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rfc:rfc2953

Network Working Group T. Ts'o Request for Comments: 2953 VA Linux Systems Category: Informational September 2000

           Telnet Encryption: DES 64 bit Output 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 data encryption standard (DES)
 encryption algorithm in output feedback mode with the telnet
 encryption option.

1. Command Names and Codes

 Encryption Type
    DES_OFB64        2
 Suboption Commands
    OFB64_IV         1
    OFB64_IV_OK      2
    OFB64_IV_BAD     3

2. Command Meanings

 IAC SB ENCRYPT IS DES_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 DES_OFB64 OFB64_IV_OK IAC SE
 IAC SB ENCRYPT REPLY DES_OFB64 OFB64_IV_BAD IAC SE

Ts'o Informational [Page 1] RFC 2953 DES 64 bit Output Feedback September 2000

   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 DES 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

 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:

Ts'o Informational [Page 2] RFC 2953 DES 64 bit Output Feedback September 2000

    V[0] = DES(V[i], key)
    V[n+1] = DES(V[n], key)
    O[n] = D[n] <exclusive or> V[n]

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_OFB64 option may 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 server to the telnet
 client, and the second 8 bytes of the key should be used as keyid 0
 for data sent by the telnet client to the telnet server.  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 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.  For this reason, the Cipher
 Feedback encryption type  should be used instead, since it provides
 limited detectability to data modification.  Neither provides true
 data integrity, however.

Ts'o Informational [Page 3] RFC 2953 DES 64 bit Output Feedback September 2000

 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

Ts'o Informational [Page 4] RFC 2953 DES 64 bit Output Feedback September 2000

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 RFC Editor function is currently provided by the
 Internet Society.

Ts'o Informational [Page 5]

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