Network Working Group J. Altman
Request for Comments: 2950 Columbia University
Category: Standards Track September 2000

Telnet Encryption: CAST-128 64 bit Cipher Feedback

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2000). All Rights Reserved.

Abstract

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

Encryption Type

CAST5_40_CFB64 8
CAST128_CFB64 10

Suboption Commands

CFB64_IV 1
CFB64_IV_OK 2
CFB64_IV_BAD 3

2. Command Meanings

IAC SB ENCRYPT IS CAST5_40_CFB64 CFB64_IV <initial vector> IAC SE
IAC SB ENCRYPT IS CAST128_CFB64 CFB64_IV <initial vector> IAC SE

The sender of this command generates a random 8 byte initial
vector, and sends it to the other side of the connection using the
CFB64_IV command. The initial vector is sent in clear text. Only
the side of the connection that is WILL ENCRYPT may send the
CFB64_IV command.

IAC SB ENCRYPT REPLY CAST5_40_CFB64 CFB64_IV_OK IAC SE
IAC SB ENCRYPT REPLY CAST128_CFB64 CFB64_IV_OK IAC SE
IAC SB ENCRYPT REPLY CAST5_40_CFB64 CFB64_IV_BAD IAC SE
IAC SB ENCRYPT REPLY CAST128_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 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
(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.

4. Algorithm

CAST 64 bit Cipher Feedback

key --->+------+
+-> CAST --+
+------+
v
INPUT --(---------->(+)+---> DATA

+--------------+

Given:
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) output.

V0 = CAST(iV, key)
On = Dn ^ Vn
V(n+1) = CAST(On, key)

5. Integration with the AUTHENTICATION telnet option

As noted in the telnet ENCRYPTION option specifications, a keyid
value 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_CFB64
(CAST5_40_CFB64) 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
initialization vector.

6. Security Considerations

Encryption using Cipher Feedback does not ensure data integrity; the
active attacker has a limited ability to modify text, if he can
predict the clear-text that was being transmitted. The limitations
faced by the attacker (that only 8 bytes can be modified at a time,

and the following 8-byte block of data will be corrupted, thus making
detection likely) are significant, but it is possible that an active
attacker still might be able to eXPloit this weakness.

The tradeoff here is that adding a message authentication code (MAC)
will significantly increase the number of bytes needed to send a
single character in the telnet protocol, which will impact
performance on slow (i.e. dialup) links.

Encryption modes using 40-bit keys are not to be considered secure.
The 40 bit key mode CAST5_40_CFB64 is listed here simply to document
the implementations that are already prevalent on the Internet but
have never been documented.

7. Acknowledgments

This document was based on the "Telnet Encryption: DES 64 bit Cipher
Feedback" document originally written by Dave Borman of Cray Research
with the assistance of the IETF Telnet Working Group.

8. References

[1] Adams, C., "The CAST-128 Encryption Algorithm", RFC2144, May
1997.

Author's Address

Jeffrey Altman, Editor
Columbia University
612 West 115th Street Room 716
New York NY 10025 USA

Phone: +1 (212) 854-1344
EMail: jaltman@columbia.edu

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