Yes, the latter. They call it a "keybag." When you sign into iMessage on a new device, a new public key is added to your "keybag" and all messages addressed to

Yes, the latter. They call it a "keybag." When you sign into iMessage on a new device, a new public key is added to your "keybag" and all messages addressed to your account are also encrypted with that key. You get a notification when a new device is signed into your iMessage account. However, Apple could presumably suppress this notification when adding a law enforcement key to your keybag. In principle, devices sending messages to you are aware of this key (they must handle it) but because iMessage is proprietary, the user has know way of knowing. In principle, iMessage's design but with open-source clients would be ideal.