downwind

appleinsideruser said:

So iMessage is e2e encrypted, but if someone snoops on the cloud it’ll tell you!? Seems like there’s a little detail missing here somewhere…

From the description, it does seems to be verifying the device you're connected from and not your user, probably using a unique secure enclave key on each device.

That way, if someone manages to log in as you to iCloud by stealing credentials or somehow breaching iCloud, they can't just use any iPhone to log in and communicate as you without the other part knowing you're using a new device, the other party will know since they've never seen your device key before.