So @Pinboard is probably right that large-scale contact tracing demands well-regulated access to mobile operators and advertising services data. But if we do go the forced-app route, how could it be minimally disclosing? I wrote up a cryptographic outline:https://gist.github.com/colmmacc/740e7db2d9c5836848c849c02f600ec9 …
Once a contact can be identified, I suspect a lot of people would then give up their location history to the epidemiologist. That's different than having a very convenient searchable trove. It does make it more work for them to identify hotspots though.