RFC2947 - Telnet Encryption: DES3 64 bit Cipher Feedback

Network Working Group J. Altman
Request for Comments: 2947 Columbia University
Category: Standards Track September 2000
Telnet Encryption: DES3 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 Triple-DES (data encryption
standard) encryption algorithm in cipher feedback mode with the
telnet encryption option.
1. Command Names and Codes
Encryption Type
DES3_CFB64 3
Suboption Commands
CFB64_IV 1
CFB64_IV_OK 2
CFB64_IV_BAD 3
2. Command Meanings
IAC SB ENCRYPT IS DES3_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 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.
4. Algorithm
DES3 64 bit Cipher Feedback
key1 key2 key3

v v v
+-------+ +-------+ +-------+
+-> DES-e -> DES-d -> DES-e -- +
+-------+ +-------+ +-------+
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 = DES-e(DES-d(DES-e(iV, key1),key2),key3)
On = Dn ^ Vn
V(n+1) = DES-e(DES-d(DES-e(On, key1),key2),key3)
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 bytes, then the DES3_CFB64 option must
not be offered or used as a valid telnet encryption option.
The following rules are to be followed for creating three DES
encryption keys based upon the available encrypt key data:
keys_to_use = bytes of key data / DES block size (8 bytes)
where the keys are labeled "key1" through "key6" with "key1" being
the first 8 bytes; "key2" the second 8 bytes; ... and "key6" being
sixth 8 bytes (if available).
When two keys are available:
. data sent from the server is encrypted with key1, decrypted with
key2, and encrypted with key1;
. data sent from the client is encrypted with key2, decrypted with
key1, and encrypted with key2
When three keys are available:
. data sent from the server is encrypted with key1, decrypted with
key2, and encrypted with key3;
. data sent from the client is encrypted with key2, decrypted with
key3, and encrypted with key1
When four keys are available:
. data sent from the server is encrypted with key1, decrypted with
key2, and encrypted with key3;
. data sent from the client is encrypted with key2, decrypted with
key4, and encrypted with key1
When five keys are available:
. data sent from the server is encrypted with key1, decrypted with
key2, and encrypted with key3;
. data sent from the client is encrypted with key2, decrypted with
key4, and encrypted with key5
When six keys are available:
. data sent from the server is encrypted with key1, decrypted with
key2, and encrypted with key3;
. data sent from the client is encrypted with key4, decrypted with
key5, and encrypted with key6
In all cases, the keys used by DES3_CFB64 must have their parity
corrected after they are determined using the above algorithm.
Note that the above algorithm assumes that it is safe to use a
non-DES key (or part of a non-DES key) as a DES key. This is not
necessarily true of all cipher systems, but we specify this
behaviour as the default since it is true for most authentication
systems in popular use today, and for compatibility with existing
implementations. New telnet AUTHENTICATION mechanisms may specify
alternative methods for determining the keys to be used for this
cipher suite in their specification, if the session key negotiated
by that authentication mechanism is not a DES key and and where
this algorithm may not be safely used.
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.
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.
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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