[ogogmad]

ChaCha20 got discovered using a computer search testing out resistance to certain attacks. Hence, the architecture came first and then the parameters came next. Any link with NN gradient descent? It would likely be an abstract one.

[tptacek]

I don't know how true this is? Salsa20 seems like pretty standard ARX design that builds a hash function in counter mode; there's a detailed paper explaining Bernstein's decisions.