A Contractual Anonymity System

We propose, develop, and implement techniques for achieving contractual anonymity. In contractual anonymity, a user and service provider enter into an anonymity contract. The user is guaranteed anonymity and message unlinkability from the contractual anonymity system unless she breaks the contract. The service provider is guaranteed that it can identify users who break the contract. The significant advantages of our system are that 1) the service provider is not able to take any action toward a particular user (such as revealing her identity or blacklisting her future authentications) unless she violates her contract, 2) our system can enforce a variety of policies, and 3) our system is efficient.