Who Watches the Watchmen? A Review of Subjective Approaches for Sybil-resistance in Proof of Personhood Protocols

Most current self-sovereign identity systems may be categorized as strictly objective, consisting of cryptographically signed statements issued by trusted third party attestors. This failure to provide an input for subjectivity accounts for a central challenge: the inability to address the question of "Who verifies the verifier?". Instead, these protocols outsource their legitimacy to mechanisms beyond their internal structure, relying on traditional centralized institutions such as national ID issuers and KYC providers to verify the claims they hold. This reliance has been employed to safeguard applications from a vulnerability previously thought to be impossible to address in distributed systems: the Sybil attack problem, which describes the abuse of an online system by creating many illegitimate virtual personas. Inspired by the progress in cryptocurrencies and blockchain technology, there has recently been a surge in networked protocols that make use of subjective inputs such as voting, vouching, and interpreting, to arrive at a decentralized and sybil-resistant consensus for identity. In this article, we will outline the approaches of these new and natively digital sources of authentication -- their attributes, methodologies strengths, and weaknesses -- and sketch out possible directions for future developments.