  The outbreak of coronavirus disease 2019 (covid-19) is imposing a severe
worldwide lock-down. Contact tracing based on smartphones' applications (apps)
has emerged as a possible solution to trace contagions and enforce a more
sustainable selective quarantine. However, a massive adoption of these apps is
required to reach the critical mass needed for effective contact tracing. As an
alternative, geo-location technologies in next generation networks (e.g., 5G)
can enable Mobile Operators (MOs) to perform passive tracing of users' mobility
and contacts with a promised accuracy of down to one meter. To effectively
detect contagions, the identities of positive individuals, which are known only
by a Governmental Authority (GA), are also required. Note that, besides being
extremely sensitive, these data might also be critical from a business
perspective. Hence, MOs and the GA need to exchange and process users'
geo-locations and infection status data in a privacy-preserving manner. In this
work, we propose a privacy-preserving protocol that enables multiple MOs and
the GA to share and process users' data to make only the final users discover
the number of their contacts with positive individuals. The protocol is based
on existing privacy-enhancing strategies that guarantee that users' mobility
and infection status are only known to their MOs and to the GA, respectively.
From extensive simulations, we observe that the cost to guarantee total privacy
(evaluated in terms of data overhead introduced by the protocol) is acceptable,
and can also be significantly reduced if we accept a negligible compromise in
users' privacy.
