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Re: [Cfrg] Adopting "AES-GCM-SIV: Nonce Misuse-Resistant Authenticated Encryption" as a CFRG document ---- Some clarifications

Adam Langley <agl@imperialviolet.org> Thu, 21 April 2016 20:28 UTC

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From: Adam Langley <agl@imperialviolet.org>
To: "Blumenthal, Uri - 0553 - MITLL" <uri@ll.mit.edu>
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Cc: Yehuda Lindell <yehuda.lindell@biu.ac.il>, "cfrg@irtf.org" <cfrg@irtf.org>, Adam Langley <agl@google.com>
Subject: Re: [Cfrg] Adopting "AES-GCM-SIV: Nonce Misuse-Resistant Authenticated Encryption" as a CFRG document ---- Some clarifications
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On Thu, Apr 21, 2016 at 1:11 PM, Blumenthal, Uri - 0553 - MITLL
<uri@ll.mit.edu> wrote:
> I’m afraid Andy is correct. Say one record has its nonce xxxx…xxxx0 (127
> bits plus 0), and another record has its nonce xxx…xxx1. The record key
> produced for both records will be the same, because it clobbers/ignores the
> LSB.

That is what I understood when I wrote those words. I might have
misunderstood Shay however!

This is the reason that the Security Considerations section mentions
that it's only safe to repeat an nonce 2^31 (rather than 2^32) times
with the AES-256 version.


Cheers

AGL

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