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Date:   Tue, 7 Aug 2018 17:15:24 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Samuel Neves <samuel.c.p.neves@...il.com>
Cc:     Paul Crowley <paulcrowley@...gle.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        linux-fscrypt@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Greg Kaiser <gkaiser@...gle.com>,
        Michael Halcrow <mhalcrow@...gle.com>,
        tomer.ashur@...t.kuleuven.be, Eric Biggers <ebiggers@...gle.com>,
        "D. J. Bernstein" <djb@...yp.to>
Subject: Re: [RFC PATCH 3/9] crypto: chacha20-generic - refactor to allow
 varying number of rounds

On Tue, Aug 07, 2018 at 02:51:21PM -0700, Eric Biggers wrote:
> On Tue, Aug 07, 2018 at 11:21:04AM +0100, Samuel Neves wrote:
> > > The best attack on ChaCha breaks 7 rounds, and that attack requires 2^248 operations.
> > 
> > This number, as far as I can tell, comes from the "New features of
> > Latin dances" paper. There have been some minor improvements in the
> > intervening 10 years, e.g., [1, 2, 3, 4], which pull back the
> > complexity of breaking ChaCha7 down to 2^235. In any case, every
> > attack so far appears to hit a wall at 8 rounds, with 12 rounds---the
> > recommended eSTREAM round number for Salsa20---seeming to offer a
> > reasonable security margin, still somewhat better than that of the
> > AES.
> > 
> > Best regards,
> > Samuel Neves
> > 
> > [1] https://eprint.iacr.org/2015/698
> > [2] https://eprint.iacr.org/2015/217
> > [3] https://eprint.iacr.org/2016/1034
> > [4] https://doi.org/10.1016/j.dam.2017.04.034
> 
> Thanks Samuel, I'll fix that number in the next iteration of the patchset.
> 

Oops, sorry, for some reason I thought you had quoted one of my commit messages,
but it was actually Paul's email.  I did mention in "crypto: chacha - add
XChaCha12 support" that "the best known attack on ChaCha makes it through only 7
rounds", but I didn't specify the complexity.

- Eric

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