Too Long; Didn't Read
Apple iMessage uses end-to-end encryption in a way that messages from the sender’s Apple device to another Apple device are only viewable by them. Each party contains a unique “key” to open the “envelope” containing the message. Only your connected devices and the Apple devices that receive your messages have the keys to unlock and read those messages. Law enforcement or any other third party cannot read your messages without access to your unlocked device, nor Apple can.