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rfc:rfc2949

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.

Altman Standards Track [Page 1] RFC 2949 CAST-128 64 bit Output Feedback September 2000

 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.

Altman Standards Track [Page 2] RFC 2949 CAST-128 64 bit Output Feedback September 2000

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.

Altman Standards Track [Page 3] RFC 2949 CAST-128 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.
 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", RFC 2144, 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

Altman Standards Track [Page 4] RFC 2949 CAST-128 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.

Altman Standards Track [Page 5]

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