Internet DRAFT - draft-altman-telnet-enc-cast128-ofb
Network Working Group Jeffrey Altman
Internet-Draft Columbia University
draft-altman-telnet-enc-cast128-ofb-00.txt February 1999
Telnet Encryption: CAST-128 64 bit Cipher Feedback
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. 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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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
This document specifies how to use the CAST-128 encryption algorithm
in cipher feedback mode with the telnet encryption option. Two key
sizes are defined: 40 bit and 128 bit.
1. Command Names and Codes
2. Command Meanings
IAC SB ENCRYPT IS CAST5_40_OFB64 OFB64_IV <initial vector> IAC SE
IAC SB ENCRYPT IS CAST128_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
OFB64_IV command. The initial vector is sent in clear text. Only
the side of the connection that is WILL ENCRYPT may send the
IAC SB ENCRYPT REPLY CAST5_40_OFB64 OFB64_IV_OK IAC SE
IAC SB ENCRYPT REPLY CAST128_OFB64 OFB64_IV_OK IAC SE
IAC SB ENCRYPT REPLY CAST5_40_OFB64 OFB64_IV_BAD IAC SE
IAC SB ENCRYPT REPLY CAST128_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. The OFB64_IV_OK command MUST be sent for
backwards compatibility with existing implementations; there real-
ly 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 CAST128 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
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-ini
tialized immediately following the end of the OFB64_IV command with
the new initial vector, and the keyid sent (received) in the last
If encryption (decryption) is not enabled when a OFB64_IV command is
sent (received), the encryption (decryption) machine must be re-ini
tialized after the next START command, with the keyid sent (received)
in that START command, and the initial vector sent (received) in this
CAST 64 bit Output Feedback
+->| CAST |--+
| +------+ |
INPUT --------->(+) ----> DATA
iV: Initial vector, 64 bits (8 bytes) long.
Dn: the nth chunk of 64 bits (8 bytes) of data to encrypt (decrypt).
On: the nth chunk of 64 bits (8 bytes) of encrypted (decrypted)
V0 = CAST(iV, key)
V(n+1) = CAST(Vn, key)
On = Dn ^ Vn
5. Integration with the AUTHENTICATION telnet option
As noted in the telnet ENCRYPTION option specifications, a keyid val-
ue of zero indicates the default encryption key, as might be derived
from the telnet AUTHENTICATION option. If the default encryption key
negotiated as a result of the telnet AUTHENTICATION option contains
less than 16 (5) bytes, then the CAST128_OFB64 (CAST5_40_OFB64)
option must not be offered or used as a valid telnet encryption option.
If there are less than 32 (10) bytes of key data, the first 16 (5)
bytes of key data are used as keyid 0 in each direction. If there
are at least 32 (10) bytes of key data, the first 16 (5) bytes of
key data are used to encrypt the data sent by the telnet client to
the telnet server; the second 16 (5) bytes of key data are used to
encrypt the data sent by the telnet server to the telnet client.
Any extra key data is used as random data to be sent as an
6. 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.
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.
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?
Encryption modes using 40-bit keys are not to be considered secure.
The 40 bit key mode CAST5_40_OFB64 is listed here simply to document
the implementations that are already prevalent on the Internet but
have never been documented.
This document was based on the "Telnet Encryption: DES 64 bit Cipher
Feedback" draft originally written by Dave Borman of Cray Research
with the assistance of the IETF Telnet Working Group.
 Adams, C., "The CAST-128 Encryption Algorithm", RFC 2144, May
Jeffrey Altman, ditor
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