Internet DRAFT - draft-cridland-sasl-tls-sessions
draft-cridland-sasl-tls-sessions
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
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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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