In Taiwan, we introduced a system called the SMS-based contact tracing. If you enter a venue, you will see a QR code and a 15-digit random code. If you scan the QR code, you’ll see exactly the same, 15 digits, and you can text it to 1922, the national number for pandemic response. But what it does is that it stores the random number the venue owner prepares into your telecom, but the state learns nothing about this. Your telecom doesn’t even know what those random numbers mean, and you just show the venue you have texted 1922, so the venue doesn’t learn about your phone number either. So nobody knows anything that they did not already know. This is called zero knowledge.