Re: Privacy-protecting contact tracer for COVID-19?

Perhaps this is a case where verifiable encryption + escrow is appropriate?

Alice encrypts some useful PII (e.g., phone number, name, address, etc).
She deposits the decryption key for this encrypted data with an escrow
service. In each interaction, she gives an encrypted copy of the data to
the other party or parties. (This giving-of-encrypted data could happen via
gossip over bluetooth, via link to cell phone geodata, etc.) If, somewhere
in the interaction chain, someone learns that Alice may have been exposed,
or may have been a carrier of infection, they go to the escrow service and
make a formal request for Alice's contact info to be unlocked.

The "verifiable" in "verifiable encryption" means that Alice's data can
have provable attributes, both at the time it's given to the escrow service
and at the time it's given to other parties -- even though the value of the
attributes is not known.

Of course, this requires an escrow service to be trusted, and that's a risk
worth pondering. However, the escrow service isn't accumulating a record of
Alice's interactions -- the only piece of data it knows is her decryption
key. And its release of such a key could be audited and surrounded with
safeguards.