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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
+
+
+
+
+T'so                         Informational                      [Page 1]
+
+RFC 2952               DES 64 bit Cipher Feedback         September 2000
+
+
+     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.
+
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+T'so                         Informational                      [Page 2]
+
+RFC 2952               DES 64 bit Cipher Feedback         September 2000
+
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+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.
+
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+
+T'so                         Informational                      [Page 3]
+
+RFC 2952               DES 64 bit Cipher Feedback         September 2000
+
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+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
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+T'so                         Informational                      [Page 4]
+
+RFC 2952               DES 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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+T'so                         Informational                      [Page 5]
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