scrypt: relax check on log_n

[aawsome]

Hi!

we are getting compatibility issues due to scrypt parameters which don't work on Rust scrypt, but work on e.g. Go (and obviously also in the scrypt C reference implementation). It seems like the RFC is too strict here by accident.

See rustic-rs/rustic#1394

There has been already a discussion about this in golang/go#33703 and the RFC errata is e.g. https://www.rfc-editor.org/errata/eid5971.

Is it possible to also relax that check in the Rust scrypt implementation to improve interoperability with software using the C or Go implementation?

[tarcieri]

Can you provide a small, self-contained reproduction of the issue?

[aawsome]

@tarcieri sure, here it is:

assert!(scrypt::Params::new(18, 1, 5, 32).is_ok())

(currently, this fails due to the check on log_n)

[aawsome]

I can provide a PR removing that check on log_n, if wanted.