And in this case, k = 381. (Though more by coincidence than design.) https://www.troyhunt.com/ive-just-launched-pwned-passwords-v...

It's much larger than that, really -- if you know that the user is definitely looking for a hash that's present, there are 381 they could have been looking for. But here we're just as interested in elements that are outside the set, and there are a lot more of them.

I was initially extremely skeptical of sites using Pwned Passwords' API until I learned how this worked/realized this.

I was super skeptical of "send Troy your whole SHA1 hash", because many many SHA1 hashes have been reversed. Then when I was looking at V2, I was considering the fact that in a given set of 381 passwords, there were probably only a couple you might be meaning to check (if you look at a range set, each one generally has a couple of repeatedly used password hashes).

But if my passwords fall outside of that set, someone might think I am maybe checking against one of the more commonly leaked passwords, when mine isn't even in that range set at all.

Partially for my own comfort, and for fun, I ended up writing my own short checker in a .NET console app: https://github.com/ocdtrekkie/PwnPsswords