Network Working Group                                           J. Myers
Internet Draft: SMTP Authentication                        February 1998
Document: draft-myers-smtp-auth-11.txt

                         SMTP Service Extension
                           for Authentication
Status of this Memo

   This document is an Internet Draft.  Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas,
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   working documents as Internet Drafts.

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   A revised version of this draft document will be submitted to the RFC
   editor as a Proposed Standard for the Internet Community.  Discussion
   and suggestions for improvement are requested.  This document will
   expire before July 1996.  Distribution of this draft is unlimited.

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].

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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"

   (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.

4. The AUTH command

   AUTH mechanism [initial-response]

         a string identifying a SASL authentication mechanism.
         an optional base64-encoded response

         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 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

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         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.

         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.
         If the initial-response argument to the AUTH command is used
         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.

         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.  In other words, the client may
         request authentication types in decreasing order of preference.

         If an AUTH command fails, the server MUST behave the same as if
         the client had not issued the AUTH command.

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         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.

         S: 220 ESMTP server ready
         C: EHLO
         S: 250 AUTH SKEY PLAIN
         C: AUTH FOOBAR
         S: 504 Unrecognized authentication type
         C: AUTH SKEY c21pdGg=
         S: 334 OTUgUWE1ODMwOA==
         C: BsAY3g4gBNo=
         S: 235 S/Key authentication successful

5. The AUTH parameter to the MAIL FROM command


       An addr-spec containing the identity which submitted the message
       to the delivery system.  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].

       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.

       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 no AUTH parameter was supplied.
       The server MAY, however, place the value of the AUTH parameter in
       a comment in the inserted Received: header and/or write it to a
       log file.

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       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.

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6. Error Codes

   The following error codes may be used to indicate various conditions
   as described.

   422 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.

   522 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.

   523 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.

   505 Authentication required

   This response may be returned by any command other than AUTH, EHLO,
   NOOP, 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 [RFC822].

   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.

   ATOM_CHAR       = <any CHAR except atom_specials>

   atom_specials   = "(" / ")" / "{" / SPACE / CTLs / "%" / "*" /
                       <"> / "\"

   auth_command    = "AUTH" SPACE auth_type [SPACE base64]
       *(CRLF base64) CRLF

   auth_param      = "AUTH=" xtext
                       ;; xtext is defined in section 5 of RFC 1891
                       ;; The decoded form of the xtext MUST be an addr-spec

   auth_type       = 1*ATOM_CHAR

   base64          = *(4base64_CHAR) [base64_terminal]

   base64_char     = "A" / "B" / "C" / "D" / "E" / "F" / "G" / "H" /
                     "I" / "J" / "K" / "L" / "M" / "N" / "O" / "P" /
                     "Q" / "R" / "S" / "T" / "U" / "V" / "W" / "X" /
                     "Y" / "Z" /
                     "a" / "b" / "c" / "d" / "e" / "f" / "g" / "h" /
                     "i" / "j" / "k" / "l" / "m" / "n" / "o" / "p" /
                     "q" / "r" / "s" / "t" / "u" / "v" / "w" / "x" /
                     "y" / "z" /
                     "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7" /
                     "8" / "9" / "+" / "/"
                       ;; Case-sensitive

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

   CHAR            = <any 7-bit US-ASCII character except NUL,
                        0x01 - 0x7f>

   continue_req    = "334" SPACE base64 CRLF

   CR              = <ASCII CR, carriage return, 0x0C>

   CRLF            = CR LF

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   CTL             = <any ASCII control character and DEL,
                      0x00 - 0x1f, 0x7f>

   LF              = <ASCII LF, line feed, 0x0A>

   SPACE           = <ASCII SP, space, 0x20>

8. References

   [ESMTP] Klensin et al, "SMTP Service Extensions", RFC 1869, November

   [ESMTP-DSN] Moore, K, "SMTP Service Extension for Delivery Status
   Notifications", RFC 1890.

   [KEYWORDS] Bradner, "Key words for use in RFCs to Indicate
   Requirement Levels", RFC 2119, March 1997.

   [SASL]  Myers, J., "Simple Authentication and Security Layer",
   draft-myers-auth-sasl-13.txt, November 1997.

   [RFC821] Postel, J., "Simple Mail Transfer Protocol", RFC 821, August

   [RFC822] Crocker, D., "Standard for the Format of ARPA Internet Text
   Messages", RFC 822, August 1982.

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.

   This extension does not provide a defined mechanism for
   authentication using a plaintext password.  This omission is

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

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   to-end message signature and encryption systems such as PEM 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.

       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


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