  Governments and researchers around the world are implementing digital contact
tracing solutions to stem the spread of infectious disease, namely COVID-19.
Many of these solutions threaten individual rights and privacy. Our goal is to
break past the false dichotomy of effective versus privacy-preserving contact
tracing. We offer an alternative approach to assess and communicate users' risk
of exposure to an infectious disease while preserving individual privacy. Our
proposal uses recent GPS location histories, which are transformed and
encrypted, and a private set intersection protocol to interface with a
semi-trusted authority.
  There have been other recent proposals for privacy-preserving contact
tracing, based on Bluetooth and decentralization, that could further eliminate
the need for trust in authority. However, solutions with Bluetooth are
currently limited to certain devices and contexts while decentralization adds
complexity. The goal of this work is two-fold: we aim to propose a
location-based system that is more privacy-preserving than what is currently
being adopted by governments around the world, and that is also practical to
implement with the immediacy needed to stem a viral outbreak.
