Relay-proof channels using UWB lasers
Alice is a hand-held device. Bob is a device providing a service, such as an ATM, an automatic door, or an anti-aircraft gun pointing at the gyro-copter in which Alice is travelling. Bob and Alice have never met, but share a key, which Alice uses to request a service from Bob (dispense cash, open door, don't shoot). Mort pretends to Bob that she is Alice, and her accomplice Cove pretends to Alice that he is Bob. Mort and Cove relay the appropriate challenges and responses to one another over a channel hidden from Alice and Bob. Meanwhile Alice waits impatiently in front of a different ATM, or the wrong door, or another gun. How can such an attack be prevented?