





Network Working Group                                          J. Altman
Request for Comments: 2949                           Columbia University
Category: Standards Track                                 September 2000


           Telnet Encryption: CAST-128 64 bit Output 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 output 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
     vector, 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.



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RFC 2949            CAST-128 64 bit Output Feedback       September 2000


   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
     connection 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
     really 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 Output 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 OFB64_IV command.

   If a new OFB64_IV command is sent (received), and encryption
   (decryption) is enabled, the encryption (decryption) machine must be
   re-initialized 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-
   initialized 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.







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RFC 2949            CAST-128 64 bit Output Feedback       September 2000


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
   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_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.




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RFC 2949            CAST-128 64 bit Output Feedback       September 2000


   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.

   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 Output
   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 2949            CAST-128 64 bit Output Feedback       September 2000


Full Copyright Statement

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Acknowledgement

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



















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