To my understanding, OP just wants websites to not expose any information (existing usernames and emails) to visitors, even in registration forms. What you sug

To my understanding, OP just wants websites to not expose any information (existing usernames and emails) to visitors, even in registration forms. What you suggest shouldn't be possible because: - multiple users can have the same password - properly hashed passwords rely on per-user/per-hash random seed, which can be only determined after user is found in the database