Since mid-June there has been a conversation several million times a day in Germany that goes something like this: “Hello,” says one of the two participants, “I’m 0xFD6F”. “Excellent,” says the other, “let’s exchange numbers.”
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And so it happens. The two keep the other’s number. And if you don’t hear each other after two weeks, delete the contact again.
The interlocutors are inherently non-human and the exchange numbers are not for a romantic date. The cell phones on which the new Corona warning app is installed speak to each other according to this model. This app should help against the spread of Corona. Many people in Germany want to take part: after only two weeks, the app had more than 14 million downloads.
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The idea is actually very simple. The common thing about the virus is that you can infect someone before you feel sick. And when a person, let’s call them Paul, gets sick, it is very difficult to find out who Paul was in contact with – who may already be infected without knowing it. Paul needs to remember where he has been in the past few days and who he has met. And then all of these people need to be informed.
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The app now does this almost completely automatically. The inventors of the app thought they would: Almost everyone almost always has their cell phone with them. If Paul’s cell phone could now remember which other cell phones are nearby, Paul wouldn’t have to diligently remember them.
The application uses bluetooth. This is a short range radio that was invented for headphones and microphones. The application now sends a signal via Bluetooth, namely: 0xFD6F. It means something like: I am a Corona app and am looking for other Corona apps near me.
When two applications are close enough to each other (e.g. in the same room), they exchange an encrypted string. You can’t tell who owns the phone. This is good for data protection. It is also not stored where the meeting took place – that is nobody’s business either. However, the app remembers how long the meeting lasted and how far the phones were at least roughly.
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For example, suppose Paul and Paula sat together on a bus for more than 15 minutes and less than three feet away – in this case, it’s easy to get infected. What if Paul has a fever and a weak virus?
Now Paul reports as infected via his app. For this he needs a code from the health department, otherwise it will not work. This rule exists so that no one simply reports sick and panic spreads. But Paul has the code and is allowed to report. Now Paul sends a secret code to all cell phones. This happens about once a day. And all cell phones can now check whether Paul’s cell phone has ever been around.
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Then Paula’s cell phone shows a warning: There was a contact, maybe you got infected. The app doesn’t say where and when it was, and certainly not who owns the phone. Nobody discovers that Paula could be infected. Now it can be tested and quarantined as a precaution.
So much for data protection. Because for the app to work well, as many people as possible have to participate. However, they only do this if they can be sure that the app will not check them. Where you are and who you meet is nobody’s business. The Corona app takes this into account.
This article was published in “Dein SPIEGEL” 08/2020.
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