Network Working Group K. Zeilenga, Ed.

Network Working Group K. Zeilenga, Ed.
Request for Comments: 4616 OpenLDAP Foundation
Updates: 2595 August 2006
Category: Standards Track

The PLAIN Simple Authentication and Security Layer (SASL) 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 Internet Society (2006).

Abstract

This document defines a simple clear-text user/password Simple
Authentication and Security Layer (SASL) mechanism called the PLAIN
mechanism. The PLAIN mechanism is intended to be used, in
combination with data confidentiality services provided by a lower
layer, in protocols that lack a simple password authentication
command.

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RFC 4616 The PLAIN SASL Mechanism August 2006

1. Introduction

Clear-text, multiple-use passwords are simple, interoperate with
almost all existing operating system authentication databases, and
are useful for a smooth transition to a more secure password-based
authentication mechanism. The drawback is that they are unacceptable
for use over network connections where data confidentiality is not
ensured.

This document defines the PLAIN Simple Authentication and Security
Layer ([SASL]) mechanism for use in protocols with no clear-text
login command (e.g., [ACAP] or [SMTP-AUTH]). This document updates
RFC 2595, replacing Section 6. Changes since RFC 2595 are detailed
in Appendix A.

The name associated with this mechanism is "PLAIN".

The PLAIN SASL mechanism does not provide a security layer.

The PLAIN mechanism should not be used without adequate data security
protection as this mechanism affords no integrity or confidentiality
protections itself. The mechanism is intended to be used with data
security protections provided by application-layer protocol,
generally through its use of Transport Layer Security ([TLS])
services.

By default, implementations SHOULD advertise and make use of the
PLAIN mechanism only when adequate data security services are in
place. Specifications for IETF protocols that indicate that this
mechanism is an applicable authentication mechanism MUST mandate that
implementations support an strong data security service, such as TLS.

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 [Keywords].

2. PLAIN SASL Mechanism

The mechanism consists of a single message, a string of [UTF-8]
encoded [Unicode] characters, from the client to the server. The
client presents the authorization identity (identity to act as),
followed by a NUL (U+0000) character, followed by the authentication
identity (identity whose password will be used), followed by a NUL
(U+0000) character, followed by the clear-text password. As with
other SASL mechanisms, the client does not provide an authorization
identity when it wishes the server to derive an identity from the
credentials and use that as the authorization identity.

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RFC 4616 The PLAIN SASL Mechanism August 2006

The formal grammar for the client message using Augmented BNF [ABNF]
follows.

message = [authzid] UTF8NUL authcid UTF8NUL passwd
authcid = 1*SAFE ; MUST accept up to 255 octets
authzid = 1*SAFE ; MUST accept up to 255 octets
passwd = 1*SAFE ; MUST accept up to 255 octets
UTF8NUL = %x00 ; UTF-8 encoded NUL character

SAFE = UTF1 / UTF2 / UTF3 / UTF4
;; any UTF-8 encoded Unicode character except NUL

UTF1 = %x01-7F ;; except NUL
UTF2 = %xC2-DF UTF0
UTF3 = %xE0 %xA0-BF UTF0 / %xE1-EC 2(UTF0) /
%xED %x80-9F UTF0 / %xEE-EF 2(UTF0)
UTF4 = %xF0 %x90-BF 2(UTF0) / %xF1-F3 3(UTF0) /
%xF4 %x80-8F 2(UTF0)
UTF0 = %x80-BF

The authorization identity (authzid), authentication identity
(authcid), password (passwd), and NUL character deliminators SHALL be
transferred as [UTF-8] encoded strings of [Unicode] characters. As
the NUL (U+0000) character is used as a deliminator, the NUL (U+0000)
character MUST NOT appear in authzid, authcid, or passwd productions.

The form of the authzid production is specific to the application-
level protocol's SASL profile [SASL]. The authcid and passwd
productions are form-free. Use of non-visible characters or
characters that a user may be unable to enter on some keyboards is
discouraged.

Servers MUST be capable of accepting authzid, authcid, and passwd
productions up to and including 255 octets. It is noted that the
UTF-8 encoding of a Unicode character may be as long as 4 octets.

Upon receipt of the message, the server will verify the presented (in
the message) authentication identity (authcid) and password (passwd)
with the system authentication database, and it will verify that the
authentication credentials permit the client to act as the (presented
or derived) authorization identity (authzid). If both steps succeed,
the user is authenticated.

The presented authentication identity and password strings, as well
as the database authentication identity and password strings, are to
be prepared before being used in the verification process. The
[SASLPrep] profile of the [StringPrep] algorithm is the RECOMMENDED
preparation algorithm. The SASLprep preparation algorithm is

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RFC 4616 The PLAIN SASL Mechanism August 2006

recommended to improve the likelihood that comparisons behave in an
expected manner. The SASLprep preparation algorithm is not mandatory
so as to allow the server to employ other preparation algorithms
(including none) when appropriate. For instance, use of a different
preparation algorithm may be necessary for the server to interoperate
with an external system.

When preparing the presented strings using [SASLPrep], the presented
strings are to be treated as "query" strings (Section 7 of
[StringPrep]) and hence unassigned code points are allowed to appear
in their prepared output. When preparing the database strings using
[SASLPrep], the database strings are to be treated as "stored"
strings (Section 7 of [StringPrep]) and hence unassigned code points
are prohibited from appearing in their prepared output.

Regardless of the preparation algorithm used, if the output of a
non-invertible function (e.g., hash) of the expected string is
stored, the string MUST be prepared before input to that function.

Regardless of the preparation algorithm used, if preparation fails or
results in an empty string, verification SHALL fail.

When no authorization identity is provided, the server derives an
authorization identity from the prepared representation of the
provided authentication identity string. This ensures that the
derivation of different representations of the authentication
identity produces the same authorization identity.

The server MAY use the credentials to initialize any new
authentication database, such as one suitable for [CRAM-MD5] or
[DIGEST-MD5].

3. Pseudo-Code

This section provides pseudo-code illustrating the verification
process (using hashed passwords and the SASLprep preparation
function) discussed above. This section is not definitive.

boolean Verify(string authzid, string authcid, string passwd) {
string pAuthcid = SASLprep(authcid, true); # prepare authcid
string pPasswd = SASLprep(passwd, true); # prepare passwd
if (pAuthcid == NULL || pPasswd == NULL) {
return false; # preparation failed
}
if (pAuthcid == "" || pPasswd == "") {
return false; # empty prepared string
}

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RFC 4616 The PLAIN SASL Mechanism August 2006

storedHash = FetchPasswordHash(pAuthcid);
if (storedHash == NULL || storedHash == "") {
return false; # error or unknown authcid
}

if (!Compare(storedHash, Hash(pPasswd))) {
return false; # incorrect password
}

if (authzid == NULL ) {
authzid = DeriveAuthzid(pAuthcid);
if (authzid == NULL || authzid == "") {
return false; # could not derive authzid
}
}

if (!Authorize(pAuthcid, authzid)) {
return false; # not authorized
}

return true;
}

The second parameter of the SASLprep function, when true, indicates
that unassigned code points are allowed in the input. When the
SASLprep function is called to prepare the password prior to
computing the stored hash, the second parameter would be false.

The second parameter provided to the Authorize function is not
prepared by this code. The application-level SASL profile should be
consulted to determine what, if any, preparation is necessary.

Note that the DeriveAuthzid and Authorize functions (whether
implemented as one function or two, whether designed in a manner in
which these functions or whether the mechanism implementation can be
reused elsewhere) require knowledge and understanding of mechanism
and the application-level protocol specification and/or
implementation details to implement.

Note that the Authorize function outcome is clearly dependent on
details of the local authorization model and policy. Both functions
may be dependent on other factors as well.

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RFC 4616 The PLAIN SASL Mechanism August 2006

4. Examples

This section provides examples of PLAIN authentication exchanges.
The examples are intended to help the readers understand the above
text. The examples are not definitive.

"C:" and "S:" indicate lines sent by the client and server,
respectively. "<NUL>" represents a single NUL (U+0000) character.
The Application Configuration Access Protocol ([ACAP]) is used in the
examples.

The first example shows how the PLAIN mechanism might be used for
user authentication.

S: * ACAP (SASL "CRAM-MD5") (STARTTLS)
C: a001 STARTTLS
S: a001 OK "Begin TLS negotiation now"
<TLS negotiation, further commands are under TLS layer>
S: * ACAP (SASL "CRAM-MD5" "PLAIN")
C: a002 AUTHENTICATE "PLAIN"
S: + ""
C: {21}
C: <NUL>tim<NUL>tanstaaftanstaaf
S: a002 OK "Authenticated"

The second example shows how the PLAIN mechanism might be used to
attempt to assume the identity of another user. In this example, the
server rejects the request. Also, this example makes use of the
protocol optional initial response capability to eliminate a round-
trip.

S: * ACAP (SASL "CRAM-MD5") (STARTTLS)
C: a001 STARTTLS
S: a001 OK "Begin TLS negotiation now"
<TLS negotiation, further commands are under TLS layer>
S: * ACAP (SASL "CRAM-MD5" "PLAIN")
C: a002 AUTHENTICATE "PLAIN" {20+}
C: Ursel<NUL>Kurt<NUL>xipj3plmq
S: a002 NO "Not authorized to requested authorization identity"

5. Security Considerations

As the PLAIN mechanism itself provided no integrity or
confidentiality protections, it should not be used without adequate
external data security protection, such as TLS services provided by
many application-layer protocols. By default, implementations SHOULD
NOT advertise and SHOULD NOT make use of the PLAIN mechanism unless
adequate data security services are in place.

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RFC 4616 The PLAIN SASL Mechanism August 2006

When the PLAIN mechanism is used, the server gains the ability to
impersonate the user to all services with the same password
regardless of any encryption provided by TLS or other confidentiality
protection mechanisms. Whereas many other authentication mechanisms
have similar weaknesses, stronger SASL mechanisms address this issue.
Clients are encouraged to have an operational mode where all
mechanisms that are likely to reveal the user's password to the
server are disabled.

General [SASL] security considerations apply to this mechanism.

Unicode, [UTF-8], and [StringPrep] security considerations also
apply.

6. IANA Considerations

The SASL Mechanism registry [IANA-SASL] entry for the PLAIN mechanism
has been updated by the IANA to reflect that this document now
provides its technical specification.

To: [email protected]
Subject: Updated Registration of SASL mechanism PLAIN

SASL mechanism name: PLAIN
Security considerations: See RFC 4616.
Published specification (optional, recommended): RFC 4616
Person & email address to contact for further information:
Kurt Zeilenga <[email protected]>
IETF SASL WG <[email protected]>
Intended usage: COMMON
Author/Change controller: IESG <[email protected]>
Note: Updates existing entry for PLAIN

7. Acknowledgements

This document is a revision of RFC 2595 by Chris Newman. Portions of
the grammar defined in Section 2 were borrowed from [UTF-8] by
Francois Yergeau.

This document is a product of the IETF Simple Authentication and
Security Layer (SASL) Working Group.

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RFC 4616 The PLAIN SASL Mechanism August 2006

8. Normative References

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

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

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

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

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

[Unicode] The Unicode Consortium, "The Unicode Standard, Version
3.2.0" is defined by "The Unicode Standard, Version
3.0" (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-
61633-5), as amended by the "Unicode Standard Annex
#27: Unicode 3.1"
(http://www.unicode.org/reports/tr27/) and by the
"Unicode Standard Annex #28: Unicode 3.2"
(http://www.unicode.org/reports/tr28/).

[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.

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

9. Informative References

[ACAP] Newman, C. and J. Myers, "ACAP -- Application
Configuration Access Protocol", RFC 2244, November
1997.

[CRAM-MD5] Nerenberg, L., Ed., "The CRAM-MD5 SASL Mechanism", Work
in Progress, June 2006.

[DIGEST-MD5] Melnikov, A., Ed., "Using Digest Authentication as a
SASL Mechanism", Work in Progress, June 2006.

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RFC 4616 The PLAIN SASL Mechanism August 2006

[IANA-SASL] IANA, "SIMPLE AUTHENTICATION AND SECURITY LAYER (SASL)
MECHANISMS",
<http://www.iana.org/assignments/sasl-mechanisms>.

[SMTP-AUTH] Myers, J., "SMTP Service Extension for Authentication",
RFC 2554, March 1999.

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RFC 4616 The PLAIN SASL Mechanism August 2006

Appendix A. Changes since RFC 2595

This appendix is non-normative.

This document replaces Section 6 of RFC 2595.

The specification details how the server is to compare client-
provided character strings with stored character strings.

The ABNF grammar was updated. In particular, the grammar now allows
LINE FEED (U+000A) and CARRIAGE RETURN (U+000D) characters in the
authzid, authcid, passwd productions. However, whether these control
characters may be used depends on the string preparation rules
applicable to the production. For passwd and authcid productions,
control characters are prohibited. For authzid, one must consult the
application-level SASL profile. This change allows PLAIN to carry
all possible authorization identity strings allowed in SASL.

Pseudo-code was added.

The example section was expanded to illustrate more features of the
PLAIN mechanism.

Editor's Address

Kurt D. Zeilenga
OpenLDAP Foundation

EMail: [email protected]

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RFC 4616 The PLAIN SASL Mechanism August 2006

Full Copyright Statement

Copyright (C) The Internet Society (2006).

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