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Network Working Group                                        D. Cridland
Internet-Draft                                             Isode Limited
Intended status: Informational                          November 9, 2007
Expires: May 12, 2008


         On the use of TLS Session resumption and SASL EXTERNAL
                  draft-cridland-sasl-tls-sessions-00

Status of this Memo

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   This Internet-Draft will expire on May 12, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   Some SASL mechanisms provide a fast reauthentication option.  TLS
   also provides this, and this memo outlines a proposal to use the TLS
   session resumption as a method for mechanism-independent fast
   reauthentication using SASL EXTERNAL.







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Table of Contents

   1.  Conventions used in this document . . . . . . . . . . . . . . . 3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Implementation  . . . . . . . . . . . . . . . . . . . . . . . . 3
     3.1.  Initial Authentication  . . . . . . . . . . . . . . . . . . 3
     3.2.  Fast Reauthentication . . . . . . . . . . . . . . . . . . . 4
   4.  Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 6
   Intellectual Property and Copyright Statements  . . . . . . . . . . 7



































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1.  Conventions used in this document

   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.  Introduction

   The [DIGEST-MD5] [SASL] mechanism provides a method of performing
   subsequent authentications using many fewer round-trips, known as
   "fast reauthentication".  Most SASL mechanisms do not, which can
   cause a relatively high number of round-trips at application protocol
   startup.

   In addition, the cost of cryptographic computation in clients can be
   quite high, leading to a slower application session setup.

   This memo proposes a use of the EXTERNAL mechanism, defined in
   [SASL], in conjunction with TLS session resumption as specified in
   [TLS], which effectively provides fast reauthentication in a generic
   manner for any SASL mechanism which supports channel binding.


3.  Implementation

3.1.  Initial Authentication

   To use this method, the client first negotiates TLS as normal, then
   uses any SASL mechanism which supports channel binding to
   authenticate as normal whilst TLS is in effect.  On successful
   authentication, the server then records the authorization identifier
   used against the SessionID used in TLS.

   If the mechanism used in this initial authentication does not
   support, or use, channel binding, then the server MUST NOT record the
   authorization identifier against the SessionID.

   The client SHOULD NOT provide a certificate during this initial TLS
   negotiation, as this would cause there to be multiple potential
   identities.

   The TLS session so created SHOULD NOT be resumed except to
   reauthenticate to obtain the same authorization identifier.







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3.2.  Fast Reauthentication

   First, a client resumes a TLS session, using the SessionID previously
   recorded as per Section 3.1.  The server, on resumption of the TLS
   session, then determines whether any authorization identifiers have
   been cached as per Section 3.1, and advertises EXTERNAL if so.  If
   EXTERNAL is advertised, then the client then authenticates using the
   EXTERNAL mechanism.

   The server verifies this by checking that the authorization
   identifier was previously used with the SessionID recorded as per
   Section 3.1.

   If this check fails, the application protocol will reject the
   authentication.  Clients SHOULD retry using a traditional SASL
   mechanism.


4.  Open Issues

   A significant problem with this method is that there is no
   negotiation to indicate the source of an externally asserted
   authorization identifier, in particular, there is no protocol by
   which a client can determine whether using EXTERNAL without
   specifying an authorization identifier will grant it the
   authorization identifier it was expecting.  In particular, this
   suggests that use of the mechanism with a TLS client certificate may
   be particularly difficult.

   Clients also cannot easily specify authorization identifiers to
   resolve this - not only do explicitly specified authorization
   identifiers tend to be treated as proxy-authentication requests, but
   the client cannot formally know what authorization identifier it was
   granted by default in the initial authentication.

   This could be addressed by definition of a new SASL mechanism which
   would explicitly use the authorization identifier previously
   associated with the TLS session.

   The problems outlined with EXTERNAL may be sufficient to consider
   replacing EXTERNAL itself with a family of mechanisms whose name
   indicates the source of the implicit authorization identifier.


5.  IANA Considerations

   This document has no actions for IANA.




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6.  Security Considerations

   This method is only suitable in the case where the SASL mechanism
   used in initial authentication is actively using channel bindings,
   and the SessionID is secure.

   In the case where the SessionID, and related session information,
   could be compromised on the wire, then the server cannot rely on this
   to provide an authorization identifier.  This is the case where the
   encryption algorithm used is NULL, for example.

   In the case where channel binding was not used by the SASL mechanism,
   then the server cannot know whether the client has been subjected to
   an MITM.  The client can know by suitable verification of the server
   certificate, if one is provided, however, the server cannot know if
   this was carried out, and for anonymous cipher suites, there is no
   certificate to verify.

   If an MITM is in effect without channel binding, use of this
   mechanism could otherwise allow the MITM to later reauthenticate.

   Caching credentials sufficient to reauthenticate non-interactively,
   whether using this method, that of [DIGEST-MD5], or merely storing
   the plaintext password, may allow a third-party unauthorized program
   to obtain the credentials and access the protected service.  Use of
   this method entirely removes the ability to recover a password from
   this cached data, however, since neither the password nor any data
   derived from it is needed to reauthenticate.

   Other security considerations applicable to this method are to be
   found in both [SASL] and [TLS].

   [The author notes that he is neither an expert on TLS nor on
   cryptography in general, hence there is probably more to consider
   than this.]


7.  Acknowledgements

   Comments were received on the idea, and/or this draft, from Sam
   Hartman, Kurt Zeilenga, Tony Finch, Alexey Melnikov, and others.
   Whether in agreement or dissent, the comments have refined and
   otherwise influenced the document.


8.  References





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8.1.  Normative References

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

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

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

8.2.  Informative References

   [DIGEST-MD5]
              Leach, P. and C. Newman, "Using Digest Authentication as a
              SASL Mechanism", RFC 2831, May 2000.


Author's Address

   Dave Cridland
   Isode Limited
   5 Castle Business Village
   36, Station Road
   Hampton, Middlesex  TW12 2BX
   GB

   Email: dave.cridland@isode.com






















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Acknowledgment

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).





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