The tech behind contact tracing apps – Part 2

Hello again dear reader!

close up photo of assorted color of push pins on map

In my previous post we took a look at how the whole exposure notification system works on iOS and Android. Looking into how each App required its own server and how the rotating ID works. These last few days I have been trying to delve further into what the phone actually does.

Like we mentioned, once you are assigned a random ID that uniquely identifies your device but has no personal information assigned it is interchanged with other devices. This is all stored on device, with an exposure log that can be viewed in your phone’s setting menu. In fact the server only handles key creation, which makes sense, because imagine the amount of traffic that would cause, it would be millions of gigabyte and unheard concurrency. Anyway, once recorded it is kept in the log with the amount of time you have been exposed to that particular user.

So, if a user actually sets off the alarm because he or she has tested positive, the App then looks into the log to see who it needs to notify and sends a special type of push notification. This is why you need specific access to what Apple calls the entitlements API, as it is the system used to relay the positive result to other phones. Think of it as a series of phone number recorded, and you call them all at once because you have their phone number, despite not knowing who they are. The algorithm used to notify the user is depicted below:

Screenshot 2020-08-14 at 01.04.05

This type of push notification is a high priority, special notification that is send with a priority flag. It is similar to a VoIP notification, but access is restricted.

Screenshot 2020-08-14 at 01.12.33

It is curious how what Apple and Google have really build is a common high priority push notification system, and a background system without the usual limits to bluetooth information interchange. But let’s not think about it this way because it kinds of takes the magic away.

Screenshot 2020-08-14 at 01.09.18

I’ll keep updating this with more info over the coming days, please let me know what you would like to know or what interests you most.

Thank for reading,

Hope you are all staying safe in this difficult times.