Network Working Group                                          J. Altman
Request for Comments: 2948                           Columbia University
Category: Standards Track                                 September 2000

Telnet Encryption: DES3 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

Abstract

   This document specifies how to use the Triple-DES (data encryption
   standard) encryption algorithm in output feedback mode with the
   telnet encryption option.

1. Command Names and Codes

   Encryption Type

      DES3_OFB64       4

   Suboption Commands

      OFB64_IV         1
      OFB64_IV_OK      2
      OFB64_IV_BAD     3

2. Command Meanings

   IAC SB ENCRYPT IS DES3_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.

   IAC SB ENCRYPT REPLY DES3_OFB64 OFB64_IV_OK IAC SE
   IAC SB ENCRYPT REPLY DES3_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 DES3 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.

4. Algorithm

   DES3 64 bit Output Feedback

                  key1       key2       key3
                   |          |          |
                   v          v          v
               +-------+  +-------+  +-------+
            +->| DES-e |->| DES-d |->| DES-e |-- +
            |  +-------+  +-------+  +-------+   |
            +------------------------------------+
                                                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 = DES-e(DES-d(DES-e(iV, key1),key2),key3)
       V(n+1) = DES-e(DES-d(DES-e(Vn, key1),key2),key3)
       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 bytes, then the DES3_OFB64 option must
   not be offered or used as a valid telnet encryption option.

   The following rules are to be followed for creating three DES
   encryption keys based upon the available encrypt key data:

     keys_to_use = bytes of key data / DES block size (8 bytes)

   where the keys are labeled "key1" through "key6" with "key1" being
   the first 8 bytes; "key2" the second 8 bytes; ... and "key6" being
   sixth 8 bytes (if available).

   When two keys are available:

    . data sent from the server is encrypted with key1, decrypted with
      key2, and encrypted with key1;

    . data sent from the client is encrypted with key2, decrypted with
      key1, and encrypted with key2

   When three keys are available:

    . data sent from the server is encrypted with key1, decrypted with
      key2, and encrypted with key3;

    . data sent from the client is encrypted with key2, decrypted with
      key3, and encrypted with key1

   When four keys are available:

    . data sent from the server is encrypted with key1, decrypted with
      key2, and encrypted with key3;

    . data sent from the client is encrypted with key2, decrypted with
      key4, and encrypted with key1

   When five keys are available:

    . data sent from the server is encrypted with key1, decrypted with
      key2, and encrypted with key3;

    . data sent from the client is encrypted with key2, decrypted with
      key4, and encrypted with key5

   When six keys are available:

    . data sent from the server is encrypted with key1, decrypted with
      key2, and encrypted with key3;

    . data sent from the client is encrypted with key4, decrypted with
      key5, and encrypted with key6

   In all cases, the keys used by DES3_OFB64 must have their parity
   corrected after they are 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.

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.

   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?

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.

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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