Re: [TLS] Comments on nonce construction and cipher text size restriction.

Ilari Liusvaara <ilariliusvaara@welho.com> Tue, 24 May 2016 18:03 UTC

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Date: Tue, 24 May 2016 21:03:17 +0300
From: Ilari Liusvaara <ilariliusvaara@welho.com>
To: Colm =?utf-8?Q?MacC=C3=A1rthaigh?= <colm@allcosts.net>
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Subject: Re: [TLS] Comments on nonce construction and cipher text size restriction.
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On Tue, May 24, 2016 at 10:44:15AM -0700, Colm MacCárthaigh wrote:
> On Tue, May 24, 2016 at 9:13 AM, Martin Thomson <martin.thomson@gmail.com>
> wrote:
> 
> > > 3. "The padded sequence number is XORed with the static client_write_iv
> > or
> > > server_write_iv, depending on the role.” I think the ivs are not needed.
> >
> > We discussed this at quite some length.  I originally took your
> > position, but the IVs add an extra layer of safety at very little
> > cost.
> >
> 
> Is this more safe? wouldn't this disclose the key in the event that the
> HKDF and the underlying hash turned out to have some cryptanalytical flaw
> that meant it wasn't back-tracking resistant? I think otherwise TLS can
> survive such a flaw because the output of the PRF is not directly
> discernible on the wire.

It seems like failing backtracking resistance would need really major
breakage in PRF-hash. And the whole TLS will fall apart pretty badly
WAY before that point with broken PRF.


-Ilari