Network Working Group T. Ts'o, Editor Internet-Draft VA Linux Systems draft-tso-telnet-enc-des-cfb-02.txt July 1999 Telnet Encryption: DES 64 bit Cipher Feedback Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 0. Abstract This document specifies how to use the DES encryption algorithm in cipher feedback mode with the telnet encryption option. 1. Command Names and Codes Encryption Type DES_CFB64 1 Suboption Commands CFB64_IV 1 CFB64_IV_OK 2 CFB64_IV_BAD 3 CFB64_CHALLENGE 4 CFB64_RESPONSE 5 Expires January 2000 [Page 1]
Internet-Draft DES 64 bit Cipher Feedback July 1999 2. Command Meanings IAC SB ENCRYPT IS DES_CFB64 CFB64_IV <initial vector> IAC SE The sender of this command generates a random 8 byte initial vec- tor, and sends it to the other side of the connection using the CFB64_IV command. The initial vector is sent in clear text. Only the side of the connection that is WILL ENCRYPT may send the CFB64_IV command. IAC SB ENCRYPT REPLY DES_CFB64 CFB64_IV_OK IAC SE IAC SB ENCRYPT REPLY DES_CFB64 CFB64_IV_BAD IAC SE The sender of these commands either accepts or rejects the initial vector received in a CFB64_IV command. Only the side of the con- nection that is DO ENCRYPT may send the CFB64_IV_OK and CFB64_IV_BAD commands. 3. Implementation Rules Once a CFB64_IV_OK command has been received, the WILL ENCRYPT side of the connection should do keyid negotiation using the ENC_KEYID command. Once the keyid negotiation has successfully identified a common keyid, then START and END commands may be sent by the side of the connection that is WILL ENCRYPT. Data will be encrypted using the DES 64 bit Cipher Feedback algorithm. If encryption (decryption) is turned off and back on again, and the same keyid is used when re-starting the encryption (decryption), the intervening clear text must not change the state of the encryption (decryption) machine. If a START command is sent (received) with a different keyid, the en- cryption (decryption) machine must be re-initialized immediately fol- lowing the end of the START command with the new key and the initial vector sent (received) in the last CFB64_IV command. If a new CFB64_IV command is sent (received), and encryption (decryp- tion) is enabled, the encryption (decryption) machine must be re-ini- tialized immediately following the end of the CFB64_IV command with the new initial vector, and the keyid sent (received) in the last START command. If encryption (decryption) is not enabled when a CFB64_IV command is sent (received), the encryption (decryption) machine must be re-ini- tialized after the next START command, with the keyid sent (received) in that START command, and the initial vector sent (received) in this CFB64_IV command. 4. Algorithm Expires January 2000 [Page 2]
Internet-Draft DES 64 bit Cipher Feedback July 1999 Given that V[i] is the initial 64 bit vector, V[n] is the nth 64 bit vector, D[n] is the nth chunk of 64 bits of data to encrypt (de- crypt), and O[n] is the nth chunk of 64 bits of encrypted (decrypted) data, then: V[0] = DES(V[i], key) O[n] = D[n] <exclusive or> V[n] V[n+1] = DES(O[n], key) 5. Security considerations Encryption using Cipher Feedback does not ensure data integrity; the active attacker has a limited ability to modify text, if he can pre- dict 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 sin- gle character in the telnet protocol, which will impact performance on slow (i.e. dialup) links. 6. Acknowledgments This document was originally written by Dave Borman of Cray Research with the assistance of the IETF Telnet Working Group. Author's Address Theodore Ts'o, Editor VA Linux Systems 43 Pleasant St. Medford, MA 02155 Phone: (781) 391-3464 EMail: tytso@valinux.com [Page 3]