Two companies that agree on very little announced a joint protocol within the same hour, which is itself a measure of how unusual the moment is. The feature will ship inside the operating systems rather than as an app.

How the matching works

Phones broadcast rotating anonymous identifiers over Bluetooth and record the ones they hear. Nothing leaves the device unless a user reports a confirmed diagnosis, at which point their recent identifiers are published for other phones to check against locally.

The design deliberately avoids a central database of who was near whom. Location is never used, and the identifiers change often enough that following one person over a day is impractical.

Whether it works is a public-health question rather than an engineering one. Uptake, not cryptography, is the variable that decides the outcome.