Triple A

We have Authentication, Authorisation and Accounting. And the least of these is Accounting, because no-one bothers about computer time these days. But its presence in the triad reminds us that username and password authentication dates back to a time when the only reason to log on was to select the account to be billed for your FORTRAN session on the teletype.

Using a password to prove identity is a later loading of this basic idea. Which is why it doesn't work.