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

Martin Thomson <martin.thomson@gmail.com> Tue, 24 May 2016 18:44 UTC

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Date: Tue, 24 May 2016 11:42:59 -0700
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From: Martin Thomson <martin.thomson@gmail.com>
To: "Dang, Quynh (Fed)" <quynh.dang@nist.gov>
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Subject: Re: [TLS] Comments on nonce construction and cipher text size restriction.
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On 24 May 2016 at 10:46, Dang, Quynh (Fed) <quynh.dang@nist.gov> wrote:
>>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.
>
> I don¹t see any extra layer here.


The argument here is that there are only 2^128 keys and some protocols
have predictable plaintext.  A predictable nonce would allow an
attacker to do some pre-calculation with a large number of keys to get
a chance of a collision (and a break).  It's a long bow, but not
entirely implausible.