    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
       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,

    Altman                      Standards Track                     [Page 3]

    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

    Altman                      Standards Track                     [Page 4]

    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
       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.

    Altman                      Standards Track                     [Page 5]