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

Yoav Nir <ynir.ietf@gmail.com> Mon, 28 March 2016 19:45 UTC

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From: Yoav Nir <ynir.ietf@gmail.com>
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Date: Mon, 28 Mar 2016 22:45:14 +0300
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To: Shay Gueron <shay.gueron@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.

> On 28 Mar 2016, at 6:41 PM, Shay Gueron <shay.gueron@gmail.com> wrote:
> 
> Hi Yoav, 
> 
> The number of nonces is not limited by 2^32. 

No. It’s limited by 2^96

> What is limited by this number is the maximum number of blocks per nonce in a single message.

We’re talking about a different part of the Security Considerations section:
                                                      Since the nonce
   size for GCM is only 12 bytes, NIST set a limit [GCM] of 2^32
   encryptions before the probability of duplicate nonces becomes too
   high.
And this limitation makes sense only if the nonces are chosen randomly. Otherwise we can generate 2^96 unique nonces with no problem. It does make sense if there are multiple senders and no sane way to partition the nonce space.

Yoav