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TCNCoalition / TCN

Specification and reference implementation of the TCN Protocol for decentralized, privacy-preserving contact tracing.
MIT License
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How to get TCN from iOS device in background from Android #79

Open gnuhel opened 4 years ago

gnuhel commented 4 years ago

Hi,

we are trying to use TCN protocol to track iOS and Android devices. iOS could retrieve TCN when advertising data from Android. But when iOS advertise the data, Android could not see any TCN data. Are we doing something wrong? We are using the implementation of Android and iOS in the repo.

In the doc it says

Android (Listener)  <=> iOS  (SENDER)
The Listener signals availability as a Bluetooth peripheral. The Sender, acting as a Bluetooth central, connects to this peripheral and writes its TCN to a field exposed by the peripheral then disconnects.

So Android opens a GATTServer and iOS can write TCN to it.

What happens if iOS is in the background? does it still try to write anything to the Android Gatt Server? I tried it but I didn't get anything? What would be the solution?

ivnsch commented 4 years ago

The last time we tested iOS -> Android, it was working. We will start a new testing round in a few days and update this issue.

gnuhel commented 4 years ago

Have you tried Android <->iOS (in background)?

ivnsch commented 4 years ago

Yeah, we tested in both directions about 1 month ago. You can try out with the CoEpi app, which uses these libraries: iOS, Android, beta branch and 0.2 tag respectively. We'll be releasing a new version in the next days so will be testing this again.

Can you specify which versions you used?

ivnsch commented 4 years ago

Confirming that with the CoEpi apps, I'm able to see the iOS app's TCN while the iOS app is in the background:

Android version: https://github.com/Co-Epi/app-android/commit/5b142d452e733cf831ff50be3ca7ccafeb7a9bb1 iOS version: https://github.com/Co-Epi/app-ios/commit/7c5880e9d0f4538dfc747cbeabc4ff150148a2a4

As for the internals (AFAIR!), while Android can connect to an iOS device advertising in the background, there was a bug in Android's BLE after doing many connections, because of which it was decided that the iOS device acts as a central and connects to the Android's peripheral to write its TCN instead. When iOS scans in the background it uses passive mode, which means that it can't request the service UUID from the peripheral if it's not provided directly in the advertisement data. But Android does send it in the advertisement data, so it's not a problem.