doesn't have to solve the wave equation to optimize the system, one only needs to know that the medium is reciprocal. Time reversal is therefore suited to applications with inhomogeneous media. An attractive aspect of time reversal signal processing is the fact that it makes use of multipath propagation. Many wireless communication systems must compensate and correct for multipath effects. Time reversal techniques use multipath to their advantage by using the energy from all paths. Fink imagines a cryptographic application based on the ergodic cavity configuration. The key would be composed of the locations of two transducers. One plays the message, the other records waves after they have bounced throughout the cavity; this recording will look like noise. When the recorded message is time reversed and played back, there is only one location to launch the waves from in order for them to focus. Given that the playback location is correct, only one other location will exhibit the focused message wave; all other locations should look noisy. See also Phase conjugation References External links Mathias Fink. Time Reversal of Ultrasonic Fields--Part 1: Basic Principles. IEEE Trans. Ultrasonics, Ferroelectrics, and Frequency Control. 39(5):pp 555--566. September 1992. Signal processing