Network Working Group R. Siemborski

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.

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RFC 5034 POP3 SASL Authentication Mechanism July 2007

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

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RFC 5034 POP3 SASL Authentication Mechanism July 2007

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.

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RFC 5034 POP3 SASL Authentication Mechanism July 2007

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

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RFC 5034 POP3 SASL Authentication Mechanism July 2007

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.

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RFC 5034 POP3 SASL Authentication Mechanism July 2007

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

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RFC 5034 POP3 SASL Authentication Mechanism July 2007

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

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RFC 5034 POP3 SASL Authentication Mechanism July 2007

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

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RFC 5034 POP3 SASL Authentication Mechanism July 2007

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.

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RFC 5034 POP3 SASL Authentication Mechanism July 2007

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.

Siemborski & Menon-Sen Standards Track [Page 10]

RFC 5034 POP3 SASL Authentication Mechanism July 2007

[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: [email protected]

Abhijit Menon-Sen
Oryx Mail Systems GmbH

EMail: [email protected]

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RFC 5034 POP3 SASL Authentication Mechanism July 2007

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