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One can only implement a HIBP check when one has access to the user's unhashed password. So, at login, registration, and password reset.

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Yes, exactly, so that's why I was asking, you mentioned the database was of hashed passwords. The database then contains the source passwords? And you're preventing the user from using one of those passwords?

Sorry, I still don't understand the procedure you mentioned and I'm genuinely curious.

Oh, I see the issue. The HIBP database is SHA-1 hashed with no salt. It was created from unhashed passwords. You can't download the unhashed version (you could of course compute it, if you really wanted to; but there's no need).

So, the procedure you need to implement is, on login/registration/pw reset, you SHA-1 hash the user's unhashed password and do a indexed lookup on your copy of HIBP's database. Or if you don't want to maintain that copy, you can use HIBP's API to do something similar.

Ah! Thanks a lot, it now makes sense. So at some point HIBP has the unhashed passwords, they obviously don’t make those public, good trick. How do you handle this from a UX perspective? Just tell the user that password is “not strong enough”?

Password managers that have HIBP integration are open about it - one says "this password appears in a list of compromised passwords"

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