Cryptographic Techniques for Privacy Preserving Identity

Currently, people have a limited range of choices in managing their identity online. They can use their real name or a long-term pseudonym, thereby lending context and credibility to information they publish but retaining no control over their privacy, or they can post anonymously, ensuring strong privacy but lending no additional credibility to their posts. In this work, we aim to develop a new type of online identity that allows users to publish information anonymously and unlinkably while simultaneously backing their posts with the credibility offered by a single, persistent identity. We show how these seemingly contradictory goals can be achieved through a series of new cryptographic techniques. Our consideration of the utility of persistent identities focuses on their ability to develop reputation. In particular, many online forums include systems for recording feedback from a users prior behavior and using it to filter spam and predict the quality of new content. However, the dependence of this reputation information on a users history of activities seems to preclude any possibility of anonymity. We demonstrate that useful reputation can in fact, coexist with strong privacy guarantees by developing a novel cryptographic primitive we call signatures of reputation which supports monotonic measures of reputation in a completely anonymous setting. In our system, users can express trust in others by voting for them, collect votes to build up their own reputation, and attach a proof of their reputation to any data they publish, all while maintaining the unlinkability of their actions. Effective use of our scheme for signatures of reputation requires a means of selectively retrieving information while hiding ones search criteria.