Even if the app encrypts everything client-side and does everything over Tor, just having the app installed on your phone (which both Google Play and the Apple

Even if the app encrypts everything client-side and does everything over Tor, just having the app installed on your phone (which both Google Play and the Apple App Store track) is sufficient to make you look suspicious from that point of view. Really, what you'd need is a very popular app to add a feature like this so that the percentage of users of the app that are potentially undocumented immigrants is the same as their proportion of the population. Then you do all the other crypto and hopefully don't store whether individual users have the feature enabled.