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Re: [TLS] "Last call" on draft-altman-tls-channel-bindings-05.txt

Simon Josefsson <simon@josefsson.org> Wed, 19 August 2009 22:25 UTC

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From: Simon Josefsson <simon@josefsson.org>
To: Nicolas Williams <Nicolas.Williams@sun.com>
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Date: Thu, 20 Aug 2009 00:25:38 +0200
In-Reply-To: <20090819212223.GN1043@Sun.COM> (Nicolas Williams's message of "Wed, 19 Aug 2009 16:22:24 -0500")
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Cc: ietf-sasl@imc.org, tls@ietf.org
Subject: Re: [TLS] "Last call" on draft-altman-tls-channel-bindings-05.txt
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Nicolas Williams <Nicolas.Williams@sun.com> writes:

> But I don't want to guess at what might happen in the future
> of digital signatures.

I agree, we could decide to not resolve this concern.

> Instead I'd rather either say either that tls-server-end-point CB is
> undefined if the cert's signature alg does not use a signature, or
> pick a hash function (e.g., SHA-512) to use in such cases.

If use of SHA-512 is hard-coded, we run into problem when it is phased
out.  Negotiation any other hash function will be tricky.  Alas, I'm not
sure leaving it undefined is any better: negotiating what hash function
to use in that situation seems equally tricky.

This is one reason where deriving channel binding data from the TLS
channel using tls-extractor appears more robust: it leaves negotiation
of the hash function to the TLS protocol.

/Simon

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