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Signal App (by Open Whisper) provides encrypted voice calls allowing two people to communicate via a data connection with secure encryption (assuming no MiTM attack). Given that Signal App uses the microphone on the mobile devise (iPhone/Android) and many other apps may also have permission to access the mic, is there a potential for the call to be listened to by other apps?

Does Signal have any method for gaining exclusive access to the mic and excluding other apps from access it while making a call? If yes, what is this mechanism?

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At the moment

Currently, the best way is to kill apps in the background that have microphone permissions, which is a bit of an ugly hack, and if you do not kill the apps fast enough they will be able to record snippets of your call.

https://stackoverflow.com/questions/17467120/exclusive-access-to-the-microphone-in-android

In the future

Android P appears to fix this, by making apps in the background act as if the microphone is silent, along with several other sensors.

https://www.theverge.com/2018/3/7/17091104/android-p-prevents-apps-using-mic-camera-idle-background

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