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Versions: 00 01 02 03 04 rfc2953                                        
Internet-Draft                     Massachusetts Institute of Technology
draft-tso-telnet-enc-des-ofb-00.txt                        February 1998


             Telnet Encryption: DES 64 bit Output Feedback

Status of this Memo

   This document is an Internet-Draft.  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
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   ftp.isi.edu (US West Coast).

1.  Command Names and Codes

   Encryption Type

      DES_OFB64        2

   Suboption Commands

      OFB64_IV         1
      OFB64_IV_OK      2
      OFB64_IV_BAD     3
      OFB64_CHALLENGE  4
      OFB64_RESPONSE   5


2.  Command Meanings

   IAC SB ENCRYPT IS DES_OFB64 OFB64_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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Internet-Draft         DES 64 bit Output Feedback          February 1998


      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

   IAC SB ENCRYPT REPLY DES_OFB64 OFB64_IV_OK IAC SE
   IAC SB ENCRYPT REPLY DES_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 con-
      nection that is DO ENCRYPT may send the OFB64_IV_OK and
      OFB64_IV_BAD commands.

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 DES 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 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 OFB64_IV command.

   If a new OFB64_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 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 (re-
   ceived) in that START command, and the initial vector sent (received)
   in this OFB64_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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Internet-Draft         DES 64 bit Output Feedback          February 1998


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


5.  Security considerations

   Encryption using Output Feedback does not ensure data integrity; an
   active attacker may be able to substitute text, if he can predict the
   clear-text that was being transmitted.

   This option was originally drafted back when CPU speeds where not
   necessarily fast enough to do allow use of CFB.  Since then, CPU's
   have gotten much faster.  Given the inherent weaknesses in Output
   Feedback mode, perhaps it should be deprecated in favor of CFB modes?

6.  Acknowledgements

   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

   EMail: tytso@mit.edu






















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