  Digital contact tracing is one of the actions useful, in combination with
other measures, to manage an epidemic diffusion of an infection disease in an
after-lock-down phase. This is a very timely issue, due to the pandemic of
COVID-19 we are unfortunately living. Apps for contact tracing aim to detect
proximity of users and to evaluate the related risk in terms of possible
contagious. Existing approaches leverage Bluetooth or GPS, or their
combination, even though the prevailing approach is Bluetooth-based and relies
on a decentralized model requiring the mutual exchange of ephemeral identifiers
among users' smartphones. Unfortunately, a number of security and privacy
concerns exist in this kind of solutions, mainly due to the exchange of
identifiers, while GPS-based solutions (inherently centralized) may suffer from
threats concerning massive surveillance. In this paper, we propose a solution
leveraging GPS to detect proximity, and Bluetooth only to improve accuracy,
without enabling exchange of identifiers. Unlike related existing solutions, no
complex cryptographic mechanism is adopted, while ensuring that the server does
not learn anything about locations of users.
