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Re: [Cfrg] Second RGLC on "AES-GCM-SIV"

Stefano Tessaro <tessaro@cs.ucsb.edu> Sat, 20 January 2018 19:26 UTC

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From: Stefano Tessaro <tessaro@cs.ucsb.edu>
Date: Sat, 20 Jan 2018 11:26:08 -0800
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To: Yoav Nir <ynir.ietf@gmail.com>
Cc: "Paterson, Kenny" <Kenny.Paterson@rhul.ac.uk>, Adam Langley <agl@imperialviolet.org>, Yehuda Lindell <Yehuda.Lindell@biu.ac.il>, "cfrg@irtf.org" <cfrg@irtf.org>
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Subject: Re: [Cfrg] Second RGLC on "AES-GCM-SIV"
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> The reason for this recommendation is that using a fixed nonce is bad, but
> using counters may lead to pairwise collisions between two encryptors, and
> that’s not ideal either.

In addition to this, one of the points highlighted by our multi-user
analysis (linked above) is that there is a great security benefit in
the multi-user/multi-key setting if we can ensure that each particular
nonce value is re-used by a small number of users only. This is of
course true if nonces are chosen with sufficient randomness. (Our
security bounds show how security degrades as nonces are re-used by
more users.) This is particularly important when using 128-bit keys,
less relevant with longer (256 bits) keys.

Best,
Stefano

-- 
Stefano Tessaro
Assistant Professor of Computer Science
University of California, Santa Barbara
http://cs.ucsb.edu/~tessaro/

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