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

Shay Gueron <shay.gueron@gmail.com> Mon, 28 March 2016 15:42 UTC

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References: <D31EFD69.68456%kenny.paterson@rhul.ac.uk> <6F0FF2D1-BE7B-4793-A872-9AE908BE2B80@gmail.com>
Date: Mon, 28 Mar 2016 18:41:43 +0300
Message-ID: <CAHP81y8hTXJJh=Cng+ZqgrpQVrHTX9bzd6c5vTLPVxpS5=GRuw@mail.gmail.com>
From: Shay Gueron <shay.gueron@gmail.com>
To: Yoav Nir <ynir.ietf@gmail.com>
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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
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Hi Yoav,

The number of nonces is not limited by 2^32.

What is limited by this number is the maximum number of blocks per nonce in
a single message. Exactly as CTR (and AES-GCM), and for the same reason:
there are only 32 bits in a 128-bit block, that are left to increment  a
counter.

On the other hand, since a new key is derived each time from the IV (and
the encryption key), GCM-SIV can be used, with a given key, practically an
"unlimited" number of times.

This details is different from the CCS paper and the (later) proposed spec.

Thanks, Shay



2016-03-28 18:22 GMT+03:00 Yoav Nir <ynir.ietf@gmail.com>:

>
> > On 28 Mar 2016, at 5:34 PM, Paterson, Kenny <Kenny.Paterson@rhul.ac.uk>
> wrote:
> >
> > Dear CFRG,
> >
> > Shay, Adam and Yehuda have asked the CFRG chairs whether their draft for
> > AES-GCM-SIV can be adopted as a CFRG document. We are minded to do so,
> but
> > first wanted to canvass members of the group for their opinions on taking
> > this step.
>
> +1: definitely take this step.
>
> That said, I think the current document is missing a convincing
> motivation. The security considerations section talks about randomly
> choosing nonces and limits the number of such nonces that can safely be
> randomly generated to 2^32. This makes sense, but for most uses (TLS,
> IPsec, SSH) we can use a non-random nonces (a counter; an LFSR if you want
> to get fancy; DES-encrypting a counter if you want to get really fancy).
> I’d be happier if there was an example where a counter is not practical.
>
> Yoav
>
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