POP3 SASL: The Post Office Protocol (POP3) Simple Authentication and Security Layer (SASL) Authentication Mechanism

Network Working Group                                      R. Siemborski
Request for Comments: 5034                                  Google, Inc.
Obsoletes: 1734                                             A. Menon-Sen
Updates: 2449                                     Oryx Mail Systems GmbH
Category: Standards Track                                      July 2007


                    The Post Office Protocol (POP3)
Simple Authentication and Security Layer (SASL) Authentication Mechanism

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 IETF Trust (2007).

Abstract

   This document defines a profile of the Simple Authentication and
   Security Layer (SASL) for the Post Office Protocol (POP3).  This
   extension allows a POP3 client to indicate an authentication
   mechanism to the server, perform an authentication protocol exchange,
   and optionally negotiate a security layer for subsequent protocol
   interactions during this session.

   This document seeks to consolidate the information related to POP3
   AUTH into a single document.  To this end, this document obsoletes
   and replaces RFC 1734, and updates the information contained in
   Section 6.3 of RFC 2449.

1.  Introduction

   The POP3 (see [RFC1939]) AUTH command (see [RFC1734]) has suffered
   several problems in its specification.  The first is that it was very
   similar to a SASL framework defined by [RFC4422], but pre-dated the
   initial SASL specification.  It was therefore missing some key
   components, such as a way to list the available authentication
   mechanisms.

   Later, [RFC2449] attempted to remedy this situation by adding the
   CAPA command and allowing an initial client response with the AUTH
   command, but problems remained in the clarity of the specification of
   how the initial client response was to be handled.

   Together, this means creating a full POP3 AUTH implementation
   requires an understanding of material in at least five different
   documents (and [RFC3206] provides additional response codes that are
   useful during authentication).

   This document attempts to combine the information in [RFC1734] and
   [RFC2449] to simplify this situation.  Additionally, it aims to
   clarify and update the older specifications where appropriate.

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

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

   Formal syntax is defined by [RFC4234].

3.  The SASL Capability

   This section supersedes the definition of the SASL Capability in
   section 6.3 of [RFC2449].

   CAPA tag:
      SASL

   Arguments:
      Supported SASL Mechanisms

   Added commands:
      AUTH
   Standard commands affected:
      None

   Announced states / possible differences:
      both / no

   Commands valid in states:
      AUTHORIZATION

   Specification reference:
      This document and [RFC4422]

   Discussion:
      The SASL capability permits the use of the AUTH command (as
      defined in Section 4 of this document) to begin a SASL negotiation
      (as defined in [RFC4422]).  The argument to the SASL capability is
      a space-separated list of SASL mechanisms that are supported.

      If a server either does not support the CAPA command or does not
      advertise the SASL capability, clients SHOULD NOT attempt the AUTH
      command.  If a client does attempt the AUTH command in such a
      situation, it MUST NOT supply the client initial response
      parameter (for backwards compatibility with [RFC1734]).

      Note that the list of available mechanisms MAY change after a
      successful STLS command (see [RFC2595]).  However, as required by
      [RFC2449], implementations MUST continue to include the SASL
      capability even after a successful AUTH command has been completed
      (even though no further AUTH commands may be issued).

   Example
      S: +OK pop.example.com BlurdyBlurp POP3 server ready
      C: CAPA
      S: +OK List of capabilities follows
      S: SASL PLAIN DIGEST-MD5 GSSAPI ANONYMOUS
      S: STLS
      S: IMPLEMENTATION BlurdyBlurp POP3 server
      S: .

4.  The AUTH Command

   AUTH mechanism [initial-response]

      Arguments:

         mechanism: A string identifying a SASL authentication
         mechanism.

         initial-response: An optional initial client response, as
         defined in Section 3 of [RFC4422].  If present, this response
         MUST be encoded as Base64 (specified in Section 4 of
         [RFC4648]), or consist only of the single character "=", which
         represents an empty initial response.

      Restrictions:

         After an AUTH command has been 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 an -ERR reply.

         The AUTH command may only be given during the AUTHORIZATION
         state.

      Discussion:

         The AUTH command initiates a SASL authentication exchange
         between the client and the server.  The client identifies the
         SASL mechanism to use with the first parameter of the AUTH
         command.  If the server supports the requested authentication
         mechanism, it performs the SASL exchange to authenticate the
         user.  Optionally, it also negotiates a security layer for
         subsequent protocol interactions during this session.  If the
         requested authentication mechanism is not supported, the server
         rejects the AUTH command with an -ERR reply.

         The authentication protocol exchange consists of a series of
         server challenges and client responses that are specific to the
         chosen SASL mechanism.

         A server challenge is sent as a line consisting of a "+"
         character, followed by a single space and a string encoded
         using Base64, as specified in Section 4 of [RFC4648].  This
         line MUST NOT contain any text other than the BASE64-encoded
         challenge.

         A client response consists of a line containing a string
         encoded as Base64.  If the client wishes to cancel the
         authentication exchange, it issues a line with a single "*".
         If the server receives such a response, it MUST reject the AUTH
         command by sending an -ERR reply.

         The optional initial-response argument to the AUTH command is
         used to save a round trip when using authentication mechanisms
         that support an initial client response.  If the initial
         response argument is omitted and the chosen mechanism requires
         an initial client response, the server MUST proceed by issuing
         an empty challenge, as defined in Section 3 of [RFC4422].  In
         POP3, an empty server challenge is defined as a line with only
         a "+", followed by a single space.  It MUST NOT contain any
         other data.

         For the purposes of the initial client response, the 255-octet
         limit on the length of a single command, defined in Section 4
         of [RFC2449], still applies.  If specifying an initial response
         would cause the AUTH command to exceed this length, the client
         MUST NOT use the initial-response parameter (and must proceed
         instead by sending its initial response after an empty
         challenge from the server, as in Section 3 of [RFC4422]).

         If the client needs to send a zero-length initial response, it
         MUST transmit the response as a single equals sign ("=").  This
         indicates that the response is present, but contains no data.

         If the client uses an initial-response argument to the AUTH
         command with a SASL mechanism that does not support an initial
         client send, the server MUST reject the AUTH command with an
         -ERR reply.

         If the server cannot Base64 decode a client response, it MUST
         reject the AUTH command with an -ERR reply.  If the client
         cannot Base64 decode any of the server's challenges, it MUST
         cancel the authentication using the "*" response.  In
         particular, servers and clients MUST reject (and not ignore)
         any character not explicitly allowed by the Base64 alphabet,
         and MUST reject any sequence of Base64 characters that contains
         the pad character ('=') anywhere other than the end of the
         string (e.g., "=AAA" and "AAA=BBB" are not allowed).

         Excepting the initial client response, these BASE64 strings may
         be of arbitrary length, depending on the authentication
         mechanism in use.  Clients and servers MUST be able to handle
         the largest encoded challenges and responses generated by the
         authentication mechanisms they support.  This requirement is
         independent of any line-length limitations the client or server
         may have in other parts of its protocol implementation.

         If the server is unable to authenticate the client, it MUST
         reject the AUTH command with an -ERR reply.  Should the client
         successfully complete the exchange, the server issues a +OK
         reply.  Additionally, upon success, the POP3 session enters the
         TRANSACTION state.

         The authorization identity generated by the SASL exchange is a
         simple username, and SHOULD use the SASLprep profile (see
         [RFC4013]) of the StringPrep algorithm (see [RFC3454]) to
         prepare these names for matching.  If preparation of the
         authorization identity fails or results in an empty string
         (unless it was transmitted as the empty string), the server
         MUST fail the authentication.

         If a security layer is negotiated during the SASL exchange, it
         takes effect for the client on the octet immediately following
         the CRLF that concludes the last response generated by the
         client.  For the server, it takes effect immediately following
         the CRLF of its success reply.

         When a security layer takes effect, the server MUST discard any
         knowledge previously obtained from the client, which was not
         obtained from the SASL negotiation itself.  Likewise, the
         client MUST discard any knowledge obtained from the server,
         such as the list of available POP3 service extensions.

         When both Transport Layer Security (TLS) (see [RFC4346]) and
         SASL security layers are in effect, the TLS encoding MUST be
         applied after the SASL encoding when sending data.  (According
         to [RFC2595], STLS can only be issued before AUTH in any case.)

         Note that POP3 does not allow for additional data to be sent
         with a message indicating a successful outcome (see Section 3.6
         of [RFC4422]).

         The service name specified by this protocol's profile of SASL
         is "pop".

         If an AUTH command fails, the client may try another
         authentication mechanism or present different credentials by
         issuing another AUTH command (or by using one of the other POP3
         authentication mechanisms).  Likewise, the server MUST behave
         as if the client had not issued the AUTH command.

         To ensure interoperability, client and server implementations
         of this extension MUST implement the PLAIN SASL mechanism
         [RFC4616] running over TLS [RFC2595].

         A server implementation MUST implement a configuration in which
         it does NOT advertise or permit any plaintext password
         mechanisms, unless the STLS command has been used to negotiate
         a TLS session (see [RFC2595]).  As described by RFC 4616, this
         configuration SHOULD be the default configuration.  Before
         using a plaintext password mechanism over a TLS session, client
         implementations MUST verify the TLS server certificate as
         required by RFC 2595, Section 2.4.  Client and server
         implementations SHOULD implement additional SASL mechanisms
         that do not send plaintext passwords, such as the GSSAPI
         [RFC4752] mechanism.

5.  Formal Syntax

   The following syntax specification uses the Augmented Backus-Naur
   Form notation as specified in [RFC4234].  The rules CRLF, ALPHA, and
   DIGIT are imported from [RFC4234].  The sasl-mech rule is from
   [RFC4422].

   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.

      auth-command     = "AUTH" SP sasl-mech [SP initial-response]
                         *(CRLF [base64]) [CRLF cancel-response] CRLF

      initial-response = base64 / "="

      cancel-response  = "*"

      base64           = base64-terminal /
                         ( 1*(4base64-CHAR) [base64-terminal] )

      base64-char      = ALPHA / DIGIT / "+" / "/"
                         ;; Case-sensitive

      base64-terminal  = (2base64-char "==") / (3base64-char "=")

      continue-req     = "+" SP [base64] CRLF

   Additionally, the ABNF specified in [RFC2449] is updated as follows:

      response         =/ continue-req

6.  Examples

   Here is an example of a client attempting AUTH PLAIN (see [RFC4616])
   under TLS and making use of the initial client response:

        S: +OK pop.example.com BlurdyBlurp POP3 server ready
        C: CAPA
        S: +OK List of capabilities follows
        S: SASL DIGEST-MD5 GSSAPI ANONYMOUS
        S: STLS
        S: IMPLEMENTATION BlurdyBlurp POP3 server
        S: .
        C: STLS
        S: +OK Begin TLS negotiation now
            (TLS negotiation proceeds, further commands protected by TLS
            layer)
        C: CAPA
        S: +OK List of capabilities follows
        S: SASL PLAIN DIGEST-MD5 GSSAPI ANONYMOUS
        S: IMPLEMENTATION BlurdyBlurp POP3 server
        S: .
        C: AUTH PLAIN dGVzdAB0ZXN0AHRlc3Q=
        S: +OK Maildrop locked and ready

   Here is another client that is attempting AUTH PLAIN under a TLS
   layer, this time without the initial response.  Parts of the
   negotiation before the TLS layer was established have been omitted:

            (TLS negotiation proceeds, further commands protected by TLS
            layer)
        C: CAPA
        S: +OK List of capabilities follows
        S: SASL PLAIN DIGEST-MD5 GSSAPI ANONYMOUS
        S: IMPLEMENTATION BlurdyBlurp POP3 server
        S: .
        C: AUTH PLAIN
            (note that there is a space following the '+' on the
            following line)
        S: +
        C: dGVzdAB0ZXN0AHRlc3Q=
        S: +OK Maildrop locked and ready

   Here is an example using a mechanism in which the exchange begins
   with a server challenge (the long lines are broken for editorial
   clarity only):

         S: +OK pop.example.com BlurdyBlurp POP3 server ready
         C: CAPA
         S: +OK List of capabilities follows
         S: SASL DIGEST-MD5 GSSAPI ANONYMOUS
         S: STLS
         S: IMPLEMENTATION BlurdyBlurp POP3 server
         S: .
         C: AUTH DIGEST-MD5
         S: + cmVhbG09ImVsd29vZC5pbm5vc29mdC5jb20iLG5vbmNlPSJPQTZNRzl0
              RVFHbTJoaCIscW9wPSJhdXRoIixhbGdvcml0aG09bWQ1LXNlc3MsY2hh
              cnNldD11dGYtOA==
         C: Y2hhcnNldD11dGYtOCx1c2VybmFtZT0iY2hyaXMiLHJlYWxtPSJlbHdvb2
            QuaW5ub3NvZnQuY29tIixub25jZT0iT0E2TUc5dEVRR20yaGgiLG5jPTAw
            MDAwMDAxLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLGRpZ2VzdC11cmk9In
            BvcC9lbHdvb2QuaW5ub3NvZnQuY29tIixyZXNwb25zZT1iMGQ1NmQyZjA1
            NGMyNGI2MjA3MjMyMjEwNjQ2OGRiOSxxb3A9YXV0aA==
         S: + cnNwYXV0aD0wYjk3MTQ2MmNlZjVlOGY5MzBkYjlhMzNiMDJmYzlhMA==
         C:
         S: +OK Maildrop locked and ready

7.  Security Considerations

   Security issues are discussed throughout this document.

8.  IANA Considerations

   The IANA has updated its site to refer to this RFC instead of
   [RFC1734] in http://www.iana.org/assignments/pop3-extension-mechanism
   (the POP3 extension registry), and also in
   http://www.iana.org/assignments/gssapi-service-names (the GSSAPI/SASL
   service name registry).

9.  Acknowledgments

   The authors would like to acknowledge the contributions of John
   Myers, Randall Gellens, Chris Newman, Laurence Lundblade, and other
   contributors to RFC 1734 and RFC 2554, on which this document draws
   heavily.

   The authors would also like to thank Ken Murchison, Randall Gellens,
   Alexey Melnikov, Mark Crispin, Arnt Gulbrandsen, Lisa Dusseault,
   Frank Ellermann, and Philip Guenther for their reviews of this
   document.

10.  Changes From RFC 1734, RFC 2449.

   1. Updated references to newer versions of various specifications,
       particularly RFC 4422.

   2. The SASL-based semantics defined in RFC 2449 are now normative for
       the AUTH extension.

   3. The proper behaviour and handling of initial client responses is
       defined, with examples and references to SASL.

   4. New minimum requirement of support for TLS+PLAIN.

   5. The SASLprep profile SHOULD be used to prepare authorization
       identities.

   6. Clarify that the TLS encoding should be applied after any encoding
       applied by SASL security layers.

   7. Note that the mechanism list can change after STLS.

   8. Explicitly mention that "=" means a zero-length initial response.

   9. Note that POP3 doesn't allow additional data to be sent with +OK.

11. Normative References

   [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2449]  Gellens, R., Newman, C., and L. Lundblade, "POP3 Extension
              Mechanism", RFC 2449, November 1998.

   [RFC2595]  Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
              2595, June 1999.

   [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              December 2002.

   [RFC4013]  Zeilenga, K., "SASLprep: Stringprep Profile for User Names
              and Passwords", RFC 4013, February 2005.

   [RFC4234]  Crocker, D., Ed., and P. Overell, "Augmented BNF for
              Syntax Specifications: ABNF", RFC 4234, October 2005.

   [RFC4422]  Melnikov, A., Ed., and K. Zeilenga, Ed., "Simple
              Authentication and Security Layer (SASL)", RFC 4422, June
              2006.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [RFC4616]  Zeilenga, K., Ed., "The PLAIN Simple Authentication and
              Security Layer (SASL) Mechanism", RFC 4616, August 2006.

12. Informative References

   [RFC1734]  Myers, J., "POP3 AUTHentication command", RFC 1734,
              December 1994.

   [RFC3206]  Gellens, R., "The SYS and AUTH POP Response Codes", RFC
              3206, February 2002.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [RFC4752]  Melnikov, A., Ed., "The Kerberos V5 ("GSSAPI") Simple
              Authentication and Security Layer (SASL) Mechanism", RFC
              4752, November 2006.

Authors' Addresses

   Robert Siemborski
   Google, Inc.
   1600 Ampitheatre Parkway
   Mountain View, CA 94043

   Phone: +1 650 623 6925
   EMail: robsiemb@google.com


   Abhijit Menon-Sen
   Oryx Mail Systems GmbH

   EMail: ams@oryx.com

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