> Is there an easy way to get a direct HTTPS download link from the Play Store?
Easy? No. But you can grab the apk from any number of third party download sites and verify the signature, and you can also just download it from https://signal.org/android/apk/ . Unless you've installed something via the Play Store, Android won't autoupdate it.
> • It's not the case for iOS.
It's still not hard to obtain iOS packages and verify their contents - it's definitely not as straightforward to use those as it is to sideload updates for Android, but it's still possible to verify that the binary matches the source, and you only need one person to notice.
> • It's not the case for WhatsApp.
Your claim was "Signal encryption doesn’t actually work". WhatsApp uses a whole bunch of Signal, but it's not Signal. This is like claiming that the IoT device I found that uses Signal in the backend but has all the key material in an unprotected Firebase bucket tells us anything about the security of Signal.
> • You have to repeat the process for every update, of which there are many.
No, someone has to. And that's something that could be automated.
> • You don't know what version other people are running, which also matters. Consider group chats!
If your position is "We should have infrastructure that makes it easy for third parties to audit Signal updates correspond to the source code", I absolutely agree! But we can build that with what currently exists, Signal's centralised infrastructure does nothing to prevent that.
> Encryption in which you rely on your adversary to encrypt messages for you is conceptually broken.
Every time someone sends an encrypted message, they're relying on a huge stack of technology that's largely outside their control. If my keyboard becomes untrustworthy, my guarantees are gone. If my video driver becomes untrustworthy, I'm in a bad place. Using any form of technology implies placing some trust in an awfully large set of people. On a daily basis, we're relying on an awful lot of faith. The Signal devs have gone out of their way to make it easier to verify whether that faith is misplaced or not.
But:
> We have no way to know, only faith.
We literally do have a way to know. We can dump every Signal APK installed on every phone and determine whether they match the source. It wouldn't be easy, but it could be done.
> If one day Signal or WhatsApp change the deal… then we can no longer really claim the service is end-to-end encrypted.
Signal doesn't appear to have changed the deal, so it seems like you're saying we can currently claim that it's end-to-end encrypted?