Your comment is close to hitting the weak point: you have to trust Apple. If you can’t, then this single / multiple devices detail doesn’t matter: they could find a million ways to silently subvert the encryption in a closed source app on a closed source OS. But if you can, then waxing on details doesn’t matter either: you trust them, so you trust them.

The thing is: it is valid to trust a company to keep their explicit word. That’s a different issue from trusting a company who never explicitly said they wouldn’t listen to your data: companies can be held liable for these sorts of public statements. If Facebook silently (knowingly) subverts Whatsapp E2E, they’re in trouble. But reading regular Facebook messages for ads? Storing them plain text? They never said they wouldn’t.

This is why Apple explicitly stating they E2E iMessage is a data point with value. It is valid to trust Apple more now, simply because they’ve upped the stakes for themselves.

Compare this to a bar bet: someone makes a claim, I am sceptical. They say, honest! Still sceptical. They say ok, bet you £300. I suddenly am much more inclined to believe them. No money changed hands :)

From: https://techcrunch.com/2014/02/27/apple-explains-exactly-how...

You’ve actually got one set of keys for each device you add to iCloud, and each iMessage is encrypted independently for each device. So if you have two devices — say, an iPad and an iPhone — each message sent to you is actually encrypted (AES-128) and stored on Apple’s servers twice. Once for each device. When you pull down a message, it’s specifically encrypted for the device you’re on.

What’s your point?

The question at hand is whether any major company would allow consumers to send messages the company can’t harvest for data. The answer is yes. Apple does.

Are you saying they don’t? —That they actually have backdoored their E2E messaging app already?

If you are just saying that it’s not entirely impossible for them to do backdoor, I don’t see what bearing that has on the discussion.

Whenever a new device (AppleID/phone number) is associated with Messages/iMessage, a notification appears on other devices saying something like "Your phone number is now being used for iMessage on <device>", along with some more text. Apple also sends an email about that to the email address associated with the iCloud account. So I don't understand what you mean by "it would be easy for Apple to simply add a new device without showing you". If that refers to Apple circumventing this kind of warning for specific users for some reason, then I guess other popular messaging platforms/apps cannot be trusted either.

Not true. They would have to have access to your iCloud credentials and you would have to confirm the new device from one of your old devices.