Why does this library reference u8? about subtle HOT 3 CLOSED

The asymmetry is intentional. Users of the library are intended to either (more commonly) receive Choice values or (less commonly) construct Choice values, and then use those Choices to do constant-time operations, not to convert them back into bools for use with branches etc.

The method is called unwrap_u8 to indicate that it "unwraps" the type-safe abstraction to provide a "raw" value. It is an escape hatch, which may be required in order to write custom implementations of the subtle traits (e.g., in the SIMD backend of curve25519-dalek), but it is not supposed to be the common-path.

Using x != 0 does not work, because that compiles into a branch – the decision to use u8s rather than bools was intentional.

from subtle.

Hmm, so if you see unwrap_u8 as unsafe, maybe it should be an unsafe method?

from subtle.

I don't think that unsafe is appropriate here, because it indicates that something is outside of the Rust abstract machine, rather than just "this API has sharp edges and can be misused".

from subtle.