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Network Working Group Jeffrey Altman
Internet-Draft Columbia University
draft-altman-telnet-enc-des3-cfb-02.txt February 1999
Telnet Encryption: DES3 64 bit Output 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
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
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.
Abstract
This document specifies how to use the Triple-DES encryption algorithm
in output feedback mode with the telnet encryption option.
1. Command Names and Codes
Encryption Type
DES3_OFB64 3
Suboption Commands
OFB64_IV 1
OFB64_IV_OK 2
OFB64_IV_BAD 3
2. Command Meanings
IAC SB ENCRYPT IS DES3_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
OFB64_IV command.
IAC SB ENCRYPT REPLY DES3_OFB64 OFB64_IV_OK IAC SE
IAC SB ENCRYPT REPLY DES3_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 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 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
START command.
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
OFB64_IV command.
4. Algorithm
DES3 64 bit Output 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)
V(n+1) = DES-e(DES-d(DES-e(Vn, key1),key2),key3)
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 bytes, then the DES3_OFB64 option must not be offered or
used as a valid telnet encryption option.
The following rules are to be followed for creating three DES encryp-
tion 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_OFB64 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 neces-
sarily true of all cipher systems, but we specify this behaviour as
the default since it is true for most authentication systems in popu-
lar use today, and for compatibility with existing implementations.
New telnet AUTHENTICATION mechanisms may specify althernative 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 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?
7. Acknowledgments
This document was based on the "Telnet Encryption: DES 64 bit Output
Feedback" draft originally written by Dave Borman of Cray Research
with the assistance of the IETF Telnet Working Group.
8. References
[FIPS-46] US National Bureau of Standards, "Data Encryption
Standard", Federal Information Processing Standard (FIPS)
Publication 46, January 1977.
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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