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Network Working Group                                              T. WuRequest for Comments: 2944                          Standford UniversityCategory: Standards Track                                 September 2000Telnet Authentication: SRPStatus 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 an authentication scheme for the Telnet   protocol under the framework described in [RFC2941], using the Secure   Remote Password Protocol (SRP) authentication mechanism.  The   specific mechanism, SRP-SHA1, is described in [RFC2945].1. Command Names and Codes   Authentication Types      SRP          5   Suboption Commands      AUTH         0      REJECT       1      ACCEPT       2      CHALLENGE    3      RESPONSE     4      EXP          8      PARAMS       9Wu                          Standards Track                     [Page 1]

RFC 2944               Telnet Authentication: SRP         September 20002. Command Meanings   IAC SB AUTHENTICATION IS <authentication-type-pair> AUTH IAC SE      This command indicates that the client has supplied the      username and is ready to receive that user's field parameters.      There is no authentication information to be sent to the remote      side of the connection yet.  This should only be sent after the      IAC SB AUTHENTICATION NAME command has been issued.  If the      modifier byte (second byte of the authentication-type-pair)      has any bits other than AUTH_WHO_MASK or AUTH_HOW_MASK set,      both bytes are included in the session key hash described later.      This ensures that the authentication type pair was correctly      negotiated, while maintaining backward-compatibility with existing      software.   IAC SB AUTHENTICATION REPLY <authentication-type-pair> PARAMS <values   of modulus, generator, and salt> IAC SE      This command is used to pass the three parameter values used      in the exponentiation to the client.  These values are often      called n, g, and s.   IAC SB AUTHENTICATION IS <authentication-type-pair> EXP <client's   exponential residue> IAC SE      This command is used to pass the client's exponential residue,      otherwise known as A, computed against the parameters exchanged      earlier.   IAC SB AUTHENTICATION REPLY <authentication-type-pair> CHALLENGE   <server's exponential residue> IAC SE      This command is used to pass the server's exponential residue,      computed against the same parameters.  This quantity is actually      the sum of two residues, i.e. g^x + g^b.  For details see [SRP]      and [RFC2945].   IAC SB AUTHENTICATION IS <authentication-type-pair> RESPONSE   <response from client> IAC SE      This command gives the server proof of the client's authenticity      with a 160-bit (20 byte) response.Wu                          Standards Track                     [Page 2]

RFC 2944               Telnet Authentication: SRP         September 2000   IAC SB AUTHENTICATION REPLY <authentication-type-pair> ACCEPT   <server's response> IAC SE      This command indicates that the authentication was successful.      The server will construct its own proof of authenticity and      include it as sub-option data.   IAC SB AUTHENTICATION REPLY <authentication-type-pair> REJECT   <optional reason for rejection> IAC SE      This command indicates that the authentication was not successful,      and if there is any more data in the sub-option, it is an ASCII      text message of the reason for the rejection.   For the PARAMS command, since three pieces of data are being   transmitted, each parameter is preceded by a 16-bit (two byte) length   specifier in network byte order.  The EXP commands do not have a   count in front of the data because there is only one piece of data in   that suboption.  The CHALLENGE, RESPONSE, and ACCEPT data also do not   have a count because they are all fixed in size.3. Implementation Rules   Currently, only AUTH_CLIENT_TO_SERVER mode is supported.  Although   the SRP protocol effectively performs implicit mutual authentication   as a result of the two-way proofs, only the AUTH_HOW_ONE_WAY   authentication mode is currently defined.  The AUTH_HOW_MUTUAL   setting is being reserved for an explicit mutual-authentication   variant of the SRP protocol to be defined in future specifications.   All large number data sent in the arguments of the PARAMS and EXP   commands must be in network byte order, i.e. most significant byte   first.  No padding is used.   The SRP-SHA1 mechanism, as described in [RFC2945] generates a 40-byte   session key, which allows implementations to use different keys for   incoming and outgoing traffic, increasing the security of the   encrypted session.  It is recommended that the Telnet ENCRYPT method,   if it is used, be able to take advantage of the longer session keys.4. Examples   User "tjw" may wish to log in on machine "foo".  The client would   send IAC SB AUTHENTICATION NAME "tjw" IAC SE IAC SB AUTHENTICATION IS   SRP AUTH IAC SE.  The server would look up the field and salt   parameters for "tjw" from its password file and send them back to the   client.  Client and server would then exchange exponential residues   and calculate their session keys (after the client prompted "tjw" forWu                          Standards Track                     [Page 3]

RFC 2944               Telnet Authentication: SRP         September 2000   his password).  Then, the client would send the server its proof that   it knows the session key.  The server would either send back an   ACCEPT or a REJECT.  If the server accepts authentication, it also   sends its own proof that it knows the session key to the client.        Client                           Server                                         IAC DO AUTHENTICATION        IAC WILL AUTHENTICATION        [ The server is now free to request authentication information.          ]                                         IAC SB AUTHENTICATION SEND                                         SRP CLIENT|ONE_WAY|                                         ENCRYPT_USING_TELOPT                                         SRP CLIENT|ONE_WAY                                         IAC SE        [ The server has requested SRP authentication.  It has indicated          a preference for ENCRYPT_USING_TELOPT, which requires the          Telnet ENCRYPT option to be negotiated once authentication          succeeds.  If the client does not support this, the server          is willing to fall back to an encryption-optional mode.          The client will now respond with the name of the          user that it wants to log in as. ]        IAC SB AUTHENTICATION NAME        "tjw" IAC SE        IAC SB AUTHENTICATION IS        SRP CLIENT|ONE_WAY|ENCRYPT_USING_TELOPT AUTH        IAC SE        [ The server looks up the appropriate information for "tjw" and          sends back the parameters in a PARAMS command.  The parameters          consist of the values N, g, and s, each preceded with a two-          byte size parameter. ]                                         IAC SB AUTHENTICATION REPLY                                         SRP CLIENT|ONE_WAY|                                         ENCRYPT_USING_TELOPT PARAMS                                         ss ss nn nn nn nn ...                                         ss ss gg gg gg gg ...                                         ss ss tt tt tt tt ...                                         IAC SEWu                          Standards Track                     [Page 4]

RFC 2944               Telnet Authentication: SRP         September 2000        [ Both sides send their exponential residues.  The client          sends its value A and the server sends its value B.  In SRP,          the CHALLENGE message may be computed but not sent before          the EXP command.  ]        IAC SB AUTHENTICATION IS        SRP CLIENT|ONE_WAY|ENCRYPT_USING_TELOPT EXP        aa aa aa aa aa aa aa aa ...        IAC SE                                         IAC SB AUTHENTICATION REPLY                                         SRP CLIENT|ONE_WAY|                                         ENCRYPT_USING_TELOPT CHALLENGE                                         bb bb bb bb bb bb bb bb ...                                         IAC SE        [ The client sends its response to the server.  This is the          message M in the SRP protocol, which proves possession of          the session key by the client.          Since ENCRYPT_USING_TELOPT is specified, the two octets          of the authentication-type-pair are appended to the          session key K before the hash for M is computed.  If          the client and server had agreed upon a mode without          the encryption flag set, nothing would be appended to K.          Both this message and the server's response are as long as          the output of the hash; the length is 20 bytes for SHA-1. ]        IAC SB AUTHENTICATION IS        SRP CLIENT|ONE_WAY|ENCRYPT_USING_TELOPT RESPONSE        xx xx xx xx xx xx xx xx ...        IAC SE        [ The server accepts the response and sends its own proof. ]                                         IAC SB AUTHENTICATION REPLY                                         SRP CLIENT|ONE_WAY|                                         ENCRYPT_USING_TELOPT ACCEPT                                         yy yy yy yy yy yy yy yy ...                                         IAC SEWu                          Standards Track                     [Page 5]

RFC 2944               Telnet Authentication: SRP         September 20005. Security Considerations   The ability to negotiate a common authentication mechanism between   client and server is a feature of the authentication option that   should be used with caution.  When the negotiation is performed, no   authentication has yet occurred.  Therefore, each system has no way   of knowing whether or not it is talking to the system it intends.  An   intruder could attempt to negotiate the use of an authentication   system which is either weak, or already compromised by the intruder.   Since SRP relies on the security of the underlying public-key   cryptosystem, the modulus "n" should be large enough to resist   brute-force attack.  A length of at least 1024 bits is recommended,   and implementations should reject attempts to use moduli that are   shorter than 512 bits, or attempts to use invalid moduli and   generator parameters (non-safe-prime "n" or non-primitive "g").6. IANA Considerations   The authentication type SRP and its associated suboption values are   registered with IANA.  Any suboption values used to extend the   protocol as described in this document must be registered with IANA   before use.  IANA is instructed not to issue new suboption values   without submission of documentation of their use.7. References   [RFC2941]   Ts'o, T. and J. Altman, "Telnet Authentication Option",RFC 2941, September 2000.   [SRP]       T. Wu, "The Secure Remote Password Protocol", In               Proceedings of the 1998 ISOC Network and Distributed               System Security Symposium, San Diego, CA, pp. 97-111.   [RFC2945]   Wu, T., "The SRP Authentication and Key Exchange System",RFC 2945, September 2000.8. Author's Address   Thomas Wu   Stanford University   Stanford, CA 94305   EMail: tjw@cs.Stanford.EDUWu                          Standards Track                     [Page 6]

RFC 2944               Telnet Authentication: SRP         September 20009.  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.Wu                          Standards Track                     [Page 7]