Network Working Group                                            T. Ts'o
Request for Comments: 2952                              VA Linux Systems
Category: Informational                                   September 2000

Telnet Encryption: DES 64 bit Cipher Feedback

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

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

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
     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.

   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
     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 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
   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.

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
   (decrypt), 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
   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 8 bytes, then the DES_CFB64 option must not
   be offered or used as a valid telnet encryption option.  If the
   encryption key negotiated 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 client to the telnet
   server, and the second 8 bytes of the key should be used as keyid 0
   for data sent by the telnet server to the telnet client.  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
   option was not a DES key, the key used by the DES_CFB64 must have its
   parity corrected after it is 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 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 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@mit.edu

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