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

Stefan Santesson <stefan@aaa-sec.com> Thu, 25 February 2010 16:55 UTC

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Date: Thu, 25 Feb 2010 17:47:40 +0100
From: Stefan Santesson <stefan@aaa-sec.com>
To: Simon Josefsson <simon@josefsson.org>
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Thread-Topic: draft-ietf-tls-cached-info-02 / New "Fast-Track" draft
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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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Simon,

Note that the requirement is MUST support, it is not MUST use.
It is perfectly allowed to use SHA-256.

Does that solve your concern?

On 10-02-25 3:46 PM, "Simon Josefsson" <simon@josefsson.org>; wrote:

> 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