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+Network Working Group                                       J. Klensin
+Request for Comments: 2095                                    R. Catoe
+Category: Standards Track                                 P. Krumviede
+                                                                   MCI
+                                                          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.
+
+Abstract
+
+   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.
+
+   Example:
+
+   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+
+     C: dGltIGI5MTNhNjAyYzdlZGE3YTQ5NWI0ZTZlNzMzNGQzODkw
+     S: A0001 OK CRAM authentication successful
+
+      In this example, the shared secret is the string
+      'tanstaaftanstaaf'.  Hence, the Keyed MD5 digest is produced by
+      calculating
+
+        MD5((tanstaaftanstaaf XOR opad),
+            MD5((tanstaaftanstaaf XOR ipad),
+            <1896.697170952@postoffice.reston.mci.net>))
+
+      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 <1896.697170952@postoffice.reston.mci.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
+
+           b913a602c7eda7a495b4e6e7334d3890
+
+      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
+
+           dGltIGI5MTNhNjAyYzdlZGE3YTQ5NWI0ZTZlNzMzNGQzODkw
+
+
+
+
+
+
+
+
+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
+   USA
+
+   EMail: klensin@mci.net
+   Phone: +1 617 960 1011
+
+   Randy Catoe
+   MCI Telecommunications
+   2100 Reston Parkway
+   Reston, VA 22091
+   USA
+
+   EMail: randy@mci.net
+   Phone: +1 703 715 7366
+
+   Paul Krumviede
+   MCI Telecommunications
+   2100 Reston Parkway
+   Reston, VA 22091
+   USA
+
+   EMail: paul@mci.net
+   Phone: +1 703 715 7251
+
+
+
+
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+Klensin, Catoe & Krumviede  Standards Track                     [Page 5]
+