  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.
