Network Working Group Jeffrey Altman Internet-Draft Columbia University draft-altman-telnet-enc-des3-cfb-01.txt February 1999 Telnet Encryption: DES3 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. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119. Abstract This document specifies how to use the Triple-DES encryption algorithm in cipher feedback mode with the telnet encryption option. 1. Command Names and Codes Encryption Type DES3_CFB64 3 Suboption Commands CFB64_IV 1 CFB64_IV_OK 2 CFB64_IV_BAD 3 2. Command Meanings IAC SB ENCRYPT IS DES3_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 DES3_CFB64 CFB64_IV_OK IAC SE IAC SB ENCRYPT REPLY DES3_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. The CFB64_IV_OK command MUST be sent for backwards compatibility with existing implementations; there real- ly isn't any reason why a sender would need to send the CFB64_IV_BAD command except in the case of a protocol violation where the IV sent was not of the correct length (i.e., 8 bytes). 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 DES3 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 DES3 64 bit Cipher Feedback key1 key2 key3 | | | v v v +-------+ +-------+ +-------+ +->| DES-e |->| DES-d |->| DES-e |-- + | +-------+ +-------+ +-------+ | | v INPUT --(-------------------------------->(+)+---> DATA | | +------------------------------------+ Given: iV: Initial vector, 64 bits (8 bytes) long. Dn: the nth chunk of 64 bits (8 bytes) of data to encrypt (decrypt). On: the nth chunk of 64 bits (8 bytes) of encrypted (decrypted) output. V0 = DES-e(DES-d(DES-e(iV, key1),key2),key3) On = Dn ^ Vn V(n+1) = DES-e(DES-d(DES-e(On, key1),key2),key3) 5. Integration with the AUTHENTICATION telnet option As noted in the telnet ENCRYPTION option specifications, a keyid val- ue 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 bytes, then the DES_CFB64 option must not be offered or used as a valid telnet encryption option. The following rules are to be followed for creating three DES encryp- tion keys based upon the available encrypt key data: keys_to_use = bytes of key data / DES block size (8 bytes) where the keys are labeled "key1" through "key6" with "key1" being the first 8 bytes; "key2" the second 8 bytes; ... and "key6" being sixth 8 bytes (if available). When two keys are available: . data sent from the server is encrypted with key1, decrypted with key2, and encrypted with key1; . data sent from the client is encrypted with key2, decrypted with key1, and encrypted with key2 When three 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 key3, and encrypted with key1 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 neces- sarily true of all cipher systems, but we specify this behaviour as the default since it is true for most authentication systems in popu- lar use today, and for compatibility with existing implementations. New telnet AUTHENTICATION mechanisms may specify althernative 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 sin- gle 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" draft 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 612 West 115th Street Room 716 New York NY 10025 USA Phone: +1 (212) 854-1344 EMail: jaltman@columbia.edu [Page 3]
Jeffrey Altman * Sr.Software Designer * Kermit-95 for Win32 and OS/2 The Kermit Project * Columbia University 612 West 115th St #716 * New York, NY * 10025 http://www.kermit-project.org/k95.html * kermit-support@kermit-project.org