How about: walkie-talkie with decent crypto?

Suppose we design walkie talkies such that two or more units can be hooked up together in a daisy chain. While the devices are daisy chained, a button can be pressed on the head walkie talkie which will generate a random symmetric key, like 256 bit AES or whatever. This is sent over the daisy-chained bus to all the others. They flash an LED or something to indicate they accepted the key. After that they can be split up and will all use that key.