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+Network Working Group                                           J. Altman
+Request for Comments: 2950                            Columbia University
+Category: Standards Track                                  September 2000
+
+
+           Telnet Encryption: CAST-128 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 CAST-128 encryption algorithm
+   in cipher 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_CFB64   8
+      CAST128_CFB64   10
+
+   Suboption Commands
+
+      CFB64_IV         1
+      CFB64_IV_OK      2
+      CFB64_IV_BAD     3
+
+2.  Command Meanings
+
+   IAC SB ENCRYPT IS CAST5_40_CFB64 CFB64_IV <initial vector> IAC SE
+   IAC SB ENCRYPT IS CAST128_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.
+
+
+
+Altman                      Standards Track                     [Page 1]
+
+RFC 2950            CAST-128 64 bit Cipher Feedback       September 2000
+
+
+   IAC SB ENCRYPT REPLY CAST5_40_CFB64 CFB64_IV_OK IAC SE
+   IAC SB ENCRYPT REPLY CAST128_CFB64 CFB64_IV_OK IAC SE
+   IAC SB ENCRYPT REPLY CAST5_40_CFB64 CFB64_IV_BAD IAC SE
+   IAC SB ENCRYPT REPLY CAST128_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 CAST128 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.
+
+
+
+
+
+
+
+Altman                      Standards Track                     [Page 2]
+
+RFC 2950            CAST-128 64 bit Cipher Feedback       September 2000
+
+
+4.  Algorithm
+
+   CAST 64 bit Cipher 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)
+   On = Dn ^ Vn
+   V(n+1) = CAST(On, 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 16 (5) bytes, then the CAST128_CFB64
+   (CAST5_40_CFB64) 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 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,
+
+
+
+Altman                      Standards Track                     [Page 3]
+
+RFC 2950            CAST-128 64 bit Cipher Feedback       September 2000
+
+
+   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.
+
+   Encryption modes using 40-bit keys are not to be considered secure.
+   The 40 bit key mode CAST5_40_CFB64 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 Cipher
+   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
+
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+Altman                      Standards Track                     [Page 4]
+
+RFC 2950            CAST-128 64 bit Cipher Feedback       September 2000
+
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+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.
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+Altman                      Standards Track                     [Page 5]
+