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Re: [Cfrg] Closing out tls1.3 "Limits on key usage" PRs (#765/#769)

"Paterson, Kenny" <Kenny.Paterson@rhul.ac.uk> Fri, 10 February 2017 09:06 UTC

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From: "Paterson, Kenny" <Kenny.Paterson@rhul.ac.uk>
To: Sean Turner <sean@sn3rd.com>
Thread-Topic: [Cfrg] Closing out tls1.3 "Limits on key usage" PRs (#765/#769)
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Date: Fri, 10 Feb 2017 09:06:07 +0000
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Cc: IRTF CFRG <cfrg@irtf.org>, "<tls@ietf.org>" <tls@ietf.org>
Subject: Re: [Cfrg] Closing out tls1.3 "Limits on key usage" PRs (#765/#769)
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Hi,

My preference is to go with the existing text, option a).

From the github discussion, I think option c) involves a less conservative
security bound (success probability for IND-CPA attacker bounded by
2^{-32} instead of 2^{-60}). I can live with that, but the WG should be
aware of the weaker security guarantees it provides.

I do not understand option b). It seems to rely on an analysis of
collisions of ciphertext blocks rather than the established security proof
for AES-GCM.

Regards,

Kenny

On 10/02/2017 05:44, "Cfrg on behalf of Martin Thomson"
<cfrg-bounces@irtf.org on behalf of martin.thomson@gmail.com> wrote:

>On 10 February 2017 at 16:07, Sean Turner <sean@sn3rd.com> wrote:
>> a) Close these two PRs and go with the existing text [0]
>> b) Adopt PR#765 [1]
>> c) Adopt PR#769 [2]
>
>
>a) I'm happy enough with the current text (I've implemented that any
>it's relatively easy).
>
>I could live with c, but I'm opposed to b. It just doesn't make sense.
>It's not obviously wrong any more, but the way it is written it is
>very confusing and easily open to misinterpretation.
>
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