Telnet Encryption: CAST-128 64 bit Output Feedback
The information below is for an old version of the document that is already published as an RFC.
This is an older version of an Internet-Draft that was ultimately published as RFC 2949.
|Author||Jeffrey E. Altman|
|Last updated||2013-03-02 (Latest revision 1999-08-11)|
|RFC stream||Legacy stream|
|RFC Editor Note||(None)|
|IESG||IESG state||RFC 2949 (Proposed Standard)|
|Send notices to||(None)|
Network Working Group Jeffrey Altman Internet-Draft Columbia University draft-altman-telnet-enc-cast128-ofb-00.txt February 1999 Telnet Encryption: CAST-128 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 CAST-128 encryption algorithm in cipher feedback mode with the telnet encryption option. Two key sizes are defined: 40 bit and 128 bit. 1. Command Names and Codes Encryption Type CAST5_40_OFB64 9 CAST128_OFB64 11 Suboption Commands OFB64_IV 1 OFB64_IV_OK 2 OFB64_IV_BAD 3 2. Command Meanings IAC SB ENCRYPT IS CAST5_40_OFB64 OFB64_IV <initial vector> IAC SE IAC SB ENCRYPT IS CAST128_OFB64 OFB64_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 OFB64_IV command. The initial vector is sent in clear text. Only the side of the connection that is WILL ENCRYPT may send the OFB64_IV command. IAC SB ENCRYPT REPLY CAST5_40_OFB64 OFB64_IV_OK IAC SE IAC SB ENCRYPT REPLY CAST128_OFB64 OFB64_IV_OK IAC SE IAC SB ENCRYPT REPLY CAST5_40_OFB64 OFB64_IV_BAD IAC SE IAC SB ENCRYPT REPLY CAST128_OFB64 OFB64_IV_BAD IAC SE The sender of these commands either accepts or rejects the initial vector received in a OFB64_IV command. Only the side of the con- nection that is DO ENCRYPT may send the OFB64_IV_OK and OFB64_IV_BAD commands. The OFB64_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 OFB64_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 OFB64_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 CAST128 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 OFB64_IV command. If a new OFB64_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 OFB64_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 OFB64_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 OFB64_IV command. 4. Algorithm CAST 64 bit Output Feedback key --->+------+ +->| CAST |--+ | +------+ | +------------+ 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 = CAST(iV, key) V(n+1) = CAST(Vn, key) On = Dn ^ Vn 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 (5) bytes, then the CAST128_OFB64 (CAST5_40_OFB64) 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 Output Feedback does not ensure data integrity; an active attacker may be able to substitute text, if he can predict the clear-text that was being transmitted. 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. This option was originally drafted back when CPU speeds where not necessarily fast enough to do allow use of CFB. Since then, CPU's have gotten much faster. Given the inherent weaknesses in Output Feedback mode, perhaps it should be deprecated in favor of CFB modes? Encryption modes using 40-bit keys are not to be considered secure. The 40 bit key mode CAST5_40_OFB64 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" draft originally written by Dave Borman of Cray Research with the assistance of the IETF Telnet Working Group. 8. References  Adams, C., "The CAST-128 Encryption Algorithm", RFC 2144, May 1997. Author's Address Jeffrey Altman, ditor Columbia Univrsity 612 West 115th Street Room 716 New York NY 10025 USA Phone: +1 (212) 854-1344 EMail: firstname.lastname@example.org 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 * email@example.com