Since Covid-19 began its spread across the world, technologists have proposed using so-called contact-tracing apps to track infections via smartphones. Now, Google and Apple are teaming up to give contact-tracers the ingredients to make that system possible—while in theory still preserving the privacy of those who use it.

On Friday, the two companies announced a rare joint project to create the groundwork for Bluetooth-based contact-tracing apps that can work across both iOS and Android phones. In mid-May, they plan to release an application programming interface that apps from public health organizations can tap into. The API will let those apps use a phone's Bluetooth radios—which have a range of about 30 feet—to keep track of whether a smartphone's owner has come into contact with someone who later turns out to have been infected with Covid-19. Once alerted, that user can then self-isolate or get tested themselves.

"We need to keep advocating to make sure this is done properly." Cristina White, Stanford University

Crucially, Google and Apple say the system won't involve tracking user locations or even collecting any identifying data that would be stored on a server. "This is a very unprecedented situation for the world," said one of the joint project's spokespeople in a phone call with WIRED. "As platform companies we’ve both been thinking hard about what we can do to help get people back to normal life and back to work effectively. We think in bringing the two platforms together we can solve digital contact tracing at scale in partnership with public health authorities and do it in a privacy-preserving way."

Unlike Apple, which has complete control over its software and hardware and can push system-wide changes with relative ease, Google faces a fragmented Android ecosystem. The company will still make the framework available to all devices running Android 6.0 or higher by delivering the update through Google Play Services, which does not require hardware partners to sign off.

Several projects, including ones led by developers at MIT, Stanford, and the governments of Singapore and Germany, have already proposed, and in some cases implemented, similar Bluetooth-based contact-tracing systems. Google and Apple declined to say which specific groups or government agencies they’ve been working with. But they argue that by building operating-level functions those applications can tap into, the apps will be far more effective and energy efficient. Most importantly, they'll be interoperable between the two dominant smartphone platforms.

In the version of the system set to roll out next month, the operating-system-level Bluetooth tracing would allow users to opt in to a Bluetooth-based proximity-detection scheme when they download a contact-tracing app. Their phone would then constantly ping out Bluetooth signals to others nearby while also listening for communications from nearby phones.

If two phones spend more than a few minutes within range of one another, they would each record contact with the other phone, exchanging unique, rotating identifier “beacon” numbers that are based on keys stored on each device. Public heath app developers would be able to "tune" both the proximity and the amount of time necessary to qualify as a contact based on current information about how Covid-19 spreads.

If a user is later diagnosed with Covid-19, they would alert their app with a tap. The app would then upload their last two weeks of keys to a server, which would then generate their recent “beacon” numbers and send them out to other phones in the system. If someone else's phone finds that one of these beacon numbers matches one stored on their phone, they would be notified that they’ve been in contact with a potentially infected person and given information about how to help prevent further spread.