3PBCS: A Privacy-Preserving, Personhood-Based Credential System
Ksandros Apostoli
M.S. thesis advised by Simone Colombo and Bryan Ford
March 22, 2022
Abstract
Recent anonymous credential schemes present major advances in the way digital
identities are treated, empowering users with fine-grained control over their
personal data. While limiting the amount of user-data that is accessible to
third parties paves the way to stronger privacy guarantees, it also leads to
new challenges in preserving accountability and Sybil-Resistance. In this work,
we present 3PBCS, a novel credential system, which integrates existing
anonymous credential schemes with the notion of Proof-of-Personhood to achieve
privacy, accountability and Sybil-Protection. Our scheme relies on
Proof-of-Personhood Tokens for bootstrapping digital identities. A combination
of classic MixNets and Secure Multiparty Computation is leveraged to achieve
Sybil-Resistance as well as accountability in a fully privacy-preserving
manner. Among usability features, we put emphasis on the support for multiple
pseudonymous accounts for a single person, without sacrificing any of the
above-mentioned guarantees. These characteristics, make our proposed work
suitable for use in a wide variety of applications, including social networks,
where privacy, accountability and Sybil-protection are becoming crucial, and
where support for multiple accounts is a core requirement.