











Network Working Group                                           J. Myers

Request for Comments: 2554                       Netscape Communications

Category: Standards Track                                     March 1999





                         SMTP Service Extension

                           for Authentication



Status 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 (1999).  All Rights Reserved.





1. Introduction



   This document defines an SMTP service extension [ESMTP] whereby an

   SMTP client may indicate an authentication mechanism to the server,

   perform an authentication protocol exchange, and optionally negotiate

   a security layer for subsequent protocol interactions.  This

   extension is a profile of the Simple Authentication and Security

   Layer [SASL].





2. Conventions Used in this Document



   In examples, "C:" and "S:" indicate lines sent by the client and

   server respectively.



   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"

   in this document are to be interpreted as defined in "Key words for

   use in RFCs to Indicate Requirement Levels" [KEYWORDS].





3. The Authentication service extension





   (1) the name of the SMTP service extension is "Authentication"



   (2) the EHLO keyword value associated with this extension is "AUTH"









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   (3) The AUTH EHLO keyword contains as a parameter a space separated

       list of the names of supported SASL mechanisms.



   (4) a new SMTP verb "AUTH" is defined



   (5) an optional parameter using the keyword "AUTH" is added to the

       MAIL FROM command, and extends the maximum line length of the

       MAIL FROM command by 500 characters.



   (6) this extension is appropriate for the submission protocol

       [SUBMIT].





4. The AUTH command



   AUTH mechanism [initial-response]



     Arguments:

         a string identifying a SASL authentication mechanism.

         an optional base64-encoded response



     Restrictions:

         After an AUTH command has successfully completed, no more AUTH

         commands may be issued in the same session.  After a successful

         AUTH command completes, a server MUST reject any further AUTH

         commands with a 503 reply.



         The AUTH command is not permitted during a mail transaction.



     Discussion:

         The AUTH command indicates an authentication mechanism to the

         server.  If the server supports the requested authentication

         mechanism, it performs an authentication protocol exchange to

         authenticate and identify the user.  Optionally, it also

         negotiates a security layer for subsequent protocol

         interactions.  If the requested authentication mechanism is not

         supported, the server rejects the AUTH command with a 504

         reply.



         The authentication protocol exchange consists of a series of

         server challenges and client answers that are specific to the

         authentication mechanism.  A server challenge, otherwise known

         as a ready response, is a 334 reply with the text part

         containing a BASE64 encoded string.  The client answer consists

         of a line containing a BASE64 encoded string.  If the client

         wishes to cancel an authentication exchange, it issues a line

         with a single "*".  If the server receives such an answer, it

         MUST reject the AUTH command by sending a 501 reply.







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         The optional initial-response argument to the AUTH command is

         used to save a round trip when using authentication mechanisms

         that are defined to send no data in the initial challenge.

         When the initial-response argument is used with such a

         mechanism, the initial empty challenge is not sent to the

         client and the server uses the data in the initial-response

         argument as if it were sent in response to the empty challenge.

         Unlike a zero-length client answer to a 334 reply, a zero-

         length initial response is sent as a single equals sign ("=").

         If the client uses an initial-response argument to the AUTH

         command with a mechanism that sends data in the initial

         challenge, the server rejects the AUTH command with a 535

         reply.



         If the server cannot BASE64 decode the argument, it rejects the

         AUTH command with a 501 reply.  If the server rejects the

         authentication data, it SHOULD reject the AUTH command with a

         535 reply unless a more specific error code, such as one listed

         in section 6, is appropriate.  Should the client successfully

         complete the authentication exchange, the SMTP server issues a

         235 reply.



         The service name specified by this protocol's profile of SASL

         is "smtp".



         If a security layer is negotiated through the SASL

         authentication exchange, it takes effect immediately following

         the CRLF that concludes the authentication exchange for the

         client, and the CRLF of the success reply for the server.  Upon

         a security layer's taking effect, the SMTP protocol is reset to

         the initial state (the state in SMTP after a server issues a

         220 service ready greeting).  The server MUST discard any

         knowledge obtained from the client, such as the argument to the

         EHLO command, which was not obtained from the SASL negotiation

         itself.  The client MUST discard any knowledge obtained from

         the server, such as the list of SMTP service extensions, which

         was not obtained from the SASL negotiation itself (with the

         exception that a client MAY compare the list of advertised SASL

         mechanisms before and after authentication in order to detect

         an active down-negotiation attack).  The client SHOULD send an

         EHLO command as the first command after a successful SASL

         negotiation which results in the enabling of a security layer.



         The server is not required to support any particular

         authentication mechanism, nor are authentication mechanisms

         required to support any security layers.  If an AUTH command

         fails, the client may try another authentication mechanism by

         issuing another AUTH command.







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         If an AUTH command fails, the server MUST behave the same as if

         the client had not issued the AUTH command.



         The BASE64 string may in general be arbitrarily long.  Clients

         and servers MUST be able to support challenges and responses

         that are as long as are generated by the authentication

         mechanisms they support, independent of any line length

         limitations the client or server may have in other parts of its

         protocol implementation.



     Examples:

         S: 220 smtp.example.com ESMTP server ready

         C: EHLO jgm.example.com

         S: 250-smtp.example.com

         S: 250 AUTH CRAM-MD5 DIGEST-MD5

         C: AUTH FOOBAR

         S: 504 Unrecognized authentication type.

         C: AUTH CRAM-MD5

         S: 334

         PENCeUxFREJoU0NnbmhNWitOMjNGNndAZWx3b29kLmlubm9zb2Z0LmNvbT4=

         C: ZnJlZCA5ZTk1YWVlMDljNDBhZjJiODRhMGMyYjNiYmFlNzg2ZQ==

         S: 235 Authentication successful.







5. The AUTH parameter to the MAIL FROM command



   AUTH=addr-spec



   Arguments:

       An addr-spec containing the identity which submitted the message

       to the delivery system, or the two character sequence "<>"

       indicating such an identity is unknown or insufficiently

       authenticated.  To comply with the restrictions imposed on ESMTP

       parameters, the addr-spec is encoded inside an xtext.  The syntax

       of an xtext is described in section 5 of [ESMTP-DSN].



   Discussion:

       The optional AUTH parameter to the MAIL FROM command allows

       cooperating agents in a trusted environment to communicate the

       authentication of individual messages.



       If the server trusts the authenticated identity of the client to

       assert that the message was originally submitted by the supplied

       addr-spec, then the server SHOULD supply the same addr-spec in an

       AUTH parameter when relaying the message to any server which

       supports the AUTH extension.









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       A MAIL FROM parameter of AUTH=<> indicates that the original

       submitter of the message is not known.  The server MUST NOT treat

       the message as having been originally submitted by the client.



       If the AUTH parameter to the MAIL FROM is not supplied, the

       client has authenticated, and the server believes the message is

       an original submission by the client, the server MAY supply the

       client's identity in the addr-spec in an AUTH parameter when

       relaying the message to any server which supports the AUTH

       extension.



       If the server does not sufficiently trust the authenticated

       identity of the client, or if the client is not authenticated,

       then the server MUST behave as if the AUTH=<> parameter was

       supplied.  The server MAY, however, write the value of the AUTH

       parameter to a log file.



       If an AUTH=<> parameter was supplied, either explicitly or due to

       the requirement in the previous paragraph, then the server MUST

       supply the AUTH=<> parameter when relaying the message to any

       server which it has authenticated to using the AUTH extension.



       A server MAY treat expansion of a mailing list as a new

       submission, setting the AUTH parameter to the mailing list

       address or mailing list administration address when relaying the

       message to list subscribers.



       It is conforming for an implementation to be hard-coded to treat

       all clients as being insufficiently trusted.  In that case, the

       implementation does nothing more than parse and discard

       syntactically valid AUTH parameters to the MAIL FROM command and

       supply AUTH=<> parameters to any servers to which it

       authenticates using the AUTH extension.



   Examples:

       C: MAIL FROM:<e=mc2@example.com> AUTH=e+3Dmc2@example.com

       S: 250 OK





























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6. Error Codes



   The following error codes may be used to indicate various conditions

   as described.



   432 A password transition is needed



   This response to the AUTH command indicates that the user needs to

   transition to the selected authentication mechanism.  This typically

   done by authenticating once using the PLAIN authentication mechanism.



   534 Authentication mechanism is too weak



   This response to the AUTH command indicates that the selected

   authentication mechanism is weaker than server policy permits for

   that user.



   538 Encryption required for requested authentication mechanism



   This response to the AUTH command indicates that the selected

   authentication mechanism may only be used when the underlying SMTP

   connection is encrypted.



   454 Temporary authentication failure



   This response to the AUTH command indicates that the authentication

   failed due to a temporary server failure.



   530 Authentication required



   This response may be returned by any command other than AUTH, EHLO,

   HELO, NOOP, RSET, or QUIT.  It indicates that server policy requires

   authentication in order to perform the requested action.





































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7. Formal Syntax



   The following syntax specification uses the augmented Backus-Naur

   Form (BNF) notation as specified in [ABNF].



   Except as noted otherwise, all alphabetic characters are case-

   insensitive.  The use of upper or lower case characters to define

   token strings is for editorial clarity only.  Implementations MUST

   accept these strings in a case-insensitive fashion.



   UPALPHA         = %x41-5A            ;; Uppercase: A-Z



   LOALPHA         = %x61-7A            ;; Lowercase: a-z



   ALPHA           = UPALPHA / LOALPHA  ;; case insensitive



   DIGIT           = %x30-39            ;; Digits 0-9



   HEXDIGIT        = %x41-46 / DIGIT    ;; hexidecimal digit (uppercase)



   hexchar         = "+" HEXDIGIT HEXDIGIT



   xchar           = %x21-2A / %x2C-3C / %x3E-7E

                     ;; US-ASCII except for "+", "=", SPACE and CTL



   xtext           = *(xchar / hexchar)



   AUTH_CHAR       = ALPHA / DIGIT / "-" / "_"



   auth_type       = 1*20AUTH_CHAR



   auth_command    = "AUTH" SPACE auth_type [SPACE (base64 / "=")]

                     *(CRLF [base64]) CRLF



   auth_param      = "AUTH=" xtext

                       ;; The decoded form of the xtext MUST be either

                       ;; an addr-spec or the two characters "<>"



   base64          = base64_terminal /

                     ( 1*(4base64_CHAR) [base64_terminal] )



   base64_char     = UPALPHA / LOALPHA / DIGIT / "+" / "/"

                     ;; Case-sensitive



   base64_terminal = (2base64_char "==") / (3base64_char "=")



   continue_req    = "334" SPACE [base64] CRLF









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   CR              = %x0C           ;; ASCII CR, carriage return



   CRLF            = CR LF



   CTL             = %x00-1F / %x7F ;; any ASCII control character and DEL



   LF              = %x0A           ;; ASCII LF, line feed



   SPACE           = %x20           ;; ASCII SP, space









8. References



   [ABNF]      Crocker, D. and P. Overell, "Augmented BNF for Syntax

               Specifications: ABNF", RFC 2234, November 1997.



   [CRAM-MD5]  Klensin, J., Catoe, R. and P. Krumviede, "IMAP/POP

               AUTHorize Extension for Simple Challenge/Response", RFC

               2195, September 1997.



   [ESMTP]     Klensin, J., Freed, N., Rose, M., Stefferud, E. and D.

               Crocker, "SMTP Service Extensions", RFC 1869, November

               1995.



   [ESMTP-DSN] Moore, K, "SMTP Service Extension for Delivery Status

               Notifications", RFC 1891, January 1996.



   [KEYWORDS]  Bradner, S., "Key words for use in RFCs to Indicate

               Requirement Levels", BCP 14, RFC 2119, March 1997.



   [SASL]      Myers, J., "Simple Authentication and Security Layer

               (SASL)", RFC 2222, October 1997.



   [SUBMIT]    Gellens, R. and J. Klensin, "Message Submission", RFC

               2476, December 1998.



   [RFC821]    Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC

               821, August 1982.



   [RFC822]    Crocker, D., "Standard for the Format of ARPA Internet

               Text Messages", STD 11, RFC 822, August 1982.

















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9. Security Considerations



   Security issues are discussed throughout this memo.



   If a client uses this extension to get an encrypted tunnel through an

   insecure network to a cooperating server, it needs to be configured

   to never send mail to that server when the connection is not mutually

   authenticated and encrypted.  Otherwise, an attacker could steal the

   client's mail by hijacking the SMTP connection and either pretending

   the server does not support the Authentication extension or causing

   all AUTH commands to fail.



   Before the SASL negotiation has begun, any protocol interactions are

   performed in the clear and may be modified by an active attacker.

   For this reason, clients and servers MUST discard any knowledge

   obtained prior to the start of the SASL negotiation upon completion

   of a SASL negotiation which results in a security layer.



   This mechanism does not protect the TCP port, so an active attacker

   may redirect a relay connection attempt to the submission port

   [SUBMIT].  The AUTH=<> parameter prevents such an attack from causing

   an relayed message without an envelope authentication to pick up the

   authentication of the relay client.



   A message submission client may require the user to authenticate

   whenever a suitable SASL mechanism is advertised.  Therefore, it may

   not be desirable for a submission server [SUBMIT] to advertise a SASL

   mechanism when use of that mechanism grants the client no benefits

   over anonymous submission.



   This extension is not intended to replace or be used instead of end-

   to-end message signature and encryption systems such as S/MIME or

   PGP.  This extension addresses a different problem than end-to-end

   systems; it has the following key differences:



      (1) it is generally useful only within a trusted enclave



      (2) it protects the entire envelope of a message, not just the

          message's body.



      (3) it authenticates the message submission, not authorship of the

          message content



      (4) it can give the sender some assurance the message was

          delivered to the next hop in the case where the sender

          mutually authenticates with the next hop and negotiates an

          appropriate security layer.









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   Additional security considerations are mentioned in the SASL

   specification [SASL].







10. Author's Address



   John Gardiner Myers

   Netscape Communications

   501 East Middlefield Road

   Mail Stop MV-029

   Mountain View, CA 94043



   EMail: jgmyers@netscape.com











































































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11.  Full Copyright Statement



   Copyright (C) The Internet Society (1999).  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.

















































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