Ah good point about overlapping location data for sick users, that would add ambiguity. This approach still puts a lot of trust in the server. Every client nee

Ah good point about overlapping location data for sick users, that would add ambiguity. This approach still puts a lot of trust in the server. Every client needs to upload their hashes to the server, so if the group running the server wanted to reverse the hashes they'd get everyone's location, not just sick people. The server would also know which hashes were popular, indicating that they are crowded areas like a city or event center. From a tracking perspective the fact that two people were in the same location is useful to start making inferences, even if the location isn't known.