Re: [RFC PATCH 3/9] crypto: chacha20-generic - refactor to allow varying number of rounds

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