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RFC2952 - Telnet Encryption: DES 64 bit Cipher Feedback

王朝other·作者佚名  2008-05-31
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Network Working Group T. Ts'o

Request for Comments: 2952 VA Linux Systems

Category: Informational September 2000

Telnet Encryption: DES 64 bit Cipher Feedback

Status of this Memo

This memo provides information for the Internet community. It does

not specify an Internet standard of any kind. 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 DES encryption algorithm in

cipher feedback mode with the telnet encryption option.

1. Command Names and Codes

Encryption Type

DES_CFB64 1

Suboption Commands

CFB64_IV 1

CFB64_IV_OK 2

CFB64_IV_BAD 3

2. Command Meanings

IAC SB ENCRYPT IS DES_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 DES_CFB64 CFB64_IV_OK IAC SE

IAC SB ENCRYPT REPLY DES_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 DES 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

Given that V[i] is the initial 64 bit vector, V[n] is the nth 64 bit

vector, D[n] is the nth chunk of 64 bits of data to encrypt

(decrypt), and O[n] is the nth chunk of 64 bits of encrypted

(decrypted) data, then:

V[0] = DES(V[i], key)

O[n] = D[n] <exclusive or> V[n]

V[n+1] = DES(O[n], 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 8 bytes, then the DES_CFB64 option must not

be offered or used as a valid telnet encryption option. If the

encryption key negotiated as a result of the telnet AUTHENTICATION

option is greater than 16 bytes the first 8 bytes of the key should

be used as keyid 0 for data sent from the telnet client to the telnet

server, and the second 8 bytes of the key should be used as keyid 0

for data sent by the telnet server to the telnet client. Otherwise,

the first 8 bytes of the encryption key is used as keyid zero for the

telnet ENCRYPTION option in both directions (with the client as WILL

ENCRYPT and the server as WILL ENCRYPT).

In all cases, if the key negotiated by the telnet AUTHENTICATION

option was not a DES key, the key used by the DES_CFB64 must have its

parity corrected after it is 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 originally written by Dave Borman of Cray Research

with the assistance of the IETF Telnet Working Group.

Author's Address

Theodore Ts'o, Editor

VA Linux Systems

43 Pleasant St.

Medford, MA 02155

Phone: (781) 391-3464

EMail: tytso@mit.edu

Full Copyright Statement

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

This document and translations of it may be copied and furnished to

others, and derivative works that comment on or otherwise explain it

or assist in its implementation may be prepared, copied, published

and distributed, in whole or in part, without restriction of any

kind, provided that the above copyright notice and this paragraph are

included on all such copies and derivative works. However, this

document itself may not be modified in any way, such as by removing

the copyright notice or references to the Internet Society or other

Internet organizations, except as needed for the purpose of

developing Internet standards in which case the procedures for

copyrights defined in the Internet Standards process must be

followed, or as required to translate it into languages other than

English.

The limited permissions granted above are perpetual and will not be

revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

Funding for the RFCEditor function is currently provided by the

Internet Society.

 
 
 
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