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VPN Gate Overview

Virtual Hub Administration Login Ultimately, such government's overprotection will cause decline in public interests, as most people in such country are kept away from the worldwide, valuable web services. Shamir and van Someren "Playing hide and seek with stored keys" have noted that a new generation of viruses can be written that will find files encrypted with long keys, making them easier to find by intruders and, therefore, more prone to attack. Many people want to sell their new algorithm and, therefore, don't want to expose the scheme to the public for fear that their idea will be stolen. In July , 14 esteemed cryptographers and computer scientists released a paper continuing the debate around cryptography and privacy. How does the recipient know that the sender is using their public key for a legitimate purpose for which they are authorized? Similarly, a bit key would require bits of key material x13 , or 52 bit words, while a bit key would require bits of key material x15 , or 60 bit words. Central Stadium , Yekaterinburg.

PacketiX.NET VPN Online Test Environment 

FIFA Rules for classification: Group stage tiebreakers H Host. Luzhniki Stadium , Moscow. Central Stadium , Yekaterinburg. Krestovsky Stadium , Saint Petersburg. Rostov Arena , Rostov-on-Don. Cosmos Arena , Samara. Getting an API Link requires you to be able to view the game first, thus you will need one of the three options on this article to initially connect to the game.

To avoid having to clear your browser cache everytime you want to connect to the game, you can use another chrome alike web browser i.

Use this browser only for KanColle and don't forget to connect to a Japanese proxy before opening it. Like this, you won't have to wait for the downloading of the game's data everytime you need to connect to the game.

There are some online kancolle login tools like ooi2 exist on internet that help users login to the game by using their server. The login process include transferring login data, i. Thus it is recommended to use these tools only if you fully trust those platform in case you are using codes made available by third parties to build your own login platform, that mean fully know what those codes are doing. Mobile gameplay is only supported on the official KanColle Android app at the moment. Use of other means to play Kancolle on mobile devices is highly discouraged, will not receive support if things go wrong, and may result in bans.

There are several other references that describe interesting algorithms and even SKC codes dating back decades. Savard's albeit old A Cryptographic Compendium page. Public key cryptography has been said to be the most significant new development in cryptography in the last years. Their paper described a two-key crypto system in which two parties could engage in a secure communication over a non-secure communications channel without having to share a secret key. PKC depends upon the existence of so-called one-way functions , or mathematical functions that are easy to compute whereas their inverse function is relatively difficult to compute.

Let me give you two simple examples:. While the examples above are trivial, they do represent two of the functional pairs that are used with PKC; namely, the ease of multiplication and exponentiation versus the relative difficulty of factoring and calculating logarithms, respectively. The mathematical "trick" in PKC is to find a trap door in the one-way function so that the inverse calculation becomes easy given knowledge of some item of information.

Generic PKC employs two keys that are mathematically related although knowledge of one key does not allow someone to easily determine the other key. One key is used to encrypt the plaintext and the other key is used to decrypt the ciphertext. The important point here is that it does not matter which key is applied first , but that both keys are required for the process to work Figure 1B.

Because a pair of keys are required, this approach is also called asymmetric cryptography. In PKC, one of the keys is designated the public key and may be advertised as widely as the owner wants.

The other key is designated the private key and is never revealed to another party. It is straight forward to send messages under this scheme. Suppose Alice wants to send Bob a message. Alice encrypts some information using Bob's public key; Bob decrypts the ciphertext using his private key.

This method could be also used to prove who sent a message; Alice, for example, could encrypt some plaintext with her private key; when Bob decrypts using Alice's public key, he knows that Alice sent the message authentication and Alice cannot deny having sent the message non-repudiation.

Public key cryptography algorithms that are in use today for key exchange or digital signatures include:. RSA today is used in hundreds of software products and can be used for key exchange, digital signatures, or encryption of small blocks of data.

RSA uses a variable size encryption block and a variable size key. The key-pair is derived from a very large number, n , that is the product of two prime numbers chosen according to special rules; these primes may be or more digits in length each, yielding an n with roughly twice as many digits as the prime factors. The public key information includes n and a derivative of one of the factors of n ; an attacker cannot determine the prime factors of n and, therefore, the private key from this information alone and that is what makes the RSA algorithm so secure.

Some descriptions of PKC erroneously state that RSA's safety is due to the difficulty in factoring large prime numbers. In fact, large prime numbers, like small prime numbers, only have two factors!

The ability for computers to factor large numbers, and therefore attack schemes such as RSA, is rapidly improving and systems today can find the prime factors of numbers with more than digits. Nevertheless, if a large number is created from two prime factors that are roughly the same size, there is no known factorization algorithm that will solve the problem in a reasonable amount of time; a test to factor a digit number took 1.

Regardless, one presumed protection of RSA is that users can easily increase the key size to always stay ahead of the computer processing curve. As an aside, the patent for RSA expired in September which does not appear to have affected RSA's popularity one way or the other. A detailed example of RSA is presented below in Section 5. D-H is used for secret-key key exchange only, and not for authentication or digital signatures.

More detail about Diffie-Hellman can be found below in Section 5. Described in FIPS A PKC algorithm based upon elliptic curves. More detail about ECC can be found below in Section 5. Incorporated into PKCS 1. Extended-Certificate Syntax Standard being phased out in favor of X.

A public key cryptosystem proposed by R. Shoup of IBM in A public key cryptosystem designed by P. Smith and based on Lucas sequences.

Can be used for encryption and signatures, using integer factoring. A public key cryptosystem based on algebraic coding theory. Vanstone CRC Press, I tried to be careful in the first paragraph of this section to state that Diffie and Hellman "first described publicly" a PKC scheme.

Although I have categorized PKC as a two-key system, that has been merely for convenience; the real criteria for a PKC scheme is that it allows two parties to exchange a secret even though the communication with the shared secret might be overheard. As shown in Section 5. And, indeed, it is the precursor to modern PKC which does employ two keys. Their method, of course, is based upon the relative ease of finding the product of two large prime numbers compared to finding the prime factors of a large number.

Diffie and Hellman and other sources credit Ralph Merkle with first describing a public key distribution system that allows two parties to share a secret, although it was not a two-key system, per se.

A Merkle Puzzle works where Alice creates a large number of encrypted keys, sends them all to Bob so that Bob chooses one at random and then lets Alice know which he has selected. An eavesdropper Eve will see all of the keys but can't learn which key Bob has selected because he has encrypted the response with the chosen key. In this case, Eve's effort to break in is the square of the effort of Bob to choose a key. While this difference may be small it is often sufficient. Merkle apparently took a computer science course at UC Berkeley in and described his method, but had difficulty making people understand it; frustrated, he dropped the course.

Merkle's method certainly wasn't published first, but he is often credited to have had the idea first. An interesting question, maybe, but who really knows? Because of the nature of the work, GCHQ kept the original memos classified. In , however, the GCHQ changed their posture when they realized that there was nothing to gain by continued silence. Documents show that a GCHQ mathematician named James Ellis started research into the key distribution problem in and that by , James Ellis, Clifford Cocks, and Malcolm Williamson had worked out all of the fundamental details of PKC, yet couldn't talk about their work.

They were, of course, barred from challenging the RSA patent! Hash functions, also called message digests and one-way encryption , are algorithms that, in essence, use no key Figure 1C.

Instead, a fixed-length hash value is computed based upon the plaintext that makes it impossible for either the contents or length of the plaintext to be recovered. Hash algorithms are typically used to provide a digital fingerprint of a file's contents, often used to ensure that the file has not been altered by an intruder or virus.

Hash functions are also commonly employed by many operating systems to encrypt passwords. Hash functions, then, provide a mechansim to ensure the integrity of a file.

This is an important distinction. Suppose that you want to crack someone's password, where the hash of the password is stored on the server. Indeed, all you then need is a string that produces the correct hash and you're in! However, you cannot prove that you have discovered the user's password, only a "duplicate key. Message Digest MD algorithms: A series of byte-oriented algorithms that produce a bit hash value from an arbitrary-length message.

Designed for systems with limited memory, such as smart cards. MD2 has been relegated to historical status, per RFC Developed by Rivest, similar to MD2 but designed specifically for fast processing in software. MD4 has been relegated to historical status, per RFC Also developed by Rivest after potential weaknesses were reported in MD4; this scheme is similar to MD4 but is slower because more manipulation is made to the original data.

MD5 has been implemented in a large number of products although several weaknesses in the algorithm were demonstrated by German cryptographer Hans Dobbertin in "Cryptanalysis of MD5 Compress". In , NIST announced that after reviewing 64 submissions, the winner was Keccak pronounced "catch-ack" , a family of hash algorithms based on sponge functions.

The NIST version can support hash output sizes of and bits. Seberry, a hash algorithm with many levels of security. HAVAL can create hash values that are , , , , or bits in length. Rijmen co-inventor of Rijndael and P. Whirlpool operates on messages less than 2 bits in length and produces a message digest of bits. The design of this hash function is very different than that of MD5 and SHA-1, making it immune to the same attacks as on those hashes. A root hash is used on peer-to-peer file transfer networks, where a file is broken into chunks; each chunk has its own MD4 hash associated with it and the server maintains a file that contains the hash list of all of the chunks.

The root hash is the hash of the hash list file. A digression on hash collisions. Hash functions are sometimes misunderstood and some sources claim that no two files can have the same hash value. This is in theory, if not in fact, incorrect. Consider a hash function that provides a bit hash value.

There are, then, 2 possible hash values. Now, while even this is theoretically correct, it is not true in practice because hash algorithms are designed to work with a limited message size, as mentioned above. Nevertheless, hopefully you get my point. The difficulty is not necessarily in finding two files with the same hash, but in finding a second file that has the same hash value as a given first file.

Since there are more than 7 billion people on earth, we know that there are a lot of people with the same number of hairs on their head.

Finding two people with the same number of hairs, then, would be relatively simple. The harder problem is choosing one person say, you, the reader and then finding another person who has the same number of hairs on their head as you have on yours. This is somewhat similar to the Birthday Problem. Alas, researchers in found that practical collision attacks could be launched on MD5, SHA-1, and other hash algorithms. Readers interested in this problem should read the following:.

For historical purposes, take a look at the situation with hash collisions, circa , in RFC In October , the SHA-1 Freestart Collision was announced; see a report by Bruce Schneier and the developers of the attack as well as the paper above by Stevens et al. See also the paper by Stevens et al. Finally, note that certain extensions of hash functions are used for a variety of information security and digital forensics applications, such as:. So, why are there so many different types of cryptographic schemes?

Why can't we do everything we need with just one? The answer is that each scheme is optimized for some specific cryptographic application s. Hash functions, for example, are well-suited for ensuring data integrity because any change made to the contents of a message will result in the receiver calculating a different hash value than the one placed in the transmission by the sender.

Since it is highly unlikely that two different messages will yield the same hash value, data integrity is ensured to a high degree of confidence.

Secret key cryptography, on the other hand, is ideally suited to encrypting messages, thus providing privacy and confidentiality. The sender can generate a session key on a per-message basis to encrypt the message; the receiver, of course, needs the same session key in order to decrypt the message. Key exchange, of course, is a key application of public key cryptography no pun intended. Asymmetric schemes can also be used for non-repudiation and user authentication; if the receiver can obtain the session key encrypted with the sender's private key, then only this sender could have sent the message.

Public key cryptography could, theoretically, also be used to encrypt messages although this is rarely done because secret key cryptography values can generally be computed about times faster than public key cryptography values. Figure 4 puts all of this together and shows how a hybrid cryptographic scheme combines all of these functions to form a secure transmission comprising a digital signature and digital envelope. In this example, the sender of the message is Alice and the receiver is Bob.

A digital envelope comprises an encrypted message and an encrypted session key. Alice uses secret key cryptography to encrypt her message using the session key , which she generates at random with each session. Alice then encrypts the session key using Bob's public key. The encrypted message and encrypted session key together form the digital envelope. Upon receipt, Bob recovers the session secret key using his private key and then decrypts the encrypted message.

The digital signature is formed in two steps. First, Alice computes the hash value of her message; next, she encrypts the hash value with her private key. Upon receipt of the digital signature, Bob recovers the hash value calculated by Alice by decrypting the digital signature with Alice's public key. Bob can then apply the hash function to Alice's original message, which he has already decrypted see previous paragraph. If the resultant hash value is not the same as the value supplied by Alice, then Bob knows that the message has been altered; if the hash values are the same, Bob should believe that the message he received is identical to the one that Alice sent.

This scheme also provides nonrepudiation since it proves that Alice sent the message; if the hash value recovered by Bob using Alice's public key proves that the message has not been altered, then only Alice could have created the digital signature. Bob also has proof that he is the intended receiver; if he can correctly decrypt the message, then he must have correctly decrypted the session key meaning that his is the correct private key.

This diagram purposely suggests a cryptosystem where the session key is used for just a single session. Even if this session key is somehow broken, only this session will be compromised; the session key for the next session is not based upon the key for this session, just as this session's key was not dependent on the key from the previous session. This is known as Perfect Forward Secrecy ; you might lose one session key due to a compromise but you won't lose all of them.

In a article in the industry literature, a writer made the claim that bit keys did not provide as adequate protection for DES at that time as they did in because computers were times faster in than in Therefore, the writer went on, we needed 56,bit keys in instead of bit keys to provide adequate protection. The conclusion was then drawn that because 56,bit keys are infeasible true , we should accept the fact that we have to live with weak cryptography false! The major error here is that the writer did not take into account that the number of possible key values double whenever a single bit is added to the key length; thus, a bit key has twice as many values as a bit key because 2 57 is two times 2 In fact, a bit key would have times more values than a bit key.

But this does bring up the question, "What is the significance of key length as it affects the level of protection? In cryptography, size does matter. The larger the key, the harder it is to crack a block of encrypted data. The reason that large keys offer more protection is almost obvious; computers have made it easier to attack ciphertext by using brute force methods rather than by attacking the mathematics which are generally well-known anyway.

With a brute force attack, the attacker merely generates every possible key and applies it to the ciphertext. Any resulting plaintext that makes sense offers a candidate for a legitimate key. Until the mids or so, brute force attacks were beyond the capabilities of computers that were within the budget of the attacker community. By that time, however, significant compute power was typically available and accessible.

General-purpose computers such as PCs were already being used for brute force attacks. Distributed attacks, harnessing the power of up to tens of thousands of powerful CPUs, are now commonly employed to try to brute-force crypto keys. This information was not merely academic; one of the basic tenets of any security system is to have an idea of what you are protecting and from whom are you protecting it!

The table clearly shows that a bit key was essentially worthless against even the most unsophisticated attacker. On the other hand, bit keys were fairly strong unless you might be subject to some pretty serious corporate or government espionage. But note that even bit keys were clearly on the decline in their value and that the times in the table were worst cases. So, how big is big enough? DES, invented in , was still in use at the turn of the century, nearly 25 years later.

If we take that to be a design criteria i. The DES proposal suggested bit keys; by , a bit key would have been required to offer equal protection and an bit key necessary by A or bit SKC key will probably suffice for some time because that length keeps us ahead of the brute force capabilities of the attackers.

Note that while a large key is good, a huge key may not always be better; for example, expanding PKC keys beyond the current or bit lengths doesn't add any necessary protection at this time. Weaknesses in cryptosystems are largely based upon key management rather than weak keys. The most effective large-number factoring methods today use a mathematical Number Field Sieve to find a certain number of relationships and then uses a matrix operation to solve a linear equation to produce the two prime factors.

The sieve step actually involves a large number of operations that can be performed in parallel; solving the linear equation, however, requires a supercomputer. In early , Shamir of RSA fame described a new machine that could increase factorization speed by orders of magnitude. There still appear to be many engineering details that have to be worked out before such a machine could be built.

Furthermore, the hardware improves the sieve step only; the matrix operation is not optimized at all by this design and the complexity of this step grows rapidly with key length, both in terms of processing time and memory requirements. Nevertheless, this plan conceptually puts bit keys within reach of being factored.

It is also interesting to note that while cryptography is good and strong cryptography is better, long keys may disrupt the nature of the randomness of data files. Shamir and van Someren "Playing hide and seek with stored keys" have noted that a new generation of viruses can be written that will find files encrypted with long keys, making them easier to find by intruders and, therefore, more prone to attack. Until the mids, export outside of North America of cryptographic products using keys greater than 40 bits in length was prohibited, which made those products essentially worthless in the marketplace, particularly for electronic commerce; today, crypto products are widely available on the Internet without restriction.

Department of Commerce Bureau of Industry and Security maintains an Encryption FAQ web page with more information about the current state of encryption registration. Without meaning to editorialize too much in this tutorial, a bit of historical context might be helpful. In the mids, the U. Department of Commerce still classified cryptography as a munition and limited the export of any products that contained crypto. For that reason, browsers in the era, such as Internet Explorer and Netscape, had a domestic version with bit encryption downloadable only in the U.

Many cryptographers felt that the export limitations should be lifted because they only applied to U. Those restrictions were lifted by or , but there is still a prevailing attitude, apparently, that U. On a related topic, public key crypto schemes can be used for several purposes, including key exchange, digital signatures, authentication, and more. The length of the secret keys exchanged via that system have to have at least the same level of attack resistance. Secure use of cryptography requires trust.

While secret key cryptography can ensure message confidentiality and hash codes can ensure integrity, none of this works without trust. PKC solved the secret distribution problem, but how does Alice really know that Bob is who he says he is?

Just because Bob has a public and private key, and purports to be "Bob," how does Alice know that a malicious person Mallory is not pretending to be Bob? There are a number of trust models employed by various cryptographic schemes. This section will explore three of them:.

Each of these trust models differs in complexity, general applicability, scope, and scalability. Pretty Good Privacy described more below in Section 5.

A PGP user maintains a local keyring of all their known and trusted public keys. The user makes their own determination about the trustworthiness of a key using what is called a "web of trust. If Alice needs Bob's public key, Alice can ask Bob for it in another e-mail or, in many cases, download the public key from an advertised server; this server might a well-known PGP key repository or a site that Bob maintains himself.

In fact, Bob's public key might be stored or listed in many places. The author's public key, for example, can be found at http: Alice is prepared to believe that Bob's public key, as stored at these locations, is valid.

Suppose Carol claims to hold Bob's public key and offers to give the key to Alice. How does Alice know that Carol's version of Bob's key is valid or if Carol is actually giving Alice a key that will allow Mallory access to messages?

The answer is, "It depends. And trust is not necessarily transitive; if Dave has a copy of Bob's key and Carol trusts Dave, it does not necessarily follow that Alice trusts Dave even if she does trust Carol.

The point here is that who Alice trusts and how she makes that determination is strictly up to Alice. PGP makes no statement and has no protocol about how one user determines whether they trust another user or not.

In any case, encryption and signatures based on public keys can only be used when the appropriate public key is on the user's keyring. Kerberos is a commonly used authentication scheme on the Internet.

Developed by MIT's Project Athena, Kerberos is named for the three-headed dog who, according to Greek mythology, guards the entrance of Hades rather than the exit, for some reason! In this model, security and authentication will be based on secret key technology where every host on the network has its own secret key. It would clearly be unmanageable if every host had to know the keys of all other hosts so a secure, trusted host somewhere on the network, known as a Key Distribution Center KDC , knows the keys for all of the hosts or at least some of the hosts within a portion of the network, called a realm.

In this way, when a new node is brought online, only the KDC and the new node need to be configured with the node's key; keys can be distributed physically or by some other secure means. The steps in establishing an authenticated session between an application client and the application server are:. While the details of their operation, functional capabilities, and message formats are different, the conceptual overview above pretty much holds for both.

One primary difference is that Kerberos V4 uses only DES to generate keys and encrypt messages, while V5 allows other schemes to be employed although DES is still the most widely algorithm used. Certificates and Certificate Authorities CA are necessary for widespread use of cryptography for e-commerce applications.

While a combination of secret and public key cryptography can solve the business issues discussed above, crypto cannot alone address the trust issues that must exist between a customer and vendor in the very fluid, very dynamic e-commerce relationship. How, for example, does one site obtain another party's public key? How does a recipient determine if a public key really belongs to the sender? How does the recipient know that the sender is using their public key for a legitimate purpose for which they are authorized?

When does a public key expire? How can a key be revoked in case of compromise or loss? The basic concept of a certificate is one that is familiar to all of us. A driver's license, credit card, or SCUBA certification, for example, identify us to others, indicate something that we are authorized to do, have an expiration date, and identify the authority that granted the certificate.

As complicated as this may sound, it really isn't. I have one issued by the State of Florida. The license establishes my identity, indicates the type of vehicles that I can operate and the fact that I must wear corrective lenses while doing so, identifies the issuing authority, and notes that I am an organ donor.

When I drive in other states, the other jurisdictions throughout the U. When I leave the U. When I am in Aruba, Australia, Canada, Israel, and many other countries, they will accept not the Florida license, per se, but any license issued in the U. This analogy represents the certificate trust chain, where even certificates carry certificates. For purposes of electronic transactions, certificates are digital documents.

The specific functions of the certificate include: Associate, or bind , a public key to an individual, organization, corporate position, or other entity. Establish what actions the holder may or may not take based upon this certificate. Secure confidential information e. VeriSign Class 3 certificate. A sample abbreviated certificate is shown in Figure 6.

While this is a certificate issued by VeriSign, many root-level certificates can be found shipped with browsers. When the browser makes a connection to a secure Web site, the Web server sends its public key certificate to the browser.

The browser then checks the certificate's signature against the public key that it has stored; if there is a match, the certificate is taken as valid and the Web site verified by this certificate is considered to be "trusted. Most certificates today comply with X. Certificate authorities are the repositories for public keys and can be any agency that issues certificates.

When a sender needs an intended receiver's public key, the sender must get that key from the receiver's CA. That scheme is straight-forward if the sender and receiver have certificates issued by the same CA. If not, how does the sender know to trust the foreign CA? One industry wag has noted, about trust: CAs, in turn, form trust relationships with other CAs. Thus, if a user queries a foreign CA for information, the user may ask to see a list of CAs that establish a "chain of trust" back to the user.

One major feature to look for in a CA is their identification policies and procedures. When a user generates a key pair and forwards the public key to a CA, the CA has to check the sender's identification and takes any steps necessary to assure itself that the request is really coming from the advertised sender. Different CAs have different identification policies and will, therefore, be trusted differently by other CAs.

Verification of identity is just one of many issues that are part of a CA's Certification Practice Statement CPS and policies; other issues include how the CA protects the public keys in its care, how lost or compromised keys are revoked, and how the CA protects its own private keys.

As a final note, CAs are not immune to attack and certificates themselves are able to be counterfeited. Problems have continued over the years; good write-ups on this can be found at " Another Certification Authority Breached the 12th! The paragraphs above describe three very different trust models. It is hard to say that any one is better than the others; it depend upon your application.

One of the biggest and fastest growing applications of cryptography today, though, is electronic commerce e-commerce , a term that itself begs for a formal definition. PGP's web of trust is easy to maintain and very much based on the reality of users as people. The model, however, is limited; just how many public keys can a single user reliably store and maintain?

And what if you are using the "wrong" computer when you want to send a message and can't access your keyring? How easy it is to revoke a key if it is compromised? PGP may also not scale well to an e-commerce scenario of secure communication between total strangers on short-notice. Kerberos overcomes many of the problems of PGP's web of trust, in that it is scalable and its scope can be very large.

In the early days of the Internet, every host had to maintain a list of every other host; the Domain Name System DNS introduced the idea of a distributed database for this purpose and the DNS is one of the key reasons that the Internet has grown as it has. While certificates and the benefits of a PKI are most often associated with electronic commerce, the applications for PKI are much broader and include secure electronic mail, payments and electronic checks, Electronic Data Interchange EDI , secure transfer of Domain Name System DNS and routing information, electronic forms, and digitally signed documents.

A single "global PKI" is still many years away, that is the ultimate goal of today's work as international electronic commerce changes the way in which we do business in a similar way in which the Internet has changed the way in which we communicate. The paragraphs above have provided an overview of the different types of cryptographic algorithms, as well as some examples of some available protocols and schemes.

The paragraphs below will show several real cryptographic applications that many of us employ knowingly or not everyday for password protection and private communication. Some of the schemes described below never were widely deployed but are still historically interesting, thus remain included here.

But passwords are not typically kept on a host or server in plaintext, but are generally encrypted using some sort of hash scheme. Note that each password is stored as a byte string. The first two characters are actually a salt , randomness added to each password so that if two users have the same password, they will still be encrypted differently; the salt, in fact, provides a means so that a single password might have different encryptions. The remaining 11 bytes are the password hash, calculated using DES.

This fact, coupled with the weak encryption of the passwords, resulted in the development of the shadow password system where passwords are kept in a separate, non-world-readable file used in conjunction with the normal password file.

In the NT case, all passwords are hashed using the MD4 algorithm, resulting in a bit byte hash value they are then obscured using an undocumented mathematical transformation that was a secret until distributed on the Internet. The password password , for example, might be stored as the hash value in hexadecimal b22d73c34bd4aa79c8b09f Passwords are not saved in plaintext on computer systems precisely so they cannot be easily compromised. For similar reasons, we don't want passwords sent in plaintext across a network.

But for remote logon applications, how does a client system identify itself or a user to the server? One mechanism, of course, is to send the password as a hash value and that, indeed, may be done. A weakness of that approach, however, is that an intruder can grab the password off of the network and use an off-line attack such as a dictionary attack where an attacker takes every known word and encrypts it with the network's encryption algorithm, hoping eventually to find a match with a purloined password hash.

In some situations, an attacker only has to copy the hashed password value and use it later on to gain unauthorized entry without ever learning the actual password.

An even stronger authentication method uses the password to modify a shared secret between the client and server, but never allows the password in any form to go across the network.

As suggested above, Windows NT passwords are stored in a security file on a server as a byte hash value. When a user logs on to a server from a remote workstation, the user is identified by the username, sent across the network in plaintext no worries here; it's not a secret anyway! The server then generates a bit random number and sends it to the client also in plaintext.

This number is the challenge. Recall that DES employs a bit key, acts on a bit block of data, and produces a bit output.

In this case, the bit data block is the random number. The client actually uses three different DES keys to encrypt the random number, producing three different bit outputs.

The first key is the first seven bytes 56 bits of the password's hash value, the second key is the next seven bytes in the password's hash, and the third key is the remaining two bytes of the password's hash concatenated with five zero-filled bytes.

So, for the example above, the three DES keys would be b22d73c34 , bd4aa79c8b0 , and 9f Each key is applied to the random number resulting in three bit outputs, which comprise the response. Thus, the server's 8-byte challenge yields a byte response from the client and this is all that would be seen on the network. The server, for its part, does the same calculation to ensure that the values match. There is, however, a significant weakness to this system.

Specifically, the response is generated in such a way as to effectively reduce byte hash to three smaller hashes, of length seven, seven, and two, respectively. Thus, a password cracker has to break at most a 7-byte hash. One Windows NT vulnerability test program that I used in the past reported passwords that were "too short," defined as "less than 8 characters.

This was, in fact, not the case at all; all the software really had to do was to look at the last eight bytes of the Windows NT LanMan hash to see that the password was seven or fewer characters. Consider the following example, showing the LanMan hash of two different short passwords take a close look at the last 8 bytes:. MS-CHAP assumes that it is working with hashed values of the password as the key to encrypting the challenge. WVKR.org is proudly powered by WordPress