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Network Working Group                                           J. AltmanRequest for Comments: 2950                            Columbia UniversityCategory: Standards Track                                  September 2000Telnet Encryption: CAST-128 64 bit Cipher 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 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     32.  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 20004.  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.eduAltman                      Standards Track                     [Page 4]

RFC 2950            CAST-128 64 bit Cipher 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 5]