  Trolls, bots, and sybils distort online discourse and compromise the security
of networked platforms. User identity is central to the vectors of attack and
manipulation employed in these contexts. However it has long seemed that, try
as it might, the security community has been unable to stem the rising tide of
such problems. We posit the Ghost Trilemma, that there are three key properties
of identity -- sentience, location, and uniqueness -- that cannot be
simultaneously verified in a fully-decentralized setting. Many
fully-decentralized systems -- whether for communication or social coordination
-- grapple with this trilemma in some way, perhaps unknowingly. In this
Systematization of Knowledge (SoK) paper, we examine the design space, use
cases, problems with prior approaches, and possible paths forward. We sketch a
proof of this trilemma and outline options for practical, incrementally
deployable schemes to achieve an acceptable tradeoff of trust in centralized
trust anchors, decentralized operation, and an ability to withstand a range of
attacks, while protecting user privacy.
