Network Working Group J. Klensin
Request for Comments: 2195 R. Catoe
Category: Standards Track P. Krumviede
Obsoletes: 2095 MCI
September 1997 IMAP/POP AUTHorize Extension for Simple Challenge/Response
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.
While IMAP4 supports a number of strong authentication mechanisms as
described in RFC 1731, it lacks any mechanism that neither passes
cleartext, reusable passwords across the network nor requires either
a significant security infrastructure or that the mail server update
a mail-system-wide user authentication file on each mail access.
This specification provides a simple challenge-response
authentication protocol that is suitable for use with IMAP4. Since
it utilizes Keyed-MD5 digests and does not require that the secret be
stored in the clear on the server, it may also constitute an
improvement on APOP for POP3 use as specified in RFC 1734.
Existing Proposed Standards specify an AUTHENTICATE mechanism for the
IMAP4 protocol [IMAP, IMAP-AUTH] and a parallel AUTH mechanism for
the POP3 protocol [POP3-AUTH]. The AUTHENTICATE mechanism is
intended to be extensible; the four methods specified in [IMAP-AUTH]
are all fairly powerful and require some security infrastructure to
support. The base POP3 specification [POP3] also contains a
lightweight challenge-response mechanism called APOP. APOP is
associated with most of the risks associated with such protocols: in
particular, it requires that both the client and server machines have
access to the shared secret in cleartext form. CRAM offers a method
for avoiding such cleartext storage while retaining the algorithmic
simplicity of APOP in using only MD5, though in a "keyed" method.
At present, IMAP [IMAP] lacks any facility corresponding to APOP.
The only alternative to the strong mechanisms identified in [IMAP-AUTH] is a presumably cleartext username and password, supported
through the LOGIN command in [IMAP]. This document describes a
simple challenge-response mechanism, similar to APOP and PPP CHAP
[PPP], that can be used with IMAP (and, in principle, with POP3).
This mechanism also has the advantage over some possible alternatives
of not requiring that the server maintain information about email
"logins" on a per-login basis. While mechanisms that do require such
per-login history records may offer enhanced security, protocols such
as IMAP, which may have several connections between a given client
and server open more or less simultaneous, may make their
implementation particularly challenging.
2. Challenge-Response Authentication Mechanism (CRAM)
The authentication type associated with CRAM is "CRAM-MD5".
The data encoded in the first ready response contains an
presumptively arbitrary string of random digits, a timestamp, and the
fully-qualified primary host name of the server. The syntax of the
unencoded form must correspond to that of an RFC 822 'msg-id'
[RFC822] as described in [POP3].
The client makes note of the data and then responds with a string
consisting of the user name, a space, and a 'digest'. The latter is
computed by applying the keyed MD5 algorithm from [KEYED-MD5] where
the key is a shared secret and the digested text is the timestamp
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
hexadecimal format, using lower-case ASCII characters.
When the server receives this client response, it verifies the digest
provided. If the digest is correct, the server should consider the
client authenticated and respond appropriately.
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 [KEYED-MD5] for precomputation
of intermediate results make it possible to avoid explicit cleartext
storage of the shared secret on the server system by instead storing
the intermediate results which are known as "contexts".
CRAM does not support a protection mechanism.
The examples in this document show the use of the CRAM mechanism with
the IMAP4 AUTHENTICATE command [IMAP-AUTH]. The base64 encoding of
the challenges and responses is part of the IMAP4 AUTHENTICATE
command, not part of the CRAM specification itself.
S: * OK IMAP4 Server
C: A0001 AUTHENTICATE CRAM-MD5
S: + PDE4OTYuNjk3MTcwOTUyQHBvc3RvZmZpY2UucmVzdG9uLm1jaS5uZXQ+
S: A0001 OK CRAM authentication successful
In this example, the shared secret is the string
'tanstaaftanstaaf'. Hence, the Keyed MD5 digest is produced by
MD5((tanstaaftanstaaf XOR opad),
MD5((tanstaaftanstaaf XOR ipad),
where ipad and opad are as defined in the keyed-MD5 Work in
Progress [KEYED-MD5] and the string shown in the challenge is the
base64 encoding of <firstname.lastname@example.org>. 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
This produces a digest value (in hexadecimal) of
The user name is then prepended to it, forming
Which is then base64 encoded to meet the requirements of the IMAP4
AUTHENTICATE command (or the similar POP3 AUTH command), yielding
[CHAP] Lloyd, B., and W. Simpson, "PPP Authentication Protocols",
RFC 1334, October 1992.
[IMAP] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 2060, University of Washington, December 1996.
[IMAP-AUTH] Myers, J., "IMAP4 Authentication Mechanisms",
RFC 1731, Carnegie Mellon, December 1994.
[KEYED-MD5] Krawczyk, Bellare, Canetti, "HMAC: Keyed-Hashing for
Message Authentication", RFC 2104, February 1997.
[MD5] Rivest, R., "The MD5 Message Digest Algorithm",
RFC 1321, MIT Laboratory for Computer Science, April 1992.
[POP3] Myers, J., and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, Carnegie Mellon, May 1996.
[POP3-AUTH] Myers, J., "POP3 AUTHentication command", RFC 1734,
Carnegie Mellon, December, 1994.
4. Security Considerations
It is conjectured that use of the CRAM authentication mechanism
provides origin identification and replay protection for a session.
Accordingly, a server that implements both a cleartext password
command and this authentication type should not allow both methods of
access for a given user.
While the saving, on the server, of "contexts" (see section 2) is
marginally better than saving the shared secrets in cleartext as is
required by CHAP [CHAP] and APOP [POP3], 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 user mailboxes and identities.
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 cleartext passwords in IMAP. 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, 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 [CERT95].
See section 1 above for additional discussion.
This memo borrows ideas and some text liberally from [POP3] and
[RFC-1731] and thanks are due the authors of those documents. Ran
Atkinson made a number of valuable technical and editorial
contributions to the document.
6. Authors' Addresses
John C. Klensin
800 Boylston St, 7th floor
Boston, MA 02199
Phone: +1 617 960 1011
2100 Reston Parkway
Reston, VA 22091
Phone: +1 703 715 7366
2100 Reston Parkway
Reston, VA 22091
Phone: +1 703 715 7251