RFC 2947 (rfc2947) - Page 2 of 6
Telnet Encryption: DES3 64 bit Cipher Feedback
Alternative Format: Original Text Document
RFC 2947 DES3 64 bit Cipher Feedback September 2000
IAC SB ENCRYPT REPLY DES3_CFB64 CFB64_IV_OK IAC SE IAC SB ENCRYPT
REPLY DES3_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 DES3 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.
Altman Standards Track