DES3 64 bit Cipher Feedback(2)

　　When four keys are available:

　　. data sent from the server is encrypted with key1, decrypted with　key2, and encrypted with key3;

　　. data sent from the client is encrypted with key2, decrypted with　key4, and encrypted with key1

　　When five keys are available:

　　. data sent from the server is encrypted with key1, decrypted with　key2, and encrypted with key3;

　　. data sent from the client is encrypted with key2, decrypted with　key4, and encrypted with key5

　　When six keys are available:

　　. data sent from the server is encrypted with key1, decrypted with　key2, and encrypted with key3;

　　. data sent from the client is encrypted with key4, decrypted with　key5, and encrypted with key6

　　In all cases, the keys used by DES3_CFB64 must have their parity　corrected after they are determined using the above algorithm.

　　Note that the above algorithm assumes that it is safe to use a　non-DES key (or part of a non-DES key) as a DES key. This is not　necessarily true of all cipher systems, but we specify this　behaviour as the default since it is true for most authentication　systems in popular use today, and for compatibility with existing　implementations. New telnet AUTHENTICATION mechanisms may specify　alternative methods for determining the keys to be used for this　cipher suite in their specification, if the session key negotiated　by that authentication mechanism is not a DES key and and where　this algorithm may not be safely used.

　　6. Security Considerations

　　Encryption using Cipher Feedback does not ensure data integrity; the　active attacker has a limited ability to modify text, if he can　predict the clear-text that was being transmitted. The limitations　faced by the attacker (that only 8 bytes can be modified at a time,　and the following 8-byte block of data will be corrupted, thus making　detection likely) are significant, but it is possible that an active　attacker still might be able to exploit this weakness.

　　The tradeoff here is that adding a message authentication code (MAC)　will significantly increase the number of bytes needed to send a　single character in the telnet protocol, which will impact　performance on slow (i.e. dialup) links.

　　7. Acknowledgments

　　This document was based on the "Telnet Encryption: DES 64 bit Cipher　Feedback" document originally written by Dave Borman of Cray Research　with the assistance of the IETF Telnet Working Group.

　　Author's Address

　　Jeffrey Altman, Editor

　　Columbia University

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　　EMail: jaltman@columbia.edu

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