Telnet Encryption: CAST-128 64 bit Cipher Feedback
RFC 2950

• Status
• IESG evaluation record
• IESG writeups
• Email expansions
• History

• Versions
• 00

Network Working Group                                           J. Altman
Request for Comments: 2950                            Columbia University
Category: Standards Track                                  September 2000

           Telnet Encryption: CAST-128 64 bit Cipher Feedback

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  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 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_CFB64   8
      CAST128_CFB64   10

   Suboption Commands

      CFB64_IV         1
      CFB64_IV_OK      2
      CFB64_IV_BAD     3

2.  Command Meanings

   IAC SB ENCRYPT IS CAST5_40_CFB64 CFB64_IV <initial vector> IAC SE
   IAC SB ENCRYPT IS CAST128_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.

Altman                      Standards Track                     [Page 1]
RFC 2950            CAST-128 64 bit Cipher Feedback       September 2000

   IAC SB ENCRYPT REPLY CAST5_40_CFB64 CFB64_IV_OK IAC SE
   IAC SB ENCRYPT REPLY CAST128_CFB64 CFB64_IV_OK IAC SE
   IAC SB ENCRYPT REPLY CAST5_40_CFB64 CFB64_IV_BAD IAC SE
   IAC SB ENCRYPT REPLY CAST128_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
     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 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
   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.

Altman                      Standards Track                     [Page 2]
RFC 2950            CAST-128 64 bit Cipher Feedback       September 2000

4.  Algorithm

   CAST 64 bit Cipher 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)
   On = Dn ^ Vn
   V(n+1) = CAST(On, 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