Telnet Encryption: DES3 64 bit Output Feedback
RFC 2948

Document Type RFC - Proposed Standard (September 2000; No errata)
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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

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

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.

Altman                      Standards Track                     [Page 1]
RFC 2948              DES3 64 bit Output Feedback         September 2000

   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.

Altman                      Standards Track                     [Page 2]
RFC 2948              DES3 64 bit Output Feedback         September 2000

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.
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