A New Two-Server Approach for Authentication with Short Secrets

Passwords and PINs continue to remain the most widespread forms of user authentication, despite growing awareness of their security limitations. This is because short secrets are convenient, particularly for an increasingly mobile user population. Many users are interested in employing a variety of computing devices with different forms of connectivity and different software platforms. Such users often find it convenient to authenticate by means of passwords and short secrets, to recover lost passwords by answering personal or “life” questions, and to make similar use of relatively weak secrets. In typical authentication methods based on short secrets, the secrets (or related values) are stored in a central database. Often overlooked is the vulnerability of the secrets to theft en bloc in the event of server compromise. With this in mind, Ford and Kaliski and others have proposed various password “hardening” schemes involving multiple servers, with password privacy assured provided that some servers remain uncompromised. In this paper, we describe a new, two-server secure roaming system that benefits from an especially lightweight new set of protocols. In contrast to previous ideas, ours can be implemented so as to require essentially no intensive cryptographic computation by clients. This and other design features render the system, in our view, the most practical proposal to date in this area. We describe in this paper the protocol and implementation challenges and the design choices underlying the system.