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Network Working Group J. Klensin Request for Comments: 2095 R. Catoe Category: Standards Track P. Krumviede

                                                        January 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.

1. Introduction

 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.

Klensin, Catoe & Krumviede Standards Track [Page 1] RFC 2095 IMAP/POP AUTHorize Extension January 1997

 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
 (including angle-brackets).
 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".

Klensin, Catoe & Krumviede Standards Track [Page 2] RFC 2095 IMAP/POP AUTHorize Extension January 1997

 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
   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 <>. 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
         tim b913a602c7eda7a495b4e6e7334d3890
    Which is then base64 encoded to meet the requirements of the IMAP4
    AUTHENTICATE command (or the similar POP3 AUTH command), yielding

Klensin, Catoe & Krumviede Standards Track [Page 3] RFC 2095 IMAP/POP AUTHorize Extension January 1997

3. References

 [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, H., "HMAC-MD5: Keyed-MD5 for Message
     Authentication", Work in Progess.
 [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

Klensin, Catoe & Krumviede Standards Track [Page 4] RFC 2095 IMAP/POP AUTHorize Extension January 1997

 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.

5. Acknowledgements

 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
 MCI Telecommunications
 800 Boylston St, 7th floor
 Boston, MA 02199
 Phone: +1 617 960 1011
 Randy Catoe
 MCI Telecommunications
 2100 Reston Parkway
 Reston, VA 22091
 Phone: +1 703 715 7366
 Paul Krumviede
 MCI Telecommunications
 2100 Reston Parkway
 Reston, VA 22091
 Phone: +1 703 715 7251

Klensin, Catoe & Krumviede Standards Track [Page 5]

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