Each device does have it's own key. When you add a new device that key is shipped to all the other devices that can receive the messages. Basically you have a r

Each device does have it's own key. When you add a new device that key is shipped to all the other devices that can receive the messages. Basically you have a ring of keys, and each time you add a new key every single device on that ring gets an updated copy of the key ring.