Re: [TLS] Computation of static secret in anonymous DH

Ilari Liusvaara <ilari.liusvaara@elisanet.fi> Fri, 31 July 2015 16:42 UTC

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Date: Fri, 31 Jul 2015 19:42:12 +0300
From: Ilari Liusvaara <ilari.liusvaara@elisanet.fi>
To: Nico Williams <nico@cryptonector.com>
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Subject: Re: [TLS] Computation of static secret in anonymous DH
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On Fri, Jun 26, 2015 at 01:41:29PM -0500, Nico Williams wrote:
> 
> tls-unique depends on the Finished message strongly binding the entire
> transcript up to that point.  I find this elegant (despite the
> resumption problem, which anyways, should be fixed by the session hash)
> and easy to understand and analyze.
> 
> If the Finished message no longer has this property in 1.3 then that's a
> problem for tls-unique, and we'd have to fix one or the other.  Surely
> 1.3 will have some handshake message that binds the transcript, and why
> that wouldn't be the Finished message is beyond me (but I am missing a
> lot of the 1.3 context, so please forgive and inform me).

Also, it turns out some are assuming tls-unique is both connection nonce
and secret value. :-/

I don't think the present construct for Finished values is appropriate
for such values, which means one would have to redefine tls-unique
so it meets the need.

(TLS-Exporter values already look to be secret and connection
nonces, and I have already seen stuff relying on both properties).

Basically, the value needs to derive from both "master secret" (to make
it secret) and session hash /w configs (to make it connection
nonce).


-Ilari