





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.



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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.







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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
   encryption key negotiated as a result of the telnet AUTHENTICATION
   option contains less than 16 (5) bytes, then the CAST128_CFB64
   (CAST5_40_CFB64) 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 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,



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RFC 2950            CAST-128 64 bit Cipher Feedback       September 2000


   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.

   Encryption modes using 40-bit keys are not to be considered secure.
   The 40 bit key mode CAST5_40_CFB64 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" document originally written by Dave Borman of Cray Research
   with the assistance of the IETF Telnet Working Group.

8.  References

   [1] Adams, C., "The CAST-128 Encryption Algorithm", RFC 2144, May
       1997.

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

















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RFC 2950            CAST-128 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
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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