The contact-tracing tool would have your smartphone log when you’ve come into close contact with other people. If one of those people later reports Covid-19 symptoms to a public health authority, your phone would send you an alert. It works a bit like exchanging contact information with everyone you meet, except everything is designed to be anonymous and automatic. Instead of contact info, your smartphone will periodically exchange anonymized tracing keys with nearby devices. Both devices maintain a list of the keys they’ve collected on a cloud server, and when one person reports an infection, they have the optin of sending an alert to people they’ve recently been in contact with. That alert will share information for what those people should do next.