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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> Tue, 26 April 2016 17:25 UTC

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From: Adam Langley <agl@imperialviolet.org>
To: Andy Lutomirski <luto@amacapital.net>
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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 Tue, Apr 26, 2016 at 9:44 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> Then you're violating one of the basic properties that even
> non-nonce-misuse-resistent schemes provide: when you encrypt two
> messages using different nonces, you shouldn't reveal whether they're
> the same message.  As proposed, this mode does *not* have that
> property.

Although the record encryption key may be the same for pairs of
messages in AES-256 mode, that doesn't lead to identical plaintexts
having identical ciphertexts because the nonce is xored into S_s,
which ends up controlling the tag and thus the initial counter.

Still, Shay and I chatted and the idea to XOR the nonce with 1 for the
second half seems like it might be cleaner. I believe that Shay and
Yehuda are working on updating the analysis and may incorporate this
change.


Cheers

AGL

-- 
Adam Langley agl@imperialviolet.org https://www.imperialviolet.org

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