  Contact tracing is an important instrument for national health services to
fight epidemics. As part of the COVID-19 situation, many proposals have been
made for scaling up contract tracing capacities with the help of smartphone
applications, an important but highly critical endeavor due to the privacy
risks involved in such solutions. Extending our previously expressed concern,
we clearly articulate in this article, the functional and non-functional
requirements that any solution has to meet, when striving to serve, not mere
collections of individuals, but the whole of a nation, as required in face of
such potentially dangerous epidemics. We present a critical information
infrastructure, PriLock, a fully-open preliminary architecture proposal and
design draft for privacy preserving contact tracing, which we believe can be
constructed in a way to fulfill the former requirements. Our architecture
leverages the existing regulated mobile communication infrastructure and builds
upon the concept of "checks and balances", requiring a majority of independent
players to agree to effect any operation on it, thus preventing abuse of the
highly sensitive information that must be collected and processed for efficient
contact tracing. This is enforced with a largely decentralised layout and
highly resilient state-of-the-art technology, which we explain in the paper,
finishing by giving a security, dependability and resilience analysis, showing
how it meets the defined requirements, even while the infrastructure is under
attack.
