RFC2949 - Telnet Encryption: CAST-128 64 bit Output Feedback

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Network Working Group J. Altman

Request for Comments: 2949 Columbia University

Category: Standards Track September 2000

Telnet Encryption: CAST-128 64 bit Output 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 output 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_OFB64 9

CAST128_OFB64 11

Suboption Commands

OFB64_IV 1

OFB64_IV_OK 2

OFB64_IV_BAD 3

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

vector, 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

OFB64_IV command.

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

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 CAST128 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

CAST 64 bit Output 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)

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

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_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

initialization vector.

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

single 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.

7. Acknowledgments

This document was based on the "Telnet Encryption: DES 64 bit Output

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

Full Copyright Statement

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Acknowledgement

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