CAST-128 64 bit Cipher Feedback(2)

　　As noted in the telnet ENCRYPTION option specifications, a keyid　value of zero indicates the default encryption key, as might be　derived from the telnet AUTHENTICATION option. If the default　encryption key negotiated as a result of the telnet AUTHENTICATION　option contains less than 16 (5) bytes, then the CAST128_CFB64　(CAST5_40_CFB64) option must not be offered or used as a valid telnet　encryption option.

　　If there are less than 32 (10) bytes of key data, the first 16 (5)　bytes of key data are used as keyid 0 in each direction. If there　are at least 32 (10) bytes of key data, the first 16 (5) bytes of key　data are used to encrypt the data sent by the telnet client to the　telnet server; the second 16 (5) bytes of key data are used to　encrypt the data sent by the telnet server to the telnet client.

　　Any extra key data is used as random data to be sent as an　initialization vector.

　　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.

　　Encryption modes using 40-bit keys are not to be considered secure.

　　The 40 bit key mode CAST5_40_CFB64 is listed here simply to document　the implementations that are already prevalent on the Internet but　have never been documented.

　　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.

　　8. References

　　[1] Adams, C., "The CAST-128 Encryption Algorithm", RFC2144, May

　　1997.

　　Author's Address

　　Jeffrey Altman, Editor

　　Columbia University

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