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

Adam Langley <agl@imperialviolet.org> Sat, 02 April 2016 23:57 UTC

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Date: Sun, 03 Apr 2016 06:57:29 +0700
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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
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On Thu, Mar 31, 2016 at 3:22 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> 4. Since this claims nonce-MR, setting nonce=0 is valid.  If someone does
> this, then I think they are vulnerable to the extra-easy parallel attack.

Ah, do you mean when the AES-GCM-SIV is used with a fixed nonce?

In that case, it breaks down in the same way as AES-GCM with random
nonces. We note that in the Security Considerations
(https://tools.ietf.org/html/draft-gueron-gcmsiv-02#section-9)

"It's worth noting that the 2^32 limit still applies as the number of
distinct messages that can be encrypted under a fixed nonce. Nonces
should be unique and the misuse-resistance of these AEADs should not
be depended on to the extent that 2^32 duplicates may occur.  (Or 2^31
duplicates in the case of AEAD_AES_256_GCM_SIV.)"

Basically our "misuse" resistance doesn't stretch to that kind of
abuse. That might not meet some definitions of nonce-MR but
AES-GCM-SIV is very strongly related to AES-GCM and so acts like it in
some cases.


Cheers

AGL