Proofs for Two-Server Password Authentication

Traditional password-based authentication and key-exchange protocols suffer from the simple fact that a single server stores the sensitive user password. In practice, when such a server is compromised, a large number of user passwords, (usually password hashes) are exposed at once. A natural solution involves splitting password between two or more servers. This work formally models the basic security requirement for two-server password authentication protocols, and in this framework provides concrete security proofs for two protocols. The first protocol considered [7] appeared at USENIX’03, but contained no security proof. For this protocol, we provide a concrete reduction to the computational Diffie-Hellman problem in the random oracle model. Next we present a second protocol, based on the same hard problem, but which is simpler, and has an easier, tighter reduction proof. Key words: password authentication, secret sharing, concrete security reduction

• Type: Conference
• Year: 2005
• Where: CTRSA
• Authors: Michael Szydlo, Burton S. Kaliski Jr.
• Keywords: Cryptology, CTRSA 2005, Password, Password Authentication, User Passwords