ripem frequently asked questions archive name ripem faq last update sun mar about this posting this is a still rather rough listing of likely questions and information about ripem a program for public key mail encryption it this faq not ripem was written and will be maintained by marc vanheyningen mvanheyn whale cs indiana edu it will be posted to a variety of newsgroups on a monthly basis follow up discussion specific to ripem is redirected to the group alt security ripem this month i have reformatted this posting in an attempt to comply with the standards for hypertext faq formatting to allow easy manipulation of this document over the world wide web let me know what you think disclaimer nothing in this faq should be considered legal advice or anything other than one person s opinion if you want real legal advice talk to a real lawyer questions and answers what is ripem ripem is a program which performs privacy enhanced mail pem using the cryptographic techniques of rsa and des it allows your electronic mail to have the properties of authentication i e who sent it can be confirmed and privacy i e nobody can read it except the intended recipient ripem was written primarily by mark riordan mrr scss cl msu edu most of the code is in the public domain except for the rsa routines which are a library called rsaref licensed from rsa data security inc how can i get ripem ripem contains the library of cryptographic routines rsaref which is considered munitions and thus is export restricted from distribution to people who are not citizens or permanent residents of the u s or canada therefore the following request is quoted from the readme file please do not export the cryptographic code in this distribution outside of the usa or canada this is a personal request from me the author of ripem and a condition of your use of ripem note that rsaref is not in the public domain and a license for it is included with the distribution you should read it before using ripem the best way to get it is to ask a friend for a copy since this will reduce the load on those sites that do carry it not to mention the humans that run them naturally this requires that you trust the friend ripem is available via anonymous ftp to citizens and permanent residents in the u s from rsa com cd to rsaref and read the readme file for info last i looked this site contains only the source tree and does not contain compiled binaries or the nice mac version ripem as well as some other crypt stuff has its home site on rpub cl msu edu which is open to non anonymous ftp for users in the u s and canada who are citizens or permanent residents to find out how to obtain access ftp there cd to pub crypt and read the file getting access for convenience binaries for many architectures are available here in addition to the full source tree will ripem run on my machine probably it has already been ported to ms dos and most flavors of unix sunos next linux aix ultrix solaris etc ports to macintosh include a standard unix style port and a rather nice mac like port written by raymond lau author of stuffit more ports are expected and help of users is invited will ripem work with my mailer probably how easy and clean the effective interface is will depend on the sophistication and modularity of the mailer though the users guide included with the distribution discusses ways to use ripem with many popular mailers including berkeley mush elm and mh code is also included in elisp to allow easy use of ripem inside gnu emacs if you make a new interface for ripem or create an improvement on one in the distribution which you believe is convenient to use secure and may be useful to others feel free to post it to alt security ripem what is rsa rsa is a crypto system which is asymmetric or public key this means that there are two different related keys one to encrypt and one to decrypt because one cannot reasonably be derived from the other you may publish your encryption or public key widely and keep your decryption or private key to yourself anyone can use your public key to encrypt a message but only you hold the private key needed to decrypt it note that the message sent with rsa is normally just the des key to the real message see what is des note that the above only provides for privacy for authentication the fingerprint of the message see what is a fingerprint like md is encrypted with the sender s private key the recipient can use the sender s public key to decrypt it and confirm that the message must have come from the sender rsa was named for the three men rivest shamir and adleman who invented it to find out more about rsa ftp to rsa com and look in pub faq or look in sci crypt what is des des is the data encryption standard a widely used symmetric or secret key crypto system unlike rsa des uses the same key to encrypt and decrypt messages however des is much faster than rsa ripem uses both des and rsa it generates a random key and encrypts your mail with des using that key it then encrypts that key with the recipient s public rsa key and includes the result in the letter allowing the recipient to recover the des key des is sometimes considered weak because it is somewhat old and uses a key length considered too short by modern standards however it should be reasonably safe against an opponent smaller than a large corporation or government agency it is not unlikely that future ripems will strengthen the symmetric cipher possibly by using multiple encryption with des what is pem and how does ripem relate pem is privacy enhanced mail a system for allowing easy transfer of encrypted electronic mail it is described in rfcs these documents have been approved and obsolete the old rfcs ripem is not really a complete implementation of pem because pem specifies certificates for authenticating keys which ripem does not handle at this time their addition is planned what s this about distributing and authenticating keys for a remote user to be able to send secure mail to you she must know your public key for you to be able to confirm that the message received came from her you must know her public key it is important that this information be accurate if a bad guy convinces her that his key is in fact yours she will send messages which he can read ripem allows for three methods of key management a central server the distributed finger servers and a flat file all three are described in the ripem users guide which is part of the distribution none of them provide perfect security why do all ripem public keys look very similar ripem public keys begin with a pkcs identifier describing various characteristics about the key so the first bunch of characters in your key may be the same as those of lots of other people s keys this does not mean your keys are similar but only that they are the same class of key were generated with the same program are of the same length etc what is a fingerprint like md md is a message digest algorithm produced by rsa data security inc it provides a bit fingerprint or cryptographically secure hash of the plaintext it is cryptographically secure because it is not possible in a reasonable amount of computation to produce a different plaintext which produces the same fingerprint thus instead of signing the entire message with the sender s private key only the md of the message needs to be signed for authentication md is sometimes used for other purposes for example it is often used to map an input of arbitrary length to bits of data as a passphrase interpreter or cookie generator md is described in its entirety including an implementation in c in rfc what is pgp pgp is another cryptographic mail program called pretty good privacy pgp has been around longer than ripem and works somewhat differently pgp is not compatible with ripem in any way though pgp does also use rsa some major differences between pgp and ripem pgp has more key management features particularly for users without a direct network connection ripem conforms to the pem rfcs and thus has a greater probability of working with other pem software pgp makes no attempt to be compatible with anything other than pgp in fact pgp is not compatible with pgp ripem uses rsaref a library of rsa routines from rsa data security inc rsaref comes with a license which allows noncommercial use pgp uses its own implementation of rsa which is not licensed thus pkp the firm holding the u s patents on the rsa algorithm claims that it is a infringement of that patent to make use or sell pgp in the u s or canada in acknowledgement of this pgp s original author phil zimmermann says in the documentation in fact if you live in the usa and you are not a federal agency you shouldn t actually run pgp on your computer because public key partners wants to forbid you from running my software pgp is contraband both pgp and ripem are export restricted and cannot be sent outside the u s and canada however pgp already exists on many ftp sites in europe and other places whether you use pgp or ripem or whatever the documentation to pgp is recommended reading to anyone interested in such issues note that the above facts both regarding patent and export restrictions are somewhat controversial many people think it shouldn t be that way and some people interpret various documents differently unfortunately discussions of it on the net inevitably seem to produce more heat than light and probably belong in misc legal computing see disclaimer what about rpem rpem stands for rabin privacy enhanced mail it was similar to ripem but used a public key cipher invented by rabin which is not rsa in an attempt to avoid the patent on rsa it was written by mark riordan the same author as ripem its distribution was halted when contrary to the beliefs of many including rabin public key partners pkp claimed that their patent was broad enough to cover any public key cipher whose strength rested in the difficulty of factoring products of large primes not just rsa this claim is not universally accepted by any means but was not challenged for pragmatic reasons rpem is not really used anymore it is not compatible with ripem or pgp what is mime mime stands for multipurpose internet mail extensions and is described in rfc you can find out about it in the newsgroup comp mail mime how pem should interact with mime is not yet entirely clear some people use the stopgap solution of having a mime type application x ripem in order to send ripem messages as mime ones i hope some standards will emerge draft internet documents exist on the matter i have this simple way to defeat the security of ripem you may wish to check the companion post ripem attacks which discusses some of the more obvious attacks on ripem s security and what procedures will minimize the risk ripem s main weak area is probably key distribution
