Abstract: A personal authentication system provides at least two levels of security for an authentication process, in addition to numerous other security features. The system operates across many different software and hardware platforms, in a client/server fashion, employing a challenge/response process that does not require users to transmit their passwords across a network. An application running on a client computer is coupled with an application running on a server computer. The client generates a response to a challenge, which is provided by the server. The response is a combined function of the server's challenge, a serial number assigned to the client, and a password provided by the user.