
delete data or entire files, cause a machine halt, or do some other damage.  
 
3.4.1.3 Trojan Horses 
 
A Trojan horse1 is a useful, or apparently useful, program or command procedure


key: chroma:document
string_value:
containing hidden code that, when invoked, performs some unwanted or harmful 
function. 
 
Trojan horse programs can be used to accomplish functions indirectly that an 
unauthorized user could not accomplish directly. For example, to gain access to the files 
of another user on a shared system, a user could create a Trojan horse program that, when


key: chroma:document
string_value:
executed, changes the invoking user’s file permissions so that the files are readable by 
any user. The author could then induce users to run the program by placing it in a common 
directory and naming it such that it appears to be a useful utility program or application. 
An example is a program that ostensibly produces a listing of the user’s files in a desirable


key: chroma:document
string_value:
format. After another user has run the program, the author of the program can then access 
the information in the user’s files. An example of a Trojan horse program that would be 
difficult to detect is a compiler that has been modified to insert additional code into 
certain programs as they are compiled, such as a system login program. The code creates


key: chroma:document
string_value:
a backdoor in the login program that permits the author to log on to the system using a 
special password. This Trojan horse can never be discovered by reading the source code 
of the login program. 
 
3.4.1.4 Zombie  
A zombie is a program that secretly takes over another Internet-attached computer and


key: chroma:document
string_value:
then uses that computer to launch attacks that are difficult to trace to the zombie's creator. 
Zombies are used in denial-of-service attacks, typically against targeted Web sites. The 
zombie is planted on hundreds of computers belonging to unsuspecting third parties, and 
then used to overwhelm the target Web site by launching an overwhelming onslaught of 
Internet traffic.


key: chroma:document
string_value:
3.4.2 The nature of viruses 
A computer virus is a piece of software that can “infect” other programs by modifying  
hem; the modification includes injecting the original program with a routine to make 
copies of the virus program, which can then go on to infect other programs. The typical


key: full_page_content
string_value:
102 
 
virus becomes embedded in a program on a computer. Then, whenever the 
infected computer comes into contact with an uninfected piece of software, a fresh copy 
of the virus passes into the new program. Thus, the infection can be spread from computer 
to computer by unsuspecting users who either swap disks or send programs to one another 
over a network. During its lifetime, a typical virus goes through the following 
four phases: 
 • Dormant phase: The virus is idle. The virus will eventually be activated by some 
event, such as a date, the presence of another program or file, or the capacity of the disk 
exceeding some limit. Not all viruses have this stage. 
• Propagation phase: The virus places a copy of itself into other programs or into 
certain 
system 
areas 
on 
the 
disk. The copy 
may 
not 
be 
identical 
to 
the propagating version; viruses often morph to evade detection. Each infected program 
will now contain a clone of the virus, which will itself enter a propagation phase. 
• Triggering phase: The virus is activated to perform the function for which it was 
intended. As with the dormant phase, the triggering phase can be caused  by a variety of 
system events, including a count of the number of times that   this copy of the virus has 
made copies of itself. 
 • Execution phase: The function is performed. The function may be harmless, such as a m
essage on the screen, or damaging, such as the destruction of programs and data files. 
3.4.2.1 virus  Structure                                                                                   
 A virus can be prepended or postpended to an executable program, or it can be 
embedded 
in 
some 
other 
fashion. The key 
to 
its 
operation 
is 
that the infected program, when invoked, will first execute the virus code and then 
execute the original code of the program. 
A very general depiction of virus structure is shown in Figure3.5  In this case, the virus c
ode, is prepended to infected programs, and it 
is assumed that the entry point to the program, when invoked, is the first line of the 
program. 
The infected program begins with the virus code and works as follows. The first 
line of code is a jump to the main virus program. The second line is a special marker 
that is used by the virus to determine whether or not a potential victim program has 
already been infected with this virus. When the program is invoked, control is imme- 
diately transferred to the main virus program. The virus program may first seek out uninf
ected 
executable 
files and infect 
them. 
Next, the virus may perform some action, usually detrimental to the system. This action c
ould be performed every time 
the program is invoked, or it could be a logic bomb that triggers only under certain 
conditions. Finally, the virus transfers control to the original program. If the infection


key: chroma:document
string_value:
102 
 
virus becomes embedded in a program on a computer. Then, whenever the 
infected computer comes into contact with an uninfected piece of software, a fresh copy 
of the virus passes into the new program. Thus, the infection can be spread from computer 
to computer by unsuspecting users who either swap disks or send programs to one another


key: chroma:document
string_value:
over a network. During its lifetime, a typical virus goes through the following 
four phases: 
 • Dormant phase: The virus is idle. The virus will eventually be activated by some 
event, such as a date, the presence of another program or file, or the capacity of the disk 
exceeding some limit. Not all viruses have this stage.


key: chroma:document
string_value:
• Propagation phase: The virus places a copy of itself into other programs or into 
certain 
system 
areas 
on 
the 
disk. The copy 
may 
not 
be 
identical 
to 
the propagating version; viruses often morph to evade detection. Each infected program 
will now contain a clone of the virus, which will itself enter a propagation phase.


key: chroma:document
string_value:
• Triggering phase: The virus is activated to perform the function for which it was 
intended. As with the dormant phase, the triggering phase can be caused  by a variety of 
system events, including a count of the number of times that   this copy of the virus has 
made copies of itself. 
 • Execution phase: The function is performed. The function may be harmless, such as a m


key: chroma:document
string_value:
essage on the screen, or damaging, such as the destruction of programs and data files. 
3.4.2.1 virus  Structure                                                                                   
 A virus can be prepended or postpended to an executable program, or it can be 
embedded 
in 
some 
other 
fashion. The key 
to 
its 
operation 
is


key: chroma:document
string_value:
its 
operation 
is 
that the infected program, when invoked, will first execute the virus code and then 
execute the original code of the program. 
A very general depiction of virus structure is shown in Figure3.5  In this case, the virus c
ode, is prepended to infected programs, and it 
is assumed that the entry point to the program, when invoked, is the first line of the


key: chroma:document
string_value:
program. 
The infected program begins with the virus code and works as follows. The first 
line of code is a jump to the main virus program. The second line is a special marker 
that is used by the virus to determine whether or not a potential victim program has 
already been infected with this virus. When the program is invoked, control is imme-


key: chroma:document
string_value:
diately transferred to the main virus program. The virus program may first seek out uninf
ected 
executable 
files and infect 
them. 
Next, the virus may perform some action, usually detrimental to the system. This action c
ould be performed every time 
the program is invoked, or it could be a logic bomb that triggers only under certain


key: full_page_content
string_value:
104 
 
 
Fig : 3.6 A compression Virus 
 
phase of the program is reasonably rapid, a user is unlikely to notice any difference 
between the execution of an infected and an uninfected program. 
A virus such as the one just described is easily detected because an infected version of a 
program 
is 
longer 
than 
the 
corresponding 
uninfected 
one. 
A 
way 
to 
thwart such a simple means of detecting a virus is to compress the executable file so that 
both the infected and uninfected versions are of identical length. Figure 3.6 
shows in general terms the logic required. The key lines in this virus are numbered, and 
Figure 3.7 illustrates the operation. We assume that program P1 is infected with the 
virus CV. When 
this 
program 
is 
invoked, 
control 
passes to its virus, which performs the following steps: 
1.                        For each uninfected file P2 that is found, the virus first compresses that 
file to produce P2 , which is shorter than the original program by the size of the virus. 
2.                        A copy of the virus is prepended to the compressed program. 
3.                        The compressed version of the original infected program, P1¿ , is uncom
pressed. 
4.                        The uncompressed original program is executed. 
In this example, the virus does nothing other than propagate. As in the previous example, 
the virus may  
include a logic bomb.


key: chroma:document
string_value:
104 
 
 
Fig : 3.6 A compression Virus 
 
phase of the program is reasonably rapid, a user is unlikely to notice any difference 
between the execution of an infected and an uninfected program. 
A virus such as the one just described is easily detected because an infected version of a 
program 
is 
longer 
than 
the 
corresponding 
uninfected 
one. 
A 
way 
to


key: chroma:document
string_value:
one. 
A 
way 
to 
thwart such a simple means of detecting a virus is to compress the executable file so that 
both the infected and uninfected versions are of identical length. Figure 3.6 
shows in general terms the logic required. The key lines in this virus are numbered, and 
Figure 3.7 illustrates the operation. We assume that program P1 is infected with the 
virus CV. When


key: chroma:document
string_value:
virus CV. When 
this 
program 
is 
invoked, 
control 
passes to its virus, which performs the following steps: 
1.                        For each uninfected file P2 that is found, the virus first compresses that 
file to produce P2 , which is shorter than the original program by the size of the virus.


key: chroma:document
string_value:
2.                        A copy of the virus is prepended to the compressed program. 
3.                        The compressed version of the original infected program, P1¿ , is uncom
pressed. 
4.                        The uncompressed original program is executed. 
In this example, the virus does nothing other than propagate. As in the previous example, 
the virus may


key: full_page_content
string_value:
105 
 
 
Fig 3.7 A Compression Virus 
In this example, the virus does nothing other than propagate. As previously mentioned, 
the virus may include a logic bomb. 
 Initial Infection 
 
Once a virus has gained entry to a system by infecting a single program, it is in a 
position to infect some or all other executable files on that system when the infected 
program executes. Thus, viral infection can be completely prevented by preventing the 
virus from gaining entry in the first place. Unfortunately, prevention is extraordinarily 
difficult because a virus can be part of any program outside a system.  
3.4.3 Types of Viruses 
The following the most significant types of viruses: 
● Parasitic virus: The traditional and still most common form of virus. A parasitic virus 
attaches itself to executable files and replicates, when the infected program is executed, 
by finding other executable files to infect. 
● Memory-resident virus: Lodges in main memory as part of a resident system program. 
From that point on, the virus infects every program that executes. 
● Boot sector virus: Infects a master boot record or boot record and spreads when a 
system is booted from the disk containing the virus. 
● Stealth virus: A form of virus explicitly designed to hide itself from detection by 
antivirus  
software. 
● Polymorphic virus: A virus that mutates with every infection, making detection by 
the  
"signature" of the virus impossible. 
● Metamorphic virus: As with a polymorphic virus, a metamorphic virus mutates with 
every infection.


key: chroma:document
string_value:
105 
 
 
Fig 3.7 A Compression Virus 
In this example, the virus does nothing other than propagate. As previously mentioned, 
the virus may include a logic bomb. 
 Initial Infection 
 
Once a virus has gained entry to a system by infecting a single program, it is in a 
position to infect some or all other executable files on that system when the infected


key: chroma:document
string_value:
program executes. Thus, viral infection can be completely prevented by preventing the 
virus from gaining entry in the first place. Unfortunately, prevention is extraordinarily 
difficult because a virus can be part of any program outside a system.  
3.4.3 Types of Viruses 
The following the most significant types of viruses:


key: chroma:document
string_value:
● Parasitic virus: The traditional and still most common form of virus. A parasitic virus 
attaches itself to executable files and replicates, when the infected program is executed, 
by finding other executable files to infect. 
● Memory-resident virus: Lodges in main memory as part of a resident system program. 
From that point on, the virus infects every program that executes.


key: chroma:document
string_value:
● Boot sector virus: Infects a master boot record or boot record and spreads when a 
system is booted from the disk containing the virus. 
● Stealth virus: A form of virus explicitly designed to hide itself from detection by 
antivirus  
software. 
● Polymorphic virus: A virus that mutates with every infection, making detection by 
the  
"signature" of the virus impossible.


key: chroma:document
string_value:
● Metamorphic virus: As with a polymorphic virus, a metamorphic virus mutates with 
every infection.


key: full_page_content
string_value:
106 
 
 
One example of a stealth virus was discussed earlier: a virus that uses compression 
so that the infected program is exactly the same length as an uninfected version. Far more 
sophisticated techniques are possible. For example, a virus can place intercept logic in 
disk I/O routines, so that when there is an attempt to read suspected portions of the disk 
using these routines, the virus will present back the original, uninfected program.  
A polymorphic virus creates copies during replication that are functionally equivalent 
but have distinctly different bit patterns. As with a stealth virus, the purpose is to defeat 
programs that scan for viruses. A more effective approach is to use encryption. A portion 
of the virus, generally called a mutation engine, creates a random encryption key to 
encrypt the remainder of the virus. The key is stored with the virus, and the mutation 
engine itself is altered.  
3.4.4 Macro Viruses 
Macro viruses are particularly threatening for a number of reasons: 
1. A macro virus is platform independent. Virtually all of the macro viruses infect 
Microsoft Word documents. Any hardware platform and operating system that supports 
Word can be infected. 
2. Macro viruses infect documents, not executable portions of code. Most of the 
information  
introduced onto a computer system is in the form of a document rather than a program. 
 3. Macro viruses are easily spread. A very common method is by electronic mail. 
 
Macro viruses take advantage of a feature found in Word and other office applications 
such as Microsoft Excel, namely the macro. In essence, a macro is an executable program 
embedded in a word processing document or other type of file. Typically, users employ 
macros to automate repetitive tasks and thereby save keystrokes.  
 
Successive releases of Word provide increased protection against macro viruses. For 
example, Microsoft offers an optional Macro Virus Protection tool that detects suspicious 
Word files and alerts the customer to the potential risk of opening a file with macros.  
3.4.5 E-mail Viruses 
 
A more recent development in malicious software is the e-mail virus. The first rapidly 
spreading e-mail viruses, such as Melissa, made use of a Microsoft Word macro 
embedded in an attachment. If the recipient opens the e-mail attachment, the Word macro 
is activated. Then 
1. The e-mail virus sends itself to everyone on the mailing list in the user's e-mail package. 
2. The virus does local damage. 
 
Thus we see a new generation of malware that arrives via e-mail and uses e-mail 
software features to replicate itself across the Internet. The virus propagates itself as soon 
as activated (either by opening an e-mail attachment of by opening the e-mail) to all of 
the e-mail addresses known to the infected host. As a result, whereas viruses used to take 
months or years to propagate, they now do so in hours.  
3.4.6 Worms


key: chroma:document
string_value:
106 
 
 
One example of a stealth virus was discussed earlier: a virus that uses compression 
so that the infected program is exactly the same length as an uninfected version. Far more 
sophisticated techniques are possible. For example, a virus can place intercept logic in 
disk I/O routines, so that when there is an attempt to read suspected portions of the disk


key: chroma:document
string_value:
using these routines, the virus will present back the original, uninfected program.  
A polymorphic virus creates copies during replication that are functionally equivalent 
but have distinctly different bit patterns. As with a stealth virus, the purpose is to defeat 
programs that scan for viruses. A more effective approach is to use encryption. A portion


key: chroma:document
string_value:
of the virus, generally called a mutation engine, creates a random encryption key to 
encrypt the remainder of the virus. The key is stored with the virus, and the mutation 
engine itself is altered.  
3.4.4 Macro Viruses 
Macro viruses are particularly threatening for a number of reasons: 
1. A macro virus is platform independent. Virtually all of the macro viruses infect


key: chroma:document
string_value:
Microsoft Word documents. Any hardware platform and operating system that supports 
Word can be infected. 
2. Macro viruses infect documents, not executable portions of code. Most of the 
information  
introduced onto a computer system is in the form of a document rather than a program. 
 3. Macro viruses are easily spread. A very common method is by electronic mail.


key: chroma:document
string_value:
Macro viruses take advantage of a feature found in Word and other office applications 
such as Microsoft Excel, namely the macro. In essence, a macro is an executable program 
embedded in a word processing document or other type of file. Typically, users employ 
macros to automate repetitive tasks and thereby save keystrokes.


key: chroma:document
string_value:
Successive releases of Word provide increased protection against macro viruses. For 
example, Microsoft offers an optional Macro Virus Protection tool that detects suspicious 
Word files and alerts the customer to the potential risk of opening a file with macros.  
3.4.5 E-mail Viruses 
 
A more recent development in malicious software is the e-mail virus. The first rapidly


key: chroma:document
string_value:
spreading e-mail viruses, such as Melissa, made use of a Microsoft Word macro 
embedded in an attachment. If the recipient opens the e-mail attachment, the Word macro 
is activated. Then 
1. The e-mail virus sends itself to everyone on the mailing list in the user's e-mail package. 
2. The virus does local damage.


key: chroma:document
string_value:
Thus we see a new generation of malware that arrives via e-mail and uses e-mail 
software features to replicate itself across the Internet. The virus propagates itself as soon 
as activated (either by opening an e-mail attachment of by opening the e-mail) to all of 
the e-mail addresses known to the infected host. As a result, whereas viruses used to take


key: full_page_content
string_value:
107 
 
 
A worm is a program that can replicate itself and send copies from computer to 
computer across network connections. Upon arrival, the worm may be activated to 
replicate and propagate again. In addition to propagation, the worm usually performs 
some unwanted function. An e-mail virus has some of the characteristics of a worm, 
because it propagates itself from system to system.  
 
Network worm programs use network connections to spread from system to system. 
Once active within a system, a network worm can behave as a computer virus or bacteria, 
or it could implant Trojan horse programs or perform any number of disruptive or 
destructive actions. 
To replicate itself, a network worm uses some sort of network vehicle. Examples include 
the following: 
• Electronic mail facility: A worm mails a copy of itself to other systems. 
● Remote execution capability: A worm executes a copy of itself on another 
system. 
● Remote login capability: A worm logs onto a remote system as a user and then 
uses  
commands to copy itself from one system to the other. 
3.4.6.1 The Morris Worm 
 
The Morris worm was designed to spread on UNIX systems and used a number of 
different techniques for propagation. When a copy began execution, its first task was to 
discover other hosts known to this host that would allow entry from this host. The worm 
performed this task by examining a variety of lists and tables, including system tables 
that declared which other machines were trusted by this host, users' mail forwarding files, 
tables by which users gave themselves permission for access to remote accounts, and 
from a program that reported the status of network connections. For each discovered host, 
the worm tried a number of methods for gaining access: 
1. It attempted to log on to a remote host as a legitimate user. In this method, the worm 
first attempted to crack the local password file, and then used the discovered passwords 
and corresponding user IDs. The assumption was that many users would use the same 
password on different systems. To obtain the passwords, the worm ran a password-
cracking program that tried 
a. Each user's account name and simple permutations of it 
b.  All the words in the local system directory  
2. It exploited a bug in the finger protocol, which reports the whereabouts of a remote 
user. 
 3. It exploited a trapdoor in the debug option of the remote process that receives and 
sends mail.  
 3.4.6.2 Recent Worm Attacks 
    
Code Red exploits a security hole in the Microsoft Internet Information Server 
(IIS) to penetrate and spread. It also disables the system file checker in Windows. The 
worm probes random IP addresses to spread to other hosts. During a certain period of


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107 
 
 
A worm is a program that can replicate itself and send copies from computer to 
computer across network connections. Upon arrival, the worm may be activated to 
replicate and propagate again. In addition to propagation, the worm usually performs 
some unwanted function. An e-mail virus has some of the characteristics of a worm,


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because it propagates itself from system to system.  
 
Network worm programs use network connections to spread from system to system. 
Once active within a system, a network worm can behave as a computer virus or bacteria, 
or it could implant Trojan horse programs or perform any number of disruptive or 
destructive actions.


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To replicate itself, a network worm uses some sort of network vehicle. Examples include 
the following: 
• Electronic mail facility: A worm mails a copy of itself to other systems. 
● Remote execution capability: A worm executes a copy of itself on another 
system. 
● Remote login capability: A worm logs onto a remote system as a user and then 
uses


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uses  
commands to copy itself from one system to the other. 
3.4.6.1 The Morris Worm 
 
The Morris worm was designed to spread on UNIX systems and used a number of 
different techniques for propagation. When a copy began execution, its first task was to 
discover other hosts known to this host that would allow entry from this host. The worm


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performed this task by examining a variety of lists and tables, including system tables 
that declared which other machines were trusted by this host, users' mail forwarding files, 
tables by which users gave themselves permission for access to remote accounts, and 
from a program that reported the status of network connections. For each discovered host,


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the worm tried a number of methods for gaining access: 
1. It attempted to log on to a remote host as a legitimate user. In this method, the worm 
first attempted to crack the local password file, and then used the discovered passwords 
and corresponding user IDs. The assumption was that many users would use the same


key: chroma:document
string_value:
password on different systems. To obtain the passwords, the worm ran a password-
cracking program that tried 
a. Each user's account name and simple permutations of it 
b.  All the words in the local system directory  
2. It exploited a bug in the finger protocol, which reports the whereabouts of a remote 
user.


key: chroma:document
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user. 
 3. It exploited a trapdoor in the debug option of the remote process that receives and 
sends mail.  
 3.4.6.2 Recent Worm Attacks 
    
Code Red exploits a security hole in the Microsoft Internet Information Server 
(IIS) to penetrate and spread. It also disables the system file checker in Windows. The


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108 
 
time, it only spreads. It then initiates a denial-of-service attack against a government Web 
site by flooding the site with packets from numerous hosts.  
  
The worm then suspends activities and reactivates periodically. In the second 
wave of attack, Code Red infected nearly 360,000 servers in 14 hours. In addition to the 
havoc it causes at the targeted server, Code Red can consume enormous amounts of 
Internet capacity, disrupting service. 
 In late 2001, a more versatile worm appeared, known as Nimda. Nimda spreads by 
multiple  
mechanisms: 
● from client to client via e-mail 
● from client to client via open network shares from Web server to client via 
browsing of compromised Web sites 
● from client to Web server via active scanning for and exploitation of various 
Microsoft IIS 4.0 / 5.0 directory traversal vulnerabilities 
● from client to Web server via scanning for the back doors left behind by the "Code 
Red II" worms The worm modifies Web documents (e.g., .htm, .html, and .asp files) and 
certain executable files found on the systems it infects and creates numerous copies of 
itself under various filenames. 
3.4.7 State of Worm Technology 
     The state of the art in worm technology includes the following: 
● Multiplatform: Newer worms are not limited to Windows machines but can attack a variety 
of  
platforms, especially the popular varieties of UNIX. 
● Multiexploit: New worms penetrate systems in a variety of ways, using exploits against 
Web  
servers, browsers, e-mail, file sharing, and other network-based applications. 
● Ultrafast spreading: One technique to accelerate the spread of a worm is to conduct a prior  
Internet scan to accumulate Internet addresses of vulnerable machines. 
● Polymorphic: To evade detection, skip past filters, and foil real-time analysis, worms adopt 
the  
virus polymorphic technique. Each copy of the worm has new code generated on the fly using  
functionally equivalent instructions and encryption techniques. 
● Metamorphic: In addition to changing their appearance, metamorphic worms have a 
repertoire  
of behavior patterns that are unleashed at different stages of propagation. 
● Transport vehicles: Because worms can rapidly compromise a large number of systems, 
they  
are ideal for spreading other distributed attack tools, such as distributed denial of service  
zombies. 
● Zero-day exploit: To achieve maximum surprise and distribution, a worm should exploit 
an  
unknown vulnerability that is only discovered by the general network community when the 
worm  
is launched. 
3.5 Virus Countermeasures


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108 
 
time, it only spreads. It then initiates a denial-of-service attack against a government Web 
site by flooding the site with packets from numerous hosts.  
  
The worm then suspends activities and reactivates periodically. In the second 
wave of attack, Code Red infected nearly 360,000 servers in 14 hours. In addition to the


key: chroma:document
string_value:
havoc it causes at the targeted server, Code Red can consume enormous amounts of 
Internet capacity, disrupting service. 
 In late 2001, a more versatile worm appeared, known as Nimda. Nimda spreads by 
multiple  
mechanisms: 
● from client to client via e-mail 
● from client to client via open network shares from Web server to client via 
browsing of compromised Web sites


key: chroma:document
string_value:
● from client to Web server via active scanning for and exploitation of various 
Microsoft IIS 4.0 / 5.0 directory traversal vulnerabilities 
● from client to Web server via scanning for the back doors left behind by the "Code 
Red II" worms The worm modifies Web documents (e.g., .htm, .html, and .asp files) and


key: chroma:document
string_value:
certain executable files found on the systems it infects and creates numerous copies of 
itself under various filenames. 
3.4.7 State of Worm Technology 
     The state of the art in worm technology includes the following: 
● Multiplatform: Newer worms are not limited to Windows machines but can attack a variety 
of  
platforms, especially the popular varieties of UNIX.


key: chroma:document
string_value:
● Multiexploit: New worms penetrate systems in a variety of ways, using exploits against 
Web  
servers, browsers, e-mail, file sharing, and other network-based applications. 
● Ultrafast spreading: One technique to accelerate the spread of a worm is to conduct a prior  
Internet scan to accumulate Internet addresses of vulnerable machines.


key: chroma:document
string_value:
● Polymorphic: To evade detection, skip past filters, and foil real-time analysis, worms adopt 
the  
virus polymorphic technique. Each copy of the worm has new code generated on the fly using  
functionally equivalent instructions and encryption techniques. 
● Metamorphic: In addition to changing their appearance, metamorphic worms have a 
repertoire


key: chroma:document
string_value:
repertoire  
of behavior patterns that are unleashed at different stages of propagation. 
● Transport vehicles: Because worms can rapidly compromise a large number of systems, 
they  
are ideal for spreading other distributed attack tools, such as distributed denial of service  
zombies. 
● Zero-day exploit: To achieve maximum surprise and distribution, a worm should exploit 
an


key: chroma:document
string_value:
an  
unknown vulnerability that is only discovered by the general network community when the 
worm  
is launched. 
3.5 Virus Countermeasures


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109 
 
3.5.1 Antivirus Approaches 
The ideal solution to the threat of viruses is prevention: Do not allow a virus to get into the 
systemin  
the first place. This goal is, in general, impossible to achieve, although prevention can reduce 
the  
number of successful viral attacks. The next best approach is to be able to do the following: 
● Detection: Once the infection has occurred, determine that it has occurred and locate the 
virus. 
● Identification: Once detection has been achieved, identify the specific virus that has 
infected a  
program. 
● Removal: Once the specific virus has been identified, remove all traces of the virus from 
the  
infected program and restore it to its original state. Remove the virus from all infected 
systems  
so that the disease cannot spread further. 
Four generations of antivirus software: 
● First generation: simple scanners 
● Second generation: heuristic scanners 
● Third generation: activity traps 
● Fourth generation: full-featured protection 
A first-generation scanner requires a virus signature to identify a virus. The virus may 
contain  
"wildcards" but has essentially the same structure and bit pattern in all copies. Such 
signature-specific scanners are limited to the detection of known viruses. Another type 
of first-generation scanner maintains a record of the length of programs and looks for 
changes in length. 
A second-generation scanner does not rely on a specific signature. Rather, the 
scanner uses heuristic rules to search for probable virus infection. One class of such 
scanners looks for fragments of code that are often associated with viruses. For example, 
a scanner may look for the beginning of an encryption loop used in a polymorphic virus 
and discover the encryption key. Once the key is discovered, the scanner can decrypt the 
virus to identify it, then remove the infection and return the program to service. 
Third-generation programs are memory-resident programs that identify a virus by 
its actions rather than its structure in an infected program. Such programs have the 
advantage that it is not necessary to develop signatures and heuristics for a wide array of 
viruses. Rather, it is necessary only to identify the small set of actions that indicate an 
infection is being attempted and then to intervene. 
 Fourth-generation products are packages consisting of a variety of antivirus 
techniques used in conjunction. These include scanning and activity trap components. In 
addition, such a package includes access control capability, which limits the ability of 
viruses to penetrate a system and then limits the ability of a virus to update files in order 
to pass on the infection.


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109 
 
3.5.1 Antivirus Approaches 
The ideal solution to the threat of viruses is prevention: Do not allow a virus to get into the 
systemin  
the first place. This goal is, in general, impossible to achieve, although prevention can reduce 
the  
number of successful viral attacks. The next best approach is to be able to do the following:


key: chroma:document
string_value:
● Detection: Once the infection has occurred, determine that it has occurred and locate the 
virus. 
● Identification: Once detection has been achieved, identify the specific virus that has 
infected a  
program. 
● Removal: Once the specific virus has been identified, remove all traces of the virus from 
the


key: chroma:document
string_value:
the  
infected program and restore it to its original state. Remove the virus from all infected 
systems  
so that the disease cannot spread further. 
Four generations of antivirus software: 
● First generation: simple scanners 
● Second generation: heuristic scanners 
● Third generation: activity traps 
● Fourth generation: full-featured protection


key: chroma:document
string_value:
A first-generation scanner requires a virus signature to identify a virus. The virus may 
contain  
"wildcards" but has essentially the same structure and bit pattern in all copies. Such 
signature-specific scanners are limited to the detection of known viruses. Another type 
of first-generation scanner maintains a record of the length of programs and looks for


key: chroma:document
string_value:
changes in length. 
A second-generation scanner does not rely on a specific signature. Rather, the 
scanner uses heuristic rules to search for probable virus infection. One class of such 
scanners looks for fragments of code that are often associated with viruses. For example, 
a scanner may look for the beginning of an encryption loop used in a polymorphic virus


key: chroma:document
string_value:
and discover the encryption key. Once the key is discovered, the scanner can decrypt the 
virus to identify it, then remove the infection and return the program to service. 
Third-generation programs are memory-resident programs that identify a virus by 
its actions rather than its structure in an infected program. Such programs have the


key: chroma:document
string_value:
advantage that it is not necessary to develop signatures and heuristics for a wide array of 
viruses. Rather, it is necessary only to identify the small set of actions that indicate an 
infection is being attempted and then to intervene. 
 Fourth-generation products are packages consisting of a variety of antivirus


key: chroma:document
string_value:
techniques used in conjunction. These include scanning and activity trap components. In 
addition, such a package includes access control capability, which limits the ability of 
viruses to penetrate a system and then limits the ability of a virus to update files in order 
to pass on the infection.


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string_value:
110 
 
3.5.2 Advanced Antivirus Techniques 
More sophisticated antivirus approaches and products continue to appear.  
Generic Decryption 
  Generic decryption (GD) technology enables the antivirus program to easily detect even 
the most complex polymorphic viruses, while maintaining fast scanning speeds 
[NACH97]. Recall that when a file containing a polymorphic virus is executed, the virus 
must decrypt itself to activate. In order to detect such a structure, executable files are run 
through a GD scanner, which contains the following elements: 
● CPU emulator: A software-based virtual computer. Instructions in an executable 
file are interpreted by the emulator rather than executed on the underlying processor. The 
emulator  
includes software versions of all registers and other processor hardware, so that the 
underlying processor is unaffected by programs interpreted on the emulator. 
● Virus signature scanner: A module that scans the target code looking for known 
virus  
signatures. 
● Emulation control module: Controls the execution of the target code 
Digital Immune System 
The digital immune system is a comprehensive approach to virus protection 
developed by IBM. The motivation for this development has been the rising threat of 
Internet-based virus propagation. Two major trends in Internet technology have had an 
increasing impact on the rate of virus propagation in recent years: 
• 
Integrated mail systems: Systems such as Lotus Notes and Microsoft Outlook 
make it very simple to send anything to anyone and to work with objects that are received. 
● Mobile-program systems: Capabilities such as Java and ActiveX allow programs 
to move on their own from one system to another. 
In response to the threat posed by these Internet-based capabilities, IBM has 
developed a prototype digital immune system.The objective of this system is to provide 
rapid response time so that viruses can be stamped out almost as soon as they are 
introduced. When a new virus enters an organization, the immune system automatically 
captures it, analyzes it, adds detection and shielding for it, removes it, and passes 
information about that virus to systems running IBM AntiVirus so that it can be detected 
before it is allowed to run elsewhere. Figure 3.8 illustrates the typical steps in digital 
immune system operation:


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110 
 
3.5.2 Advanced Antivirus Techniques 
More sophisticated antivirus approaches and products continue to appear.  
Generic Decryption 
  Generic decryption (GD) technology enables the antivirus program to easily detect even 
the most complex polymorphic viruses, while maintaining fast scanning speeds


key: chroma:document
string_value:
[NACH97]. Recall that when a file containing a polymorphic virus is executed, the virus 
must decrypt itself to activate. In order to detect such a structure, executable files are run 
through a GD scanner, which contains the following elements: 
● CPU emulator: A software-based virtual computer. Instructions in an executable


key: chroma:document
string_value:
file are interpreted by the emulator rather than executed on the underlying processor. The 
emulator  
includes software versions of all registers and other processor hardware, so that the 
underlying processor is unaffected by programs interpreted on the emulator. 
● Virus signature scanner: A module that scans the target code looking for known 
virus  
signatures.


key: chroma:document
string_value:
signatures. 
● Emulation control module: Controls the execution of the target code 
Digital Immune System 
The digital immune system is a comprehensive approach to virus protection 
developed by IBM. The motivation for this development has been the rising threat of 
Internet-based virus propagation. Two major trends in Internet technology have had an


key: chroma:document
string_value:
increasing impact on the rate of virus propagation in recent years: 
• 
Integrated mail systems: Systems such as Lotus Notes and Microsoft Outlook 
make it very simple to send anything to anyone and to work with objects that are received. 
● Mobile-program systems: Capabilities such as Java and ActiveX allow programs 
to move on their own from one system to another.


key: chroma:document
string_value:
In response to the threat posed by these Internet-based capabilities, IBM has 
developed a prototype digital immune system.The objective of this system is to provide 
rapid response time so that viruses can be stamped out almost as soon as they are 
introduced. When a new virus enters an organization, the immune system automatically


key: chroma:document
string_value:
captures it, analyzes it, adds detection and shielding for it, removes it, and passes 
information about that virus to systems running IBM AntiVirus so that it can be detected 
before it is allowed to run elsewhere. Figure 3.8 illustrates the typical steps in digital 
immune system operation:


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111 
 
 
Figure 3.8. Digital Immune System 
Fig 3.8 illustrates the typical steps in digital immune system operation: 
1. A monitoring program on each PC uses a variety of heuristics based on system 
behavior, suspicious changes to programs, or family signature to infer that a virus may 
be present.  
2. The administrative machine encrypts the sample and sends it to a central virus 
analysis machine.  
3. This machine creates an environment in which the infected program can be safely 
run for analysis. Techniques used for this purpose include emulation, or the creation of a 
protected environment within which the suspect program can be executed and monitored.  
4. The resulting prescription is sent back to the administrative machine.  
5. The administrative machine forwards the prescription to the infected client.  
6. The prescription is also forwarded to other clients in the organization.  
7. Subscribers around the world receive regular antivirus updates that protect them 
from the new virus. 
The success of the digital immune system depends on the ability of the virus analysis 
machine to detect new and innovative virus strains. 
 
3.5.3 Behavior-Blocking Software 
Behavior-blocking software integrates with the operating system of a host computer 
and monitors program behavior in real-time for malicious actions. The behavior blocking 
software then blocks potentially malicious actions before they have a chance to affect the 
system. Monitored behaviors can include the following: 
● Attempts to open, view, delete, and/or modify files; 
● Attempts to format disk drives and other unrecoverable disk operations; 
● Modifications to the logic of executable files or macros; 
● Modification of critical system settings, such as start-up settings; 
● Scripting of e-mail and instant messaging clients to send executable content; and 
● Initiation of network communications.


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111 
 
 
Figure 3.8. Digital Immune System 
Fig 3.8 illustrates the typical steps in digital immune system operation: 
1. A monitoring program on each PC uses a variety of heuristics based on system 
behavior, suspicious changes to programs, or family signature to infer that a virus may 
be present.


key: chroma:document
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be present.  
2. The administrative machine encrypts the sample and sends it to a central virus 
analysis machine.  
3. This machine creates an environment in which the infected program can be safely 
run for analysis. Techniques used for this purpose include emulation, or the creation of a 
protected environment within which the suspect program can be executed and monitored.


key: chroma:document
string_value:
4. The resulting prescription is sent back to the administrative machine.  
5. The administrative machine forwards the prescription to the infected client.  
6. The prescription is also forwarded to other clients in the organization.  
7. Subscribers around the world receive regular antivirus updates that protect them 
from the new virus.


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The success of the digital immune system depends on the ability of the virus analysis 
machine to detect new and innovative virus strains. 
 
3.5.3 Behavior-Blocking Software 
Behavior-blocking software integrates with the operating system of a host computer 
and monitors program behavior in real-time for malicious actions. The behavior blocking


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software then blocks potentially malicious actions before they have a chance to affect the 
system. Monitored behaviors can include the following: 
● Attempts to open, view, delete, and/or modify files; 
● Attempts to format disk drives and other unrecoverable disk operations; 
● Modifications to the logic of executable files or macros;


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● Modification of critical system settings, such as start-up settings; 
● Scripting of e-mail and instant messaging clients to send executable content; and 
● Initiation of network communications.


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112 
 
If the behavior blocker detects that a program is initiating would-be malicious 
behaviors as it runs, it can block these behaviors in real-time and/or terminate the 
offending software. This gives it a fundamental advantage over such established antivirus 
detection techniques as fingerprinting or heuristics.  
The ability to watch software as it runs in real time clearly confers a huge benefit to 
the behavior blocker; however, it also has drawbacks. Since the malicious code must 
actually run on the target machine before all its behaviors can be identified, it can cause 
a great deal of harm to the system before it has been detected and blocked by the behavior 
blocking system. For instance, a new virus might shuffle a number of seemingly 
unimportant files around the hard drive before infecting a single file and being blocked.  
3.6 Distributed Denial of Service Attacks 
 
Distributed denial of service (DDoS) attacks present a significant security threat to 
corporations, and the threat appears to be growing . In one study, covering a three-week 
period in 2001, investigators observed more than 12,000 attacks against more than 5000 
distinct targets, ranging from well-known ecommerce companies such as Amazon and 
Hotmail to small foreign ISPs and dial-up connections. DDoS attacks make computer 
systems inaccessible by flooding servers, networks, or even end user systems with useless 
traffic so that legitimate users can no longer gain access to those resources.  
 
 A denial of service (DoS) attack is an attempt to prevent legitimate users of a service 
from using that service. When this attack comes from a single host or network node, then 
it is simply referred to as a DoS attack. A more serious threat is posed by a DDoS attack. 
In a DDoS attack, an attacker is able to recruit a number of hosts throughout the Internet 
to simultaneously or in a coordinated fashion launch an attack upon the target. This 
section is concerned with DDoS attacks.  
3.6.1 DDoS Attack Description 
 
A DDoS attack attempts to consume the target's resources so that it cannot provide 
service. One way to classify DDoS attacks is in terms of the type of resource that is 
consumed. Broadly speaking, the resource consumed is either an internal host resource 
on the target system or data transmission capacity in the local network to which the target 
is attacked. 
A simple example of an internal resource attack is the SYN flood attack. Figure 19.5a 
shows the steps involved: 
1. The attacker takes control of multiple hosts over the Internet, instructing them to 
contact the target Web server.  
2. The slave hosts begin sending TCP/IP SYN (synchronize/initialization) packets, with 
erroneous return IP address information, to the target. 
3. Each SYN packet is a request to open a TCP connection. For each such packet, the 
Web server responds with a SYN/ACK (synchronize/acknowledge) packet, trying to 
establish a TCP connection with a TCP entity at a spurious IP address. The Web server 
maintains a data structure for each SYN request waiting for a response back and becomes 
bogged down as more traffic floods in. The result is that legitimate connections are denied 
while the victim machine is waiting to complete bogus "half-open" connections.


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112 
 
If the behavior blocker detects that a program is initiating would-be malicious 
behaviors as it runs, it can block these behaviors in real-time and/or terminate the 
offending software. This gives it a fundamental advantage over such established antivirus 
detection techniques as fingerprinting or heuristics.


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The ability to watch software as it runs in real time clearly confers a huge benefit to 
the behavior blocker; however, it also has drawbacks. Since the malicious code must 
actually run on the target machine before all its behaviors can be identified, it can cause 
a great deal of harm to the system before it has been detected and blocked by the behavior


key: chroma:document
string_value:
blocking system. For instance, a new virus might shuffle a number of seemingly 
unimportant files around the hard drive before infecting a single file and being blocked.  
3.6 Distributed Denial of Service Attacks 
 
Distributed denial of service (DDoS) attacks present a significant security threat to


key: chroma:document
string_value:
corporations, and the threat appears to be growing . In one study, covering a three-week 
period in 2001, investigators observed more than 12,000 attacks against more than 5000 
distinct targets, ranging from well-known ecommerce companies such as Amazon and 
Hotmail to small foreign ISPs and dial-up connections. DDoS attacks make computer


key: chroma:document
string_value:
systems inaccessible by flooding servers, networks, or even end user systems with useless 
traffic so that legitimate users can no longer gain access to those resources.  
 
 A denial of service (DoS) attack is an attempt to prevent legitimate users of a service 
from using that service. When this attack comes from a single host or network node, then


key: chroma:document
string_value:
it is simply referred to as a DoS attack. A more serious threat is posed by a DDoS attack. 
In a DDoS attack, an attacker is able to recruit a number of hosts throughout the Internet 
to simultaneously or in a coordinated fashion launch an attack upon the target. This 
section is concerned with DDoS attacks.  
3.6.1 DDoS Attack Description


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string_value:
A DDoS attack attempts to consume the target's resources so that it cannot provide 
service. One way to classify DDoS attacks is in terms of the type of resource that is 
consumed. Broadly speaking, the resource consumed is either an internal host resource 
on the target system or data transmission capacity in the local network to which the target 
is attacked.


