Re: [TLS] Wrapping up cached info

Martin Rex <mrex@sap.com> Mon, 17 May 2010 19:16 UTC

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From: Martin Rex <mrex@sap.com>
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To: jsalowey@cisco.com (Joseph Salowey)
Date: Mon, 17 May 2010 21:16:01 +0200 (MEST)
In-Reply-To: <AC1CFD94F59A264488DC2BEC3E890DE50A67C326@xmb-sjc-225.amer.cisco.com> from "Joseph Salowey" at May 17, 10 09:29:44 am
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Cc: tls@ietf.org
Subject: Re: [TLS] Wrapping up cached info
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Joseph Salowey wrote:
> 
> I agree with Uri, that if you determine you need SHA-256 then you should
> plan for hash agility.  TLS 1.2 plans for hash agility.  
> 
> What about Nico's proposal where a checksum is used to identify the
> cached data and the actual handshake contains the actual data hashed
> with the algorithm used in the PRF negotiated with the cipher suite? 
> 
> This way we don't have to introduce hash agility into the extension, but
> we have cryptographic hash agility where it matters in the Finished
> computation.  Does it solve the problem?  

Yes, I think so.
This approach should solve the issue at the technical level.

Going more into detail, one would hash/mac only the data that got
actually replaced in the handshake, each prefixed by a (locally computed)
length field.

-Martin