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

Colm MacCárthaigh <colm@allcosts.net> Tue, 24 May 2016 17:44 UTC

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Date: Tue, 24 May 2016 10:44:15 -0700
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From: =?UTF-8?Q?Colm_MacC=C3=A1rthaigh?= <colm@allcosts.net>
To: Martin Thomson <martin.thomson@gmail.com>
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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 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.

-- 
Colm