It is user facing in several places. Play Store on the web shows it as the URL for the application page. Settings app shows the app id too. It's in multiple other places too, but it's mostly an under the hood unique identifier for an app. It's how the OS identifies app packages.
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If you try to install an older release of an app, meaning installing a package or set of package (split apks) with a lower versionCode and same app id, you get an error from the downgrade prevention which is both an important security and data corruption prevention feature.
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If you try to install an app with the same app id as one you have installed in any profile which has a mismatched signing key NOT authorized by the original key via a key rotation proof, you get a signature validation error explaining that the signing key is mismatched.
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Downgrade protection error could be improved by making it describe the specific problem. Signing key mismatch already has a specific error that's worded properly in AOSP but it's still confusing when you don't realize what F-Droid is doing and think devs publish their apps on it.
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The problem is that they are reusing the app id while using their own signing key for the app so it conflicts with the app from the developer across profiles. If they changed the app id to their own for their own builds as is intended none of that would be an issue.
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I thought that was determined by the dev who publishes the app on the store?
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Developers do not publish their apps on F-Droid. F-Droid publishes their builds of apps. You're confusing how the Play Store works with how F-Droid works.
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Wait, you can't publish your app on f droid? I'm not at all familiar with how this process works, I'm not a dev.
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You can technically only request them to publish. If F-droid thinks your app is open source enough, they will build it from source on their own and publish it with their own signing keys.
This is one of the key reasons Moxie also hates to push Signal onto F-droid.
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Only if one of them is interested in packaging it and it doesn't get blocked for one reason or another by someone else.