Android
To inform Android users about incoming messages in the background, you can use Threema’s own and independent push service “Threema Push,” which doesn’t generate metadata for third parties.
Otherwise, Threema uses the push service already installed on the device, i.e., Firebase Cloud Messaging (FCM) by Google (or Huawei’s push kit HMS). The app then fetches messages directly from the Threema servers, decrypts them, and displays a local notification. Neither contents nor details about messages are transmitted via FCM (the FCM payload is empty), and all of Firebase’s tracking and analysis components have been removed.
iOS
Threema uses the Apple Push Notification Service (APNS) to inform recipients with iOS about new messages while the app is closed or in the background. The APNS message contains a payload that has been encrypted with a symmetric key, which is negotiated between the app and the Threema servers and is not known to Apple.
Within this encrypted payload, the Threema ID and nickname of the sender, the message ID, and the fact whether it is a direct or a group message, are transmitted.
The Threema app is started in the background for each incoming push notification, decrypts the push payload, downloads the corresponding message directly from the Threema servers, decrypts it, and shows a local message preview (if enabled) and the contact name of the sender.
