Re: [TLS] draft-ietf-tls-cached-info-02 / New "Fast-Track" draft

Simon Josefsson <simon@josefsson.org> Thu, 25 February 2010 14:44 UTC

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From: Simon Josefsson <simon@josefsson.org>
To: Stefan Santesson <stefan@aaa-sec.com>
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Date: Thu, 25 Feb 2010 15:46:28 +0100
In-Reply-To: <C7AC395B.892B%stefan@aaa-sec.com> (Stefan Santesson's message of "Thu, 25 Feb 2010 14:27:39 +0100")
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Cc: "Kemp, David P." <DPKemp@missi.ncsc.mil>, "tls@ietf.org" <tls@ietf.org>
Subject: Re: [TLS] draft-ietf-tls-cached-info-02 / New "Fast-Track" draft
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Stefan Santesson <stefan@aaa-sec.com>; writes:

> I think this issue is blown totally out of any reasonable proportions.
>
> This is not the first time a hash is used to provide an identifier where no
> strong collision resistance is required. I don't want to break my back to
> avoid using a perfectly suitable hash algorithm just for political reasons.

I agree that the political reasons shouldn't be a serious concern here.

There is still the technical concern that at some point in the future
SHA-1 may no longer be easily available in libraries and we want to
transition to some other algorithm.  Algorithm agility is useful even
for non-crypto algorithms.

Here is a proposal: Say SHA-1 is a MUST when TLS < 1.2 is negotiated but
later TLS versions MUST use the same hash as the one used by the PRF?

Btw, is the intention that cached info can be used with TLS versions <
1.2?  If it is only for TLS >= 1.2, then SHA-256 seems to make more
sense than SHA-1 because TLS 1.2 uses SHA-256.

/Simon