A really fascinating read. Here's an excerpt. Click the title for the link.
It turns out that one obvious weakness I have seen mentioned in some blog posts and presentations isn’t actually a weakness at all, thanks to a design decision.
iMessage is a centralized system with a central directory server. If someone could compromise that server, they could add “phantom devices” to tap conversations (or completely reroute them to a new destination). To limit this Apple sends you a notification every time a device is added to your iCloud account.
I always thought Apple’s server detected a new entry and then pushed out a notification, which would mean that if they were deeply compromised (okay, forced by a government) to alter their system, the notification could be faked, but that isn’t how it works. Your device checks its own registry of keys, and pops up an alert if it sees a new one tied to your account.
According to the Johns Hopkins paper, they managed to block the push notifications on a local network which prevented checking the directory and creating the alert. That’s easy to fix, and I expect a fix in a future update (but I have no confirmation).
Once in place that will make it impossible to place a ‘tap’ using a phantom device without at least someone in the conversation receiving an alert. The way the current system works, you also cannot add a phantom recipient because your own devices keep checking for new recipients on your account.