Privacy protection for authentication protocols

In our highly computerized and networked society, privacy of individuals is precious and becomes increasingly important. Problems particularly arise in the context of authentication protocols where, as a general rule, entities actively reveal their respective identities to each other. To encounter this issue, different privacy-preserving authentication methods have been developed in the last decades. The list of these techniques comprises, apart from identity escrow, ring authentication, hidden and anonymous credentials, and several others, the concept of affiliation-hiding authentication (AHA). Such protocols offer the appealing and seemingly contradictory service to enable users to authenticate each other as members of a certain group without revealing their affiliation to group outsiders. In AHA protocols (also known as Secret Handshakes), users become group members by registering with group authorities (GAs) and obtaining individual membership credentials. Group members then use their credentials to privately authenticate each other, optionally also establishing a secure session key. The pivotal privacy property that contrasts AHA with classical authentication or authenticated key establishment is that parties learn each other's affiliations to groups and compute common session keys if and only if their groups match. Prior work has succeeded in constructing AHA protocols that offer different degrees of security, privacy, and efficiency. However, a set of essential problems have been left open. These include a close study of the level of trust that intrinsically has to be placed into participants of such systems (including into GAs), the extension of the single-group setting with only one GA to a setting where users are affiliated to multiple groups and, through AHA, want to discover matching ones, and certainly the question of efficient implementability. We argue that all these topics are highly relevant for practical deployment of privacy-preserving authentication in general, and AHA in particular. In this thesis, the author concretizes and cryptographically models these challenges, and offers provably secure solutions. Furthermore, this thesis treats privacy-related challenges that are posed in the context of network-based social interactions. Without doubt, online social networks, that help participants to build and reflect their social relations to other participants, have taken an essential role in people's daily life. A key step in the constitution of new links between participants consists of the reconciliation of shared contacts or friends. The author develops techniques to discover common contacts in social networks in a privacy-aware manner, i.e., without disclosing non-matching contacts. Besides formalizing this task and offering appropriate solutions, the thesis analyzes an interesting connection between AHA protocols and the challenge of private discovery of common contacts. By identifying and solving a variety of relevant open problems in the context of privacy-aware authentication, this thesis contributes to wide-scale deployment of methods that respect and regain user privacy in p2p systems, mobile ad hoc networks, and social networking applications.