RFC 2948 (rfc2948) - Page 2 of 6
Telnet Encryption: DES3 64 bit Output Feedback
Alternative Format: Original Text Document
RFC 2948 DES3 64 bit Output Feedback September 2000
IAC SB ENCRYPT REPLY DES3_OFB64 OFB64_IV_OK IAC SE
IAC SB ENCRYPT REPLY DES3_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
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 DES3 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.
Altman Standards Track