Public-key cryptography and password protocols

Abstract

We study protocols for strong authentication and key exchange in asymmetric scenarios where the authentication server possesses a pair of private and public keys while the client has only a weak human-memorizable password as its authentication key. We present and analyze several simple password protocols in this scenario, and show that under the choice of suitable public key encryption functions the security of these protocols can be formally proven based on standard cryptographic assumptions. In particular, our analysis shows optimal resistance to off-line password guessing attacks. In addition to user authentication, we enhance our protocols to provide two-way authentication, authenticated key exchange, defense against server's compromise, and user anonymity. We complement these results with a proof that public key techniques are unavoidable for password protocols that resist off-line guessing attacks. As a further contribution, we introduce the notion of public passwords that allow for the use of such protocols in situations where the client's machine does not have the means to validate the server's public key. Public passwords serve as `hand-held certificates' that the user can carry without the need for special computing devices.