This is done by keeping the old hashes around. The new password hash is compared to prior hashes to be sure it doesn't match any of them. This only catches exac

This is done by keeping the old hashes around. The new password hash is compared to prior hashes to be sure it doesn't match any of them. This only catches exact matches on re-used passwords. Or, the current plaintext password is compared to the new plaintext password (normally a password change requires the current password) so you can do more sophisticated similarity checks, but only compared to the current passord, not any older ones.