Re: [TLS] RESOLVED (Re: [sasl] lasgt call comments (st Call:

<Pasi.Eronen@nokia.com> Wed, 04 November 2009 14:13 UTC

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From: <Pasi.Eronen@nokia.com>
To: <mrex@sap.com>, <simon@josefsson.org>
Date: Wed, 4 Nov 2009 15:13:42 +0100
Thread-Topic: [TLS] RESOLVED (Re: [sasl] lasgt call comments (st Call:
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References: <87hbtcc457.fsf@mocca.josefsson.org> from "Simon Josefsson" at Nov 2, 9 04:55:48 pm <200911021635.nA2GZAEp004639@fs4113.wdf.sap.corp>
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Cc: channel-binding@ietf.org, Nicolas.Williams@sun.com, sasl@ietf.org, tls@ietf.org
Subject: Re: [TLS] RESOLVED (Re: [sasl] lasgt call comments (st Call:
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Martin Rex wrote:

> > That was my conclusion as well, hence
> > http://tools.ietf.org/html/draft-josefsson-sasl-tls-cb-00
> > which uses the TLS PRF interface.
> >
> > For -02 I also added hashing the Finished message, to match the
> > semantics for connection/session (regardless of its definition) of
> > draft-altman-tls-channel-bindings, but I'd prefer to avoid it
> > completely.
> 
> If you refer to the TLS-extractor interface with TLS PRF, that does
> unconditionally include the client.random and server.random in the
> computation and therefore the output will differ for different
> incarnations (resumes) of the same TLS session.  That is comparable
> to keying to the _most_recent_ finished message -- with __NO__
> special cased for TLS session resume and TLS renogiation.

Note that just extracting some bytes from the TLS master secret (with
TLS extractor) is not sufficient to produce a channel binding,
because with RSA cipher suites, a man-in-the-middle could cause two
TLS sessions to have the same TLS master secret (this is explained in
draft-ietf-tls-extractor-07, Section 5). But those two sessions would
have different Finished messages.

(Version -00 of draft-josefsson-sasl-tls-cb had this problem. Newer
versions avoid it by including the handshake messages (either hashed, 
or via the Finished message) in the calculation.)

Best regards,
Pasi