[Deimorz]

It depends what purpose you're using this for. If it's something like a password manager where you're checking the user's existing passwords to see if any have been breached, then yes, you should make sure not to hit false positives.

But if you're using it as a way to prevent people from using known-breached passwords on a site/service, it's really not worth worrying about. False positives would only be a problem when the user is using bad password practices anyway. If it's a non-shared, random password like it should be, a tiny chance of blocking an acceptable one is fine. They can just generate another one, it's an extremely minor inconvenience at worst.

Just include a note in the error message saying something like, "In very rare cases, this could be a false positive. Even if it is, you must choose a different password." The 0.1% of users it impacts (or whatever your error rate is) will be fine.

[ascar]

While I absolutely agree you shouldn't reuse passwords across services, it's a reality for many users and I'm convinced it's not an acceptable point of view to tell your user his password might be leaked, if there is no indication for it. This is not a minor inconvenience, this might trigger a major panic and it's inconsiderate to ignore it.

Even if you display there is a minor chance of a false positive, your user must now think 0,1% this was a false positive or 99,9% my password was leaked! I don't want users to panic if there is no reason to and I don't want users to blame false positives if there is reason to panic. I think it's definitely worth worrying about.

Also research has shown that forcing users to regularly change passwords leads to weaker passwords. And as many (most?) users are not using password managers (yet) expecting a different secure (i.e. long enough, non dictionary) password is too far from reality.

[dwaite]

You probably shouldn't indicate to them that their password has been leaked.

The database only lists the SHA hash of passwords which have been found in various datasets and the occurrence count. It does not indicate that the matching password (one of half a billion) has ever been associated with the user account.

The fault in knowledge schemes is oversharing the secret. However, there is no way to know for sure that the user has done this by reusing passwords - you will have false positives (say, one other person on the internet chose this password by chance) and false negatives (the user has reused the password all over the place, but has managed to not be part of a known breach dataset).