Peter Wayner says:
What's to prevent me from swapping phones with a friend or buying a used phone at a garage sale? Whooa. The secret registered keys just became unsynchronized. When the government comes to listen in, they only receive gobbledly-gook because the secret key registered under my name isn't the right one.
Perhaps you can deduce which chip is doing the encryption by identification data transmitted by the chip down the line -- they might identify themselves, making it impossible for you to avoid having them figure out which pair of keys registered with the Ministry of Truth and the Ministry of Love are to be used to listen in on your conversation. After all, they keys are registered by the manufacturer... Perry PS We all remember the Ministry of Love and the Ministry of Truth, don't we? This proposal was, of course, created by a group spun off from the Ministry of Peace, a.k.a. No Such Agency.