PDM: A New Strong Password-Based Protocol

In this paper we present PDM (Password Derived Moduli), a new approach to strong password-based protocols usable either for mutual authentication or for downloading security information such as the user’s private key. We describe how the properties desirable for strong password mutual authentication differ from the properties desirable for credentials download. In particular, a protocol used solely for credentials download can be simpler and less expensive than one used for mutual authentication since some properties (such as authentication of the server) are not necessary for credentials download. The features necessary for mutual authentication can be easily added to a credentials download protocol, but many of the protocols designed for mutual authentication are not as desirable for use in credentials download as protocols like PDM and basic EKE and SPEKE because they are unnecessarily expensive when used for that purpose. PDM’s performance is vastly more expensive at the client than any of the protocols in the literature, but it is more efficient at the server. We claim that performance at the server, since a server must handle a large and potentially unpredictable number of clients, is more important than performance at the client, assuming that client performance is “good enough”. We describe PDM for credentials download, and then show how to enhance it to have the properties desirable for mutual authentication. In particular, the enhancement we advocate for allowing PDM to avoid storing a password-equivalent at the server is less expensive than existing schemes, and our approach can be used as a more efficient (at the server) variant of augmented EKE and SPEKE than the currently published schemes. PDM is important because it is a very different approach to the problem than any in the literature, we believe it to be unencumbered by patents, and because it can be a lot less expensive at the server than existing schemes.