  Successful containment of the Coronavirus pandemic rests on the ability to
quickly and reliably identify those who have been in close proximity to a
contagious individual. Existing tools for doing so rely on the collection of
exact location information of individuals over lengthy time periods, and
combining this information with other personal information. This unprecedented
encroachment on individual privacy at national scales has created an outcry and
risks rejection of these tools.
  We propose an alternative: an extremely simple scheme for providing
fine-grained and timely alerts to users who have been in the close vicinity of
an infected individual. Crucially, this is done while preserving the anonymity
of all individuals, and without collecting or storing any personal information
or location history. Our approach is based on using short-range communication
mechanisms, like Bluetooth, that are available in all modern cell phones. It
can be deployed with very little infrastructure, and incurs a relatively low
false-positive rate compared to other collocation methods. We also describe a
number of extensions and tradeoffs.
  We believe that the privacy guarantees provided by the scheme will encourage
quick and broad voluntary adoption. When combined with sufficient testing
capacity and existing best practices from healthcare professionals, we hope
that this may significantly reduce the infection rate.
