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Not sure why you would think that was necessary or at all likely. We have these things called hashes...

that doesn't explain anything. they shouldn't be uploading unsalted hashes either. and if it's salted, it won't match with any database

True, the best solution is to download the list of hashed "bad passwords" and check if the one entered matches against that.

if salted hash is leaked then it doesn't matter and does not need to be detected anyway.