As I understand, there are no restrictions on the password input to GenerateFromPassword, so a password of length 0 is considered valid input and this is working as intended. But maybe something else should be done.

CC @FiloSottile, @katiehockman, @rolandshoemaker.

Unfortunately there isn't really a bcrypt specification, all we have to go off is the original paper from the OpenBSD folks, which doesn't specify what you do in this case, and other implementations.

The OpenBSD libc implementation appears to accept empty input, and can probably be considered the reference implementation. PHP does the same thing, and is likely the place where bcrypt is most often used in the wild.

I don't think there is a particularly strong security argument to reject empty inputs, but there is perhaps a UX one to be made. Given we appear to match the community implementation consensus on this matter though, I would lean towards just saying this works as intended.

Not the safest interface, but I think here backwards compatibility and matching the ecosystem are good enough reasons not to make a change.