Taming Big Brother Ambitions: More Privacy for Secret Handshakes

In Secret Handshakes (SH) and Affiliation-Hiding Authenticated Key Exchange (AH-AKE) schemes, users become group members by registering with Group Authorities (GAs) and obtaining membership credentials. Group members then use their membership credentials to privately authenticate each other and communicate securely. The distinguishing privacy property of SH and AH-AKE is that parties learn each other’s groups affiliations and compute common session keys only if their groups match. Current SH and AH-AKE schemes consider GAs to be fully trusted, especially, with regard to (i) security of the registration phase (no phantom members), (ii) secrecy of established session keys, and (iii) privacy. The impact of possible “big brother” ambitions of malicious GAs has not been investigated so far. In this paper, we discuss implications on group members’ privacy and security of their communication in the presence of possible GA corruptions. We demonstrate problems arising from relaxed GA trust assumptions and propose an efficient — yet provably secure — AH-AKE protocol with enhanced privacy properties.