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From:   Samuel Neves <samuel.c.p.neves@gmail.com>
Date:   Tue, 7 Aug 2018 11:21:04 +0100
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Subject: Re: [RFC PATCH 3/9] crypto: chacha20-generic - refactor to allow
 varying number of rounds
To:     Paul Crowley <paulcrowley@google.com>
Cc:     "Jason A. Donenfeld" <Jason@zx2c4.com>, ebiggers@kernel.org,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        linux-fscrypt@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Greg Kaiser <gkaiser@google.com>,
        Michael Halcrow <mhalcrow@google.com>,
        tomer.ashur@esat.kuleuven.be, Eric Biggers <ebiggers@google.com>,
        "D. J. Bernstein" <djb@cr.yp.to>
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> 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