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3PBCS: A Privacy-Preserving, Personhood-Based Credential System

Ksandros Apostoli
M.S. thesis advised by Simone Colombo and Bryan Ford
March 22, 2022


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.

M.S. Thesis: PDF

Slides: PDF

Topics: Privacy Anonymity Identity Personhood Cryptography Social Networks Bryan Ford