Network Working Group                                    T. Ts'o, Editor
Internet-Draft                     Massachusetts Institute of Technology
draft-tso-telnet-enc-des-cfb-01.txt                        November 1998

             Telnet Encryption: DES 64 bit Cipher Feedback

Status of this Memo

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1.  Command Names and Codes

   Encryption Type

      DES_CFB64        1

   Suboption Commands

      CFB64_IV         1
      CFB64_IV_OK      2
      CFB64_IV_BAD     3
      CFB64_CHALLENGE  4
      CFB64_RESPONSE   5

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

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      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.


      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.

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-
   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 (re-
   ceived) 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:

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

5.  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 sin-
   gle character in the telnet protocol, which will impact performance
   on slow (i.e. dialup) links.

6.  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
   Massachusetts Institute of Technology
   MIT Room E40-343
   77 Massachusetts Ave.
   Cambridge, MA 02139

   Phone: (617) 253-8091


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