Network Working Group T. Ts'o
Request for Comments: 2952 VA Linux Systems
Category: Informational September 2000
Telnet Encryption: DES 64 bit Cipher Feedback
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
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
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
vector, 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
T'so Informational [Page 1]
RFC 2952 DES 64 bit Cipher Feedback September 2000 The sender of these commands either accepts or rejects the initial vector received in a CFB64_IV command. Only the side of the connection 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 really 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 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 encryption (decryption) machine must be re-initialized immediately following 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 (decryption) is enabled, the encryption (decryption) machine must be re-initialized 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- initialized 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. T'so Informational [Page 2]
RFC 2952 DES 64 bit Cipher Feedback September 2000 4. Algorithm 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 (decrypt), 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. Integration with the AUTHENTICATION telnet option 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 8 bytes, then the DES_CFB64 option must not be offered or used as a valid telnet encryption option. If the encryption key negotiated as a result of the telnet AUTHENTICATION option is greater than 16 bytes the first 8 bytes of the key should be used as keyid 0 for data sent from the telnet client to the telnet server, and the second 8 bytes of the key should be used as keyid 0 for data sent by the telnet server to the telnet client. Otherwise, the first 8 bytes of the encryption key is used as keyid zero for the telnet ENCRYPTION option in both directions (with the client as WILL ENCRYPT and the server as WILL ENCRYPT). In all cases, if the key negotiated by the telnet AUTHENTICATION option was not a DES key, the key used by the DES_CFB64 must have its parity corrected after it is 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. T'so Informational [Page 3]
RFC 2952 DES 64 bit Cipher Feedback September 2000 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 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@mit.edu T'so Informational [Page 4]
RFC 2952 DES 64 bit Cipher Feedback September 2000
Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
T'so Informational [Page 5]
mirror server hosted at Truenetwork, Russian Federation.