Affiliation-Hiding Key Exchange with Untrusted Group Authorities

Privacy-preserving techniques are increasingly important in our highly computerized society where privacy is both precious and elusive. Affiliation-Hiding Authenticated Key Exchange (AH-AKE) protocols offer an appealing service: authenticated key agreement coupled with privacy of group memberships of protocol participants. This type of service is essential in privacy-conscious p2p systems, mobile ad hoc networks and social networking applications. Prior work has succeeded in constructing a number of secure and efficient AH-AKE protocols which all assume full trust in the Group Authority (GA) -- the entity that sets up the group as well as registers and (optionally) revokes members. In this paper, we argue that, for many anticipated application scenarios, the trusted GA model should be relaxed to allow for certain types of malicious behavior. We examine the consequences of malicious GAs and explore the design of stronger AH-AKE protocols that withstand GA attacks. Our results demonstrate that such protocols are both feasible and practical.