There is no "backdoor" when the browser is sandboxed. "backdoor" is a specific thing, I think you need to read up on it before you keep using it incorrectly:
https://en.wikipedia.org/wiki/Backdoor_(computing)
>On the other hand, an app is sandboxed, too (on mobile OSes like Android and iOS). When you download it, you can check a hash that you can (if you want to) compare with a friend to see if they got the same app.
That isn't what "sandboxed" means, it has nothing to do with checking hashes. And no, mobile apps are not really sandboxed, they have full access to your mobile device once you install it and give it access - and let's be real, most people are just going to blindly click "allow" for anything the app requests after installing an app.
>With an app, there is intermediary (the "app store") that would need to collude with the developers to send a backdoor just for you, and even then you would still have the app binary as proof.
You keep referring to "backdoor", and I don't think you really know what that means.
>That's always a question I have with "secure" web services: if you use ProtonMail, you trust that Proton doesn't send you a web page that leaks your key. But if you trust Proton for that, what's the point of the end-to-end encryption? When you use the Signal app, the whole idea is that you don't have to trust Signal for the end-to-end encryption, at all.
That isn't how any of this works. The main value proposition of Signal is that we do trust its end-to-end encryption. Protonmail sending a "web page" that "leaks your key"? WTF?
