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The Challenge of Proximity Apps For COVID-19 Contact Tracing

Around the world, a diverse and growing chorus is calling for the use of smartphone proximity technology to fight COVID-19. In particular, public health experts and others argue that smartphones could provide a solution to an urgent need for rapid, widespread contact tracing—that is, tracking who infected people come in contact with as they move through the world. Proponents of this approach point out that many people already own smartphones, which are frequently used to track users’ movements and interactions in the physical world.

But it is not a given that smartphone tracking will solve this problem, and the risks it poses to individual privacy and civil liberties are considerableLocation tracking—using GPS and cell site information, for example—is not suited to contact tracing because it will not reliably reveal the close physical interactions that experts say are likely to spread the disease. Instead, developers are rapidly coalescing around applications for proximity tracing, which measures Bluetooth signal strength to determine whether two smartphones were close enough together for their users to transmit the virus. In this approach, if one of the users becomes infected, others whose proximity has been logged by the app could find out, self-quarantine, and seek testing. Just today, Apple and Google announced joint application programming interfaces (APIs) using these principles that will be rolled out in iOS and Android in May. A number of similarly designed applications are now available or will launch soon.

As part of the nearly unprecedented societal response to COVID-19, such apps raise difficult questions about privacy, efficacy, and responsible engineering of technology to advance public health. Above all, we should not trust any application—no matter how well-designed—to solve this crisis or answer all of these questions. Contact tracing applications cannot make up for shortages of effective treatment, personal protective equipment, and rapid testing, among other challenges.

COVID-19 is a worldwide crisis, one which threatens to kill millions and upend society, but history has shown that exceptions to civil liberties protections made in a time of crisis often persist much longer than the crisis itself. With technological safeguards, sophisticated proximity tracking apps may avoid the common privacy pitfalls of location tracking. Developers and governments should also consider legal and policy limits on the use of these apps. Above all, the choice to use them should lie with individual users, who should inform themselves of the risks and limitations, and insist on necessary safeguards. Some of these safeguards are discussed below. 

How Do Proximity Apps Work?

There are many different proposals for Bluetooth-based proximity tracking apps, but at a high level, they begin with a similar approach. The app broadcasts a unique identifier over Bluetooth that other, nearby phones can detect. To protect privacy, many proposals, including the Apple and Google APIs, have each phone’s identifier rotated frequently to limit the risk of third-party tracking.

When two users of the app come near each other, both apps estimate the distance between each other using Bluetooth signal strength. If the apps estimate that they are less than approximately six feet (or two meters) apart for a sufficient period of time, the apps exchange identifiers. Each app logs an encounter with the other’s identifier. The users’ location is not necessary, as the application need only know if the users are sufficiently close together to create a risk of infection.

When a user of the app learns that they are infected with COVID-19, other users can be notified of their own infection risk. This is where different designs for the app significantly diverge.

Some apps rely on one or more central authorities that have privileged access to information about users’ devices. For example, TraceTogether, developed for the government of Singapore, requires all users to share their contact information with the app’s administrators. In this model, the authority keeps a database that maps app identifiers to contact information. When a user tests positive, their app uploads a list of all the identifiers it has come into contact with over the past two weeks. The central authority looks up those identifiers in its database, and uses phone numbers or email addresses to reach out to other users who may have been exposed. This places a lot of user information out of their own control, and in the hands of the government. This model creates unacceptable risks of pervasive tracking of individuals’ associations and should not be employed by other public health entities.

Other models rely on a database that doesn’t store as much information about the app’s users. For example, it’s not actually necessary for an authority to store real contact information. Instead, infected users can upload their contact logs to a central database, which stores anonymous identifiers for everyone who may have been exposed. Then, the devices of users who are not infected can regularly ping the authority with their own identifiers. The authority responds to each ping with whether the user has been exposed. With basic safeguards in place, this model could be more protective of user privacy. Unfortunately, it may still allow the authority to learn the real identities of infected users. With more sophisticated safeguards, like cryptographic mixing, the system could offer slightly stronger privacy guarantees.

Some proposals go further, publishing the entire database publicly. For example, Apple and Google’s proposal, published April 10, would broadcast a list of keys associated with infected individuals to nearby people with the app. This model places less trust in a central authority, but it creates new risks to users who share their infection status that must be mitigated or accepted.

Some apps require authorities, like health officials, to certify that an individual is infected before they may alert other app users. Other models could allow users to self-report infection status or symptoms, but those may result in significant numbers of false positives, which could undermine the usefulness of the app.

In short, while there is early promise in some of the ideas for engineering proximity tracking apps, there are many open questions.

Read more at Source – EFF

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