International Association
for Cryptologic Research

Password-Authenticated Key Exchange allows users to generate a strong cryptographic key based

on a shared \\human-memorable\" password without requiring a public-key infrastructure. It is one of the most

widely used and fundamental cryptographic primitives. Unfortunately, mass password theft from organizations

is continually in the news and, even if passwords are salted and hashed, brute force breaking of password

hashing is usually very successful in practice.

In this paper, we propose two ecient protocols where the password database is somehow shared among two

servers (or more), and authentication requires a distributed computation involving the client and the servers.

In this scenario, even if a server compromise is doable, the secret exposure is not valuable to the adversary since

it reveals only a share of the password database and does not permit to brute force guess a password without

further interactions with the parties for each guess. Our protocols rely on smooth projective hash functions and

are proven secure under classical assumption in the standard model (i.e. do not require idealized assumption,

such as random oracles).