Contact tracing is on the radar of the majority of European privacy professionals these days. More specifically, the focus is on whether tracing apps should function based on centralized or decentralized systems.

The debate over the better approach boils down to effectiveness versus the current and future risks associated with the potential use of personally identifiable data. Member of European Parliament Alexandra Geese recently hosted a webinar on the matter, seeking to sort out the privacy implications of the centralized and decentralized methods.

Carmella Troncoso, leader of the Decentralized Privacy-Preserving Proximity Tracing project within the Pan-European Privacy-Preserving Proximity Tracing initiative, was on hand to explain the decentralized approach and its privacy benefits.

"When we started all of this, our philosophy was that it seems like this is necessary," said Troncoso, who was joined on the webinar by European Data Protection Supervisor Wojciech Wiewiórowski and Panoptykon Foundation Co-Founder and President Katarzyna Szymielewicz. "But if we’re going to deploy a technology with such a brutal invasiveness, we need to make sure it hasn’t done any harm. After the pandemic, we don’t want there to be a trace of it or have it create a new problem."

According to Troncoso, the decentralized method was built with a "big effort to make the server powerless," alluding to the approach employing Bluetooth tracking that doesn't requires personal information and leaves no trail back to participants. In contrast, she said a centralized system, which functions off participants' annonymized data, “puts the server in a position of trust, where it won’t misuse” people's information.

"There’s a lot of information there about how often people meet, who has common friends and other things that were not needed for the purpose of the system," Troncoso said regarding a potential social graph being curated from the identifiers generated by the centralized system on both infected individual and those who come in contact with them.

The social graph would be a mere starting point for potentially invasive uses stemming from the centralized system, which unlike the decentralized systems comes with a high degree of data retention. Long-term storage of citizen's data could lead to added surveillance or misuse by government agencies.

Governments in Poland and the U.K. have indicated they prefer a centralized system, according to Szymielewicz, who is on the fence regarding a preferred app system. Other member states, including Germany and France, have shifted their attention to the decentralized approach.

She said her "red line" on the matter would come with the disclosure of personal identifiers.

"Obviously there are principles and then there is real life,” Szymielewicz said. “If we go with the decentralized approach we gain all privacy protections, but we don’t get anonymous aggregated data for public health authorities. On the other hand ... at no point can numbers or identifiers be attributed to phone numbers, addresses or location."

Despite its pitfalls, Wiewiórowski and the EU data protection authorities aren't opposed to centralized system if its deployed and maintained properly.

"There is definitely a preference among data protection authorities in a decentralized approach, but we understand some countries and developers see the feasibility of some functionalities being carried centrally," Wiewiórowski said. "We think it’s something that’s worth a try, but it has to be done with perfect control over the governments and authorities that will have a role in the solution."

Wiewiórowski also took time to touch on the guidance from the European Data Protection Board regarding the apps, noting the guidelines and accompanying annexes for app developers “are not final” and will "continue to evolve as the apps evolve." He added the board is amenable to supporting an app and its system under the conditions of temporary use, known purpose and details on data access.

"These are simply the conditions that need to be fulfilled to do something extraordinary in an extraordinary time." Wiewiórowski said. "We can’t save civilization by resigning from fundamental values."

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