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Versions: 00 01 02 03 RFC 2952

Network Working Group                                    T. Ts'o, Editor
Internet-Draft                                          VA Linux Systems
draft-tso-telnet-enc-des-cfb-03.txt                          August 1999

             Telnet Encryption: DES 64 bit Cipher Feedback

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
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   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.

0.  Abstract

   This document specifies how to use the DES encryption algorithm in
   cipher feedback mode with the telnet encryption option.

1.  Command Names and Codes

   Encryption Type

      DES_CFB64        1

   Suboption Commands

      CFB64_IV         1
      CFB64_IV_OK      2
      CFB64_IV_BAD     3

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2.  Command Meanings

   IAC SB ENCRYPT IS DES_CFB64 CFB64_IV <initial vector> IAC SE

      The sender of this command generates a random 8 byte initial vec-
      tor, 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.

   IAC SB ENCRYPT REPLY DES_CFB64 CFB64_IV_OK IAC SE
   IAC SB ENCRYPT REPLY DES_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 con-
      nection 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 real-
      ly 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 DES 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 en-
   cryption (decryption) machine must be re-initialized immediately fol-
   lowing 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 (decryp-
   tion) is enabled, the encryption (decryption) machine must be re-ini-
   tialized 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-ini-

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   tialized 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.

4.  Algorithm

   Given that V[i] is the initial 64 bit vector, V[n] is the nth 64 bit
   vector, D[n] is the nth chunk of 64 bits of data to encrypt (de-
   crypt), and O[n] is the nth chunk of 64 bits of encrypted (decrypted)
   data, then:

      V[0] = DES(V[i], key)
      O[n] = D[n] <exclusive or> V[n]
      V[n+1] = DES(O[n], key)

5.  Integration with the AUTHENTICATION telnet option

   As noted in the telnet ENCRYPTION option specifications, a keyid val-
   ue 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 8 bytes, then the DES_CFB64 option may not be offered or
   used as a valid telnet encryption option.  If the encryption key ne-
   gotiated as a result of the telnet AUTHENTICATION option is greater
   than 16 bytes the first 8 bytes of the key should be used as keyid 0
   for data sent from the telnet server to the telnet client, and the
   second 8 bytes of the key should be used as keyid 0 for data sent by
   the telnet client to the telnet server.  Otherwise, the first 8 bytes
   of the encryption key is used as keyid zero for the telnet ENCRYPTION
   option in both directions (with the client as WILL ENCRYPT and the
   server as WILL ENCRYPT).

   In all cases, if the key negotiated by the telnet AUTHENTICATION op-
   tion was not a DES key, the key used by the DES_CFB64 must have its
   parity corrected after it is detrmined 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 neces-
   sarily true of all cipher systems, but we specify this behaviour as
   the default since it is true for most authentication systems in popu-
   lar use today, and for compatibility with existing implementations.
   New telnet AUTHENTICATION mechanisms may specify althernative 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

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   Encryption using Cipher Feedback does not ensure data integrity; the
   active attacker has a limited ability to modify text, if he can pre-
   dict 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 sin-
   gle character in the telnet protocol, which will impact performance
   on slow (i.e. dialup) links.

7.  Acknowledgments

   This document was originally written by Dave Borman of Cray Research
   with the assistance of the IETF Telnet Working Group.

Author's Address

   Theodore Ts'o, Editor
   VA Linux Systems
   43 Pleasant St.
   Medford, MA 02155

   Phone: (781) 391-3464

   EMail: tytso@valinux.com

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