RFC 2953 (rfc2953) - Page 2 of 5

Telnet Encryption: DES 64 bit Output Feedback

RFC 2953               DES 64 bit Output Feedback         September 2000


     The sender of these commands either accepts or rejects the initial
     vector received in a OFB64_IV command.  Only the side of the
     connection that is DO ENCRYPT may send the OFB64_IV_OK and
     OFB64_IV_BAD commands.  The OFB64_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 OFB64_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 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
   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 OFB64_IV command.

   If a new OFB64_IV command is sent (received), and encryption
   (decryption) 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
   (received) 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
   (decrypt), and O[n] is the nth chunk of 64 bits of encrypted
   (decrypted) data, then:





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