  We study the mutual coupling of chaotic lasers and observe both
experimentally and in numeric simulations, that there exists a regime of
parameters for which two mutually coupled chaotic lasers establish isochronal
synchronization, while a third laser coupled unidirectionally to one of the
pair, does not synchronize. We then propose a cryptographic scheme, based on
the advantage of mutual-coupling over unidirectional coupling, where all the
parameters of the system are public knowledge. We numerically demonstrate that
in such a scheme the two communicating lasers can add a message signal
(compressed binary message) to the transmitted coupling signal, and recover the
message in both directions with high fidelity by using a mutual chaos pass
filter procedure. An attacker however, fails to recover an errorless message
even if he amplifies the coupling signal.
