  We propose a decentralized digital contact tracing service that preserves the
users' privacy by design while complying to the highest security standards. Our
approach is based on Bluetooth and measures actual encounters of people, the
contact time period, and estimates the proximity of the contact. We trace the
users' contacts and the possible spread of infectious diseases while preventing
location tracking of users, protecting their data and identity. We verify and
improve the impact of tracking based on epidemiological models. We compare a
centralized and decentralized approach on a legal perspective and find a
decentralized approach preferable considering proportionality and data
minimization.
