Network Working Group                                           J. Altman
Request for Comments: 2947                            Columbia University
Category: Standards Track                                  September 2000


            Telnet Encryption: DES3 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 Triple-DES (data encryption
  standard) encryption algorithm in cipher feedback mode with the
  telnet encryption option.

1.  Command Names and Codes

  Encryption Type

     DES3_CFB64       3

  Suboption Commands

     CFB64_IV         1
     CFB64_IV_OK      2
     CFB64_IV_BAD     3

2.  Command Meanings

  IAC SB ENCRYPT IS DES3_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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  IAC SB ENCRYPT REPLY DES3_CFB64 CFB64_IV_OK IAC SE IAC SB ENCRYPT
  REPLY DES3_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 DES3 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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4.  Algorithm

  DES3 64 bit Cipher 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)
  On = Dn ^ Vn
  V(n+1) = DES-e(DES-d(DES-e(On, key1),key2),key3)

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_CFB64 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).









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   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_CFB64 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



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    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 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,
  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.

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.

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: [email protected]















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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.



















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