iMessage New Device Sec

You know when you add a new Apple device to your lineup, you inevitably get a message like this one? Seems ridiculous doesn’t it? I know I added the device, I was there. I always thought it was a bug, a remnant in iOS and OS X from the early days when iMessage was being developed. As they say, it’s not a bug, it’s a feature. Rich Mogull explains on Securosis:

It turns out you can’t add devices to an iCloud account without triggering an alert because that analysis happens on your device, and doesn’t rely (totally) on a push notification from the server. Apple put the security logic in each device, even though the system still needs a central authority. Basically, they designed the system to not trust them.

So you get that alert on every device, because every device sees the new device on it’s own, independent from all the other Apple devices you own. Here’s why:

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.

Because of how the devices are set up (each notifying you when a new iMessage device is added), the group of them act as your own personal iMessage canary.

…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.

If you added a new device and didn’t observe this popup, you could naturally assume that iMessage, or at the very least your iMessage account had been compromised.

For reference, every iMessage you send is encrypted end-to-end using public key crypto. You don’t share a single key among your devices, each device has its own keys. Every message you send is encrypted with public keys for each iMessage device you own. This means that a given message may have 4 or 5 keys attached to it. When you add a new device, it send its public key to Apple, and when Apple pushes that key to your other devices - thus you get this popup.

Typed on AEKII