  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.
