A framework for public-channel cryptography using chaotic lasers
  Two mutually coupled chaotic diode lasers with individual external feedback, are shown to establish chaos synchronization in the low-frequency fluctuations regime. A third laser with identical external feedback but coupled unidirectionally to one of the pair does not synchronize. Both experiments and simulations reveal the existence of a window of parameters for which synchronization by mutual coupling is possible but synchronization by unidirectional coupling is not. This parameter space forms the basis of a proposed public-channel cryptographic scheme and is robust to various possible attacks. 
Noam Gross, Einat Klein, Michael Rosenbluh, Wolfgang Kinzel, Lev   Khaykovich, Ido Kanter