SoK: Anonymous Credentials

Anonymous credentials are a powerful tool for making assertions about identity while maintaining privacy and have been the subject of study for many years. The interest in anonymous credentials has intensified in recent years as the Internet and Web have become more and more interwoven into the fabric of our daily lives, causing large scale concerns about privacy. In particular, users are now wanting to reduce the amount of personal information they share in order to gain access to services. Since their introduction by Chaum (Comm. ACM 1985) there have been a plethora of results attempting to instantiate them with the first fully anonymous scheme being realised by Camenish and Lysyanskaya (EUROCRYPT 2001). Since this breakthrough result, there have been several newer schemes that have been proposed that not only improve on the Camenish-Lysyanskaya scheme but also introduce new features. In fact there have been a large variety of extensions proposed that have led to a seemingly incomparable landscape of schemes. In this paper, we review the many properties of anonymous credential systems, and systematically categorise and compare the approaches in the literature. Our analysis allows us to highlight gaps, open questions and directions for future research in the space of anonymous credentials.