One document matched: draft-ietf-sasl-crammd5-06.txt
Differences from draft-ietf-sasl-crammd5-05.txt
SASL Working Group L. Nerenberg, Ed.
Internet-Draft Orthanc Systems
Obsoletes: RFC2195 (if approved) March 1, 2006
Expires: September 2, 2006
The CRAM-MD5 SASL Mechanism
draft-ietf-sasl-crammd5-06
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on September 2, 2006.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document defines a simple challenge-response authentication
mechanism, using a keyed MD5 digest, for use with the Simple
Authentication and Security Layer (SASL).
Nerenberg Expires September 2, 2006 [Page 1]
Internet-Draft The CRAM-MD5 SASL Mechanism March 2006
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The CRAM-MD5 SASL Mechanism . . . . . . . . . . . . . . . . . 3
3. Formal Grammar . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Normative References . . . . . . . . . . . . . . . . . . . 5
5.2. Informative References . . . . . . . . . . . . . . . . . . 6
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 6
A.1. IMAP4 . . . . . . . . . . . . . . . . . . . . . . . . . . 6
A.1.1. Example 1: Simple IMAP . . . . . . . . . . . . . . . . 6
A.1.2. Example 2: IMAP4 with embedded spaces . . . . . . . . 7
A.1.3. Example 3: IMAP4 with Unicode characters . . . . . . . 7
A.2. ACAP . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
A.2.1. Example 4: Simple ACAP . . . . . . . . . . . . . . . . 8
Appendix B. IANA Considerations . . . . . . . . . . . . . . . . . 8
Appendix C. Contributors . . . . . . . . . . . . . . . . . . . . 8
Appendix D. Changes since RFC 2195 . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 11
Nerenberg Expires September 2, 2006 [Page 2]
Internet-Draft The CRAM-MD5 SASL Mechanism March 2006
1. Introduction
This document defines a simple challenge-response authentication
method, using a keyed MD5 [RFC2104] digest, for use with the Simple
Security and Authentication Layer (SASL) [I-D.ietf-sasl-rfc2222bis].
The mechanism name associated with CRAM-MD5 is 'CRAM-MD5'.
This mechanism is an improvement over plain text authentication
schemes, such as the SASL PLAIN [I-D.ietf-sasl-plain] mechanism, in
that it transmits the clients' authentication credentials in a secure
manner.
This mechanism does not provide a security layer.
2. The CRAM-MD5 SASL Mechanism
The mechanism starts with the server issuing a <challenge>. The data
encoded in the challenge contains a presumptively arbitrary string of
random data.
The client makes note of the data and then responds with a <response>
consisting of the <username>, a space, and a <digest>. The digest is
computed by applying the keyed MD5 algorithm from [RFC2104] where the
key is a shared secret and the digested text is the challenge
(including angle-brackets). The client MUST NOT interpret or attempt
to validate the contents of the challenge in any way.
This shared secret is a string known only to the client and server.
The digest parameter itself is a 16-octet value which is sent in a
restricted hexadecimal format (see the <digest> production in
Section 3).
When the server receives this client response, it verifies the digest
provided. Since the user name may contain the space character, the
server must take care to ensure the right-most space is recognised as
the token separating the user name from the digest. If the digest is
correct, the server should consider the client authenticated.
The client MUST prepare the user name and shared secret strings using
the SASLprep [RFC4013] profile of the Stringprep [RFC3454] algorithm.
The resulting values MUST be encoded as UTF-8 [RFC2279] strings. The
server may store the prepared string instead of, or as well as, the
unprepared string, so that it does not have to prepare it every time
it is needed for computation. However, if the original, unprepared
string, is not stored, it may render the computed secret to be
incompatible with a future revisions of SASLprep that support
currently unassigned code points (compare section 7 of Stringprep).
Nerenberg Expires September 2, 2006 [Page 3]
Internet-Draft The CRAM-MD5 SASL Mechanism March 2006
It is therefor recommended to store the unprepared string in the
database.
3. Formal Grammar
The following grammar specification uses the Augmented Backus-Naur
Form (ABNF) as specified in [RFC2234], and incorporates by reference
the Core Rules defined in that document.
challenge = "<" 3*(%x21-3B / %x3D / %x3F-7E) ">"
; a bracketed string of printing ASCII characters, not
; containing embedded "<" or ">"
digest = 32(DIGIT / %x61-66)
; A hexadecimal string, using ONLY lower-case
; letters
response = username SP digest
username = 1*OCTET
; Must be well-formed UTF-8.
4. Security Considerations
It is conjectured that use of the CRAM-MD5 authentication mechanism
provides replay protection for a session.
This mechanism does not obscure the user name in any way.
Accordingly, a server that implements both a clear-text password
command and this authentication type should not allow both methods of
access for a given user name.
Keyed MD5 is chosen for this application because of the greater
security imparted to authentication of short messages. In addition,
the use of the techniques described in [RFC2104] for pre-computation
of intermediate results make it possible to avoid explicit clear-text
storage of the shared secret on the server system by instead storing
the intermediate results which are known as "contexts." While the
saving, on the server, of the MD5 context is marginally better than
saving the shared secrets in clear-text, it is not sufficient to
protect the secrets if the server itself is compromised.
Consequently, servers that store the secrets or contexts must both be
protected to a level appropriate to the potential information value
in the data and services protected by this mechanism. In other
words, techniques like this one involve a trade-off between
vulnerability to network sniffing and I/O buffer snooping and
Nerenberg Expires September 2, 2006 [Page 4]
Internet-Draft The CRAM-MD5 SASL Mechanism March 2006
vulnerability of the server host's databases. If one believes that
the host and its databases are subject to compromise, and the network
is not, this technique (and all others like it) is unattractive. It
is perhaps even less attractive than clear-text passwords, which are
typically stored on hosts in one-way hash form. On the other hand,
if the server databases are perceived as reasonably secure, and one
is concerned about client-side or network interception of the
passwords (secrets), then this (and similar) techniques are
preferable to clear-text passwords by a wide margin.
As the length of the shared secret increases, so does the difficulty
of deriving it.
While there are now suggestions in the literature that the use of MD5
and keyed MD5 in authentication procedures probably has a limited
effective lifetime, the technique is now widely deployed and widely
understood. It is believed that this general understanding may
assist with the rapid replacement, by CRAM-MD5, of the current uses
of permanent clear-text passwords in many protocols. This document
has been deliberately written to permit easy upgrading to use SHA (or
whatever alternatives emerge) when they are considered to be widely
available and adequately safe.
Even with the use of CRAM-MD5, users are still vulnerable to active
attacks. An example of an increasingly common active attack is 'TCP
Session Hijacking' as described in CERT Advisory CA-95:01.
CRAM-MD5 does not authenticate the server and does not include a
client-supplied nonce. As a result, it is possible to construct a
server with a fixed challenge string that has pre-computed the hashes
for all possible passwords up to a certain length (or from a
dictionary). Such a server could then immediately determine the
user's password if it is sufficiently short.
5. References
5.1. Normative References
[I-D.ietf-sasl-rfc2222bis]
Melnikov, A. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", draft-ietf-sasl-rfc2222bis-15
(work in progress), January 2006.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
Nerenberg Expires September 2, 2006 [Page 5]
Internet-Draft The CRAM-MD5 SASL Mechanism March 2006
[RFC2234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
[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.
5.2. Informative References
[I-D.ietf-sasl-plain]
Zeilenga, K., "The Plain SASL Mechanism",
draft-ietf-sasl-plain-08 (work in progress), March 2005.
[RFC2244] Newman, C. and J. Myers, "ACAP -- Application
Configuration Access Protocol", RFC 2244, November 1997.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
Appendix A. Examples
The examples in this appendix DO NOT form part of the specification.
Where conflicts exist between the examples and the formal grammar or
the normative text in Section 2, the latter are authoritative.
A.1. IMAP4
These examples show the use of the CRAM-MD5 mechanism with the IMAP4
[RFC3501] AUTHENTICATE command. The base64 encoding of the
challenges and responses is part of the IMAP4 AUTHENTICATE command,
and not part of the CRAM-MD5 specification itself.
A.1.1. Example 1: Simple IMAP
In this example the shared secret is the string 'tanstaaftanstaaf'.
S: * OK [CAPABILITY IMAP4rev1 STARTTLS LOGINDISABLED AUTH=CRAM-MD5]
C: A0001 AUTHENTICATE CRAM-MD5
S: + PDE4OTYuNjk3MTcwOTUyQHBvc3RvZmZpY2UuZXhhbXBsZS5uZXQ+
C: am9lIDNkYmM4OGYwNjI0Nzc2YTczN2IzOTA5M2Y2ZWI2NDI3
S: A0001 OK CRAM-MD5 authentication successful
Nerenberg Expires September 2, 2006 [Page 6]
Internet-Draft The CRAM-MD5 SASL Mechanism March 2006
Hence, the keyed MD5 digest is produced by calculating
MD5((SASLprep(tanstaaftanstaaf) XOR opad),
MD5((SASLprep(tanstaaftanstaaf) XOR ipad),
<1896.697170952@postoffice.example.net>))
where ipad and opad are as defined in RFC 2104 and the string shown
in the challenge is the base64 encoding of
'<1896.697170952@postoffice.example.net>'. The shared secret is
null-padded to a length of 64 bytes. If the shared secret is longer
than 64 bytes, the MD5 digest of the shared secret is used as a 16
byte input to the keyed MD5 calculation.
This produces a digest value (in hexadecimal) of
'3dbc88f0624776a737b39093f6eb6427'. The user name is then prepended
to it, forming 'joe 3dbc88f0624776a737b39093f6eb6427', which is then
base64 encoded to meet the requirements of the IMAP4 AUTHENTICATE
command yielding 'am9lIDNkYmM4OGYwNjI0Nzc2YTczN2IzOTA5M2Y2ZWI2NDI3'.
A.1.2. Example 2: IMAP4 with embedded spaces
This example uses the user name 'Ali Baba' and the shared secret
'Open, Sesame'. It illustrates that both user names and passwords
may contain non-alphanumeric characters.
S: <68451038525716401353.0@localhost>
C: Ali Baba 6fa32b6e768f073132588e3418e00f71
A.1.3. Example 3: IMAP4 with Unicode characters
This example demonstrates the processing of Unicode strings. The raw
user name is 'Al<U+00AA>dd<U+00AD>in<U+00AE>' where <U+00AA> is the
Unicode Latin symbol <FEMININE ORDINAL INDICATOR>, <U+00AD> is <SOFT
HYPHEN>, and <U+00AE> is the <REGISTERED SIGN>. Preparing the raw
user name with SASLprep returns 'Aladdin<U+00AE>' which we then
encode into the UTF-8 string 'Aladdin\xC2\xAE' (shown here and below
using C-style string format notation). As before, the shared secret
is 'Open, Sesame'.
S: <92230559549732219941.0@localhost>
C: Aladdin\xC2\xAE 9950ea407844a71e2f0cd3284cbd912d
A.2. ACAP
An example of using CRAM-MD5 with ACAP [RFC2244].
Nerenberg Expires September 2, 2006 [Page 7]
Internet-Draft The CRAM-MD5 SASL Mechanism March 2006
A.2.1. Example 4: Simple ACAP
This example uses the user name 'joe' and the shared secret
'tanstaaftanstaaf'.
S: * ACAP (IMPLEMENTATION "Infotrope ACAP Server, version 0.1.3,
Copyright 2002-2004 Dave Cridland <dave@cridland.net>")
(SASL "PLAIN" "DIGEST-MD5" "CRAM-MD5" "ANONYMOUS") (STARTTLS)
C: AUTH AUTHENTICATE "CRAM-MD5"
S: + {43}
S: <2262304172.6455022@gw2.gestalt.entity.net>
C: {36+}
C: joe 2aa383bf320a941d8209a7001ef6aeb6
S: AUTH OK "You're logged in as joe. Frooby."
Appendix B. IANA Considerations
It is requested that the Internet Assigned Numbers Authority (IANA)
update the SASL Mechanism Registry entry for CRAM-MD5 to refer to
this document.
To: iana@iana.org
Subject: Updated Registration of SASL CRAM-MD5 mechanism.
SASL mechanism name: CRAM-MD5
Security considerations: See RFC XXXX
Published specification: RFC XXXX
Person & email address to contact for further information:
Lyndon Nerenberg <lyndon+rfc-crammd5@orthanc.ca>
IETF SASL WG <ietf-sasl@imc.org>
Appendix C. Contributors
The CRAM-MD5 mechanism was originally specified in RFC 2095, IMAP/POP
AUTHorize Extension for Simple Challenge/Response. The authors of
that document -- John C. Klensin, Paul Krumviede, and Randy Catoe --
are to be credited with the design and specification of CRAM-MD5, and
they are the original authors of the majority of the text in this
document. This memo serves only to re-state CRAM-MD5 within the
formal context of SASL, which specification it preceded by several
months.
Dave Cridland and Simon Josefsson contributed updated examples.
Nerenberg Expires September 2, 2006 [Page 8]
Internet-Draft The CRAM-MD5 SASL Mechanism March 2006
Appendix D. Changes since RFC 2195
The syntax of the <challenge> has been relaxed.
Both the user name and the shared secret (password) must be prepared
using SASLprep, and the resulting values encoded as UTF-8 strings.
Nerenberg Expires September 2, 2006 [Page 9]
Internet-Draft The CRAM-MD5 SASL Mechanism March 2006
Author's Address
Lyndon Nerenberg (editor)
Orthanc Systems
304 - 1755 Robson Street
Vancouver, BC V6G 3B7
Canada
Email: lyndon+rfc-crammd5@orthanc.ca
Nerenberg Expires September 2, 2006 [Page 10]
Internet-Draft The CRAM-MD5 SASL Mechanism March 2006
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM 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.
Copyright Statement
Copyright (C) The Internet Society (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Nerenberg Expires September 2, 2006 [Page 11]
| PAFTECH AB 2003-2026 | 2026-04-23 09:53:56 |