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Network Working Group                                          J. AltmanRequest for Comments: 2948                           Columbia UniversityCategory: Standards Track                                 September 2000Telnet Encryption: DES3 64 bit Output FeedbackStatus 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 Triple-DES (data encryption   standard) encryption algorithm in output feedback mode with the   telnet encryption option.1.  Command Names and Codes   Encryption Type      DES3_OFB64       4   Suboption Commands      OFB64_IV         1      OFB64_IV_OK      2      OFB64_IV_BAD     32.  Command Meanings   IAC SB ENCRYPT IS DES3_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 2948              DES3 64 bit Output Feedback         September 2000   IAC SB ENCRYPT REPLY DES3_OFB64 OFB64_IV_OK IAC SE   IAC SB ENCRYPT REPLY DES3_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 DES3 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 2948              DES3 64 bit Output Feedback         September 20004.  Algorithm   DES3 64 bit Output Feedback                  key1       key2       key3                   |          |          |                   v          v          v               +-------+  +-------+  +-------+            +->| DES-e |->| DES-d |->| DES-e |-- +            |  +-------+  +-------+  +-------+   |            +------------------------------------+                                                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 = DES-e(DES-d(DES-e(iV, key1),key2),key3)       V(n+1) = DES-e(DES-d(DES-e(Vn, key1),key2),key3)       On = Dn ^ Vn5.  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 bytes, then the DES3_OFB64 option must   not be offered or used as a valid telnet encryption option.   The following rules are to be followed for creating three DES   encryption keys based upon the available encrypt key data:     keys_to_use = bytes of key data / DES block size (8 bytes)   where the keys are labeled "key1" through "key6" with "key1" being   the first 8 bytes; "key2" the second 8 bytes; ... and "key6" being   sixth 8 bytes (if available).   When two keys are available:    . data sent from the server is encrypted with key1, decrypted with      key2, and encrypted with key1;Altman                      Standards Track                     [Page 3]

RFC 2948              DES3 64 bit Output Feedback         September 2000    . data sent from the client is encrypted with key2, decrypted with      key1, and encrypted with key2   When three keys are available:    . data sent from the server is encrypted with key1, decrypted with      key2, and encrypted with key3;    . data sent from the client is encrypted with key2, decrypted with      key3, and encrypted with key1   When four keys are available:    . data sent from the server is encrypted with key1, decrypted with      key2, and encrypted with key3;    . data sent from the client is encrypted with key2, decrypted with      key4, and encrypted with key1   When five keys are available:    . data sent from the server is encrypted with key1, decrypted with      key2, and encrypted with key3;    . data sent from the client is encrypted with key2, decrypted with      key4, and encrypted with key5   When six keys are available:    . data sent from the server is encrypted with key1, decrypted with      key2, and encrypted with key3;    . data sent from the client is encrypted with key4, decrypted with      key5, and encrypted with key6   In all cases, the keys used by DES3_OFB64 must have their parity   corrected after they are 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.Altman                      Standards Track                     [Page 4]

RFC 2948              DES3 64 bit Output Feedback         September 20006.  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.   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?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.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.eduAltman                      Standards Track                     [Page 5]

RFC 2948              DES3 64 bit Output Feedback         September 2000Full 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 6]