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Network Working Group                                             T. Ts'oRequest for Comments: 2953                               VA Linux SystemsCategory: Informational                                    September 2000Telnet Encryption: DES 64 bit Output FeedbackStatus 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     32.  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 SETs'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.eduTs'o                         Informational                      [Page 4]

RFC 2953               DES 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.Ts'o                         Informational                      [Page 5]