Re: [Cfrg] Data limit to achieve Indifferentiability for ciphertext with TLS 1.3 GCM, and the 2nd paragraph of Section 5.5

Martin Thomson <martin.thomson@gmail.com> Sun, 13 November 2016 23:54 UTC

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From: Martin Thomson <martin.thomson@gmail.com>
Date: Mon, 14 Nov 2016 08:54:21 +0900
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To: "Dang, Quynh (Fed)" <quynh.dang@nist.gov>
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Subject: Re: [Cfrg] Data limit to achieve Indifferentiability for ciphertext with TLS 1.3 GCM, and the 2nd paragraph of Section 5.5
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These are intentionally very conservative.  Having implemented this, I
find it OK.  The text cites its sources.  Reading those sources
corrects any misapprehension.

The key point here is that we want to ensure that the initial - maybe
uninformed - inferences need to be for the safe thing.  We don't want
to have the situation where we have looser text and that leads to
mistakes.

For instance, someone could deploy code that assumes a certain
"average" record size based on a particular deployment and hence a
larger limit.  If the deployment characteristics change without the
code changing we potentially have an issue.

You really need to demonstrate that there is harm with the current
text.  if rekeying happens on that timescale (which is still very
large), that's not harmful.  I'm concerned that we aren't going to
rekey often enough.  I don't agree that it will create any negative
perception of GCM.

On 14 November 2016 at 05:48, Dang, Quynh (Fed) <quynh.dang@nist.gov> wrote:
> Hi Eric and all,
>
>
> Regardless of the actual record size, each 128-bit block encryption is
> performed with a unique 128-bit counter which is formed by the 96-bit IV and
> the 32-bit counter_block value called CB in NIST SP 800-38D under a given
> key as long as the number of encrypted records is not more than 2^64.
>
> Assuming a user would like to limit the probability of a collision among
> 128-bit ciphertext-blocks under 1/2^32, the data limit of the ciphertext (
> or plaintext) is 2^(96/2) (= 2^48) 128-bit blocks which is 2^64 bytes.
>
> Reading the 2nd paragraph of section 5.5, a user might feel that he/she
> needs to rekey a lot more quicker than he/she needs. Putting an
> unnecessarily low data limit of 2^24.5 full-size records (2^38.5 bytes) also
> creates an incorrect negative impression (in my opinion) about GCM.
>
> I would like to request the working group to consider to revise the text.
>
> Regards,
> Quynh.
>
>
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