During the pandemic, in Taiwan, we used a privacy-preserving contact-tracing method. Basically, a venue prints a random number on the QR code on the front door, a person scans it and sends it to a well-known number, 1922, but the telecom knows nothing about what this random number means, and the venue learns nothing — not even the phone number of the visitor — and the state learns nothing whatsoever. If an infection happens, we can do contact tracing and use recursive notification, again, without sacrificing any of the privacy of the people who are not in the affected area. I hope this illustration shows a little bit of the flavour of how a zero-knowledge data-knowledge sharing arrangement can actually work.