This paper presents a type of verification technique that securely protects access to any restricted material by showing different random challenge on each access attempt. Each challenge generates a ...

An authentication method that proves the identity of users logging into a network. When a user logs in, the access server or authentication server sends a random number (the challenge) to the client.

It only works in IE - if you are using Netscape or another browser you're SOL. If you have to support other browsers your best bet is Basic Authentication with SSL.