> 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
