Apple and Google are working together on an ambitious plan to alert people who have come in contact with coronavirus patients - here's how it will work

The project is being rolled out in two phases.

The first will require that those who want to opt into the program download an app distributed by a public health authority. Apple and Google are making their application programming interfaces, or tools that enable apps to use certain aspects of an operating system, available for public health authorities in May.

The second phase will involve Apple and Google building the ability to participate in the program directly into their respective smartphone operating systems, eliminating the need to download an app. This stage is expected to arrive in the coming months.

Both steps will roll out to iOS and Android devices through software updates.

But users will have to provide their explicit consent by opting into the program when it launches if they wish to participate.

Apple and Google have shared some clarity on how they plan to address older devices that may not be supported by future software updates. Google has said that it will be distributing these updates through Google Play Services updates rather than traditional operating system updates, which should allow it to address a broad swath of Android devices.

Apple has said it hopes to address as many devices as it possibly can, although it has an advantage in the fact that iOS is far less fragmented and many iPhones in use already run on the same software. As of January 2020, 77% of devices introduced in the last four years were running on iOS 13, Apple's latest software, according to the company.

The idea of contact tracing through smartphones isn't new.

Contact tracing is the process of monitoring the people that an infected person may have come into contact with so that those people can take the appropriate steps should they develop symptoms.

This is typically done manually, but there's been a growing interest in utilizing smartphones for this purpose recently as the coronavirus has spread. Singapore recently launched a contact tracing app called TraceTogether that also uses Bluetooth signals, and the French government also said its working on an app to warn users if they've come into contact with an infected individual.

The nonprofit Pan-European Privacy-Preserving Proximity Tracing project, which was developed by 130 European technologists and scientists and unveiled earlier this month, also uses Bluetooth low energy signals to enable smartphones to anonymously communicate and alert users when they may have been exposed to the virus.

With their system, Apple and Google hope to encourage widespread adoption by making one large network that works across iOS and Android and doesn't require the user to download an app.

Here's how Apple and Google's system will work.

Apple and Google's contact tracing system will use Bluetooth low energy signals to enable smartphones to simultaneously broadcast and scan for beacons coming from nearby devices.

That means the system would never use location data since it relies on these anonymous keys being sent out from nearby smartphones to determine your proximity to a potentially contagious person.

Here's an example of what a scenario in which the system is used could look like. A man and a woman are sitting on a park bench and have a brief, 10-minute conversation. Their phones exchange anonymous keys via Bluetooth that don't include any identifiable information.

The man is later diagnosed with COVID-19 after testing positive, and he enters this information into the app he's downloaded from a public health authority. The man gives the app permission to send the last 14 days worth of keys for his broadcast beacons to a server, where they're stored for 14 days.

Meanwhile, the woman's phone periodically checks keys from the server and downloads them to her phone. That list of keys from the server is then checked against the keys that the woman's phone has collected while in proximity to other devices.

Since she was recently in proximity to the man who tested positive who chose to share his keys to the server, the system would then find a match in the list of keys her phone collected and the list of keys from downloaded from the server. This matching process is done locally on her device.

The woman would then receive a notification from a public health authority alerting her that she's been in contact with someone who has tested positive for COVID-19. That notification would include a link directing her to a health authority app or website where she can find more information.

The companies say the system has been built with privacy in mind.

Apple and Google say the system has many protections in place to ensure that privacy isn't compromised.

First, the user will have to opt in to use the service, even when the second phase rolls out and it's baked into the operating system. Both companies say the system is designed to be opt-in only, and governments won't be able to mandate use of the technology.

The keys being broadcast from the user's device do not contain any identifiable information; they're just a string of numbers. The identities of people who have tested positive would not be revealed to other users or to Apple and Google.

These keys also change every 15 minutes, so it wouldn't be possible to track a device for an extended period of time. And again, the system doesn't use any location data.

There also isn't a centralized server where the keys are stored long-term, and the key-matching process also occurs on the user's device locally.

Apple and Google are only offering these APIs to public health authorities, and the companies can disable the beacon broadcasting system when it's no longer needed.

They've also thought about how to prevent false positives, although it's not entirely clear how the system will do so in practice.

Namely, what's to stop someone from saying they have COVID-19 when they in fact don't.

A Google representative said the team has considered using a system in which some external verification from a healthcare professional is required. For example, one idea could involve scanning a QR code from your doctor after testing positive for COVID-19 in order to verify this within the app.

But this will only work if a lot of people adopt it and widespread testing becomes available.

For Apple and Google's system to meaningfully help in slowing the spread of COVID-19, two key things need to happen: The system must be widely-used, and there must be widespread testing.

The former point is a big part of the reason why Apple and Google have stressed in their joint announcement that the system was built with privacy in mind. The latter will likely be a significant challenge in getting the system to work to its full potential, as the United States has struggled to make COVID-19 testing available broadly.