[Etzos]

Wow, I completely missed that (largely because I assumed a Go implementation could not guarantee constant time), sorry about that and thanks for looking deeper than I did.

On that note, how much of a guarantee is there in the Go implementation? I assume in most cases it's going to be constant time, but isn't that a little harder to guarantee when compared to the asm version? And if not why not just use the Go implementation everywhere for consistency?

[wbl]

The asm version is much more performant. The Go version is a port of the version in NSS, but the compiler is free to screw it up in various ways. Furthermore correctness is always a concern: cleverness in unsaturated arithmetic code can be hard.