  We study the secrecy of an optical communication system with two scattering
layers, to hide both the sender and receiver, by measuring the correlation of
the intermediate speckle generated between the two layers. The binary message
is modulated as spatially shaped wavefronts, and the high number of
transmission modes of the scattering layers allows for many uncorrelated
incident wavefronts to send the same message, making it difficult for an
attacker to intercept or decode the message and thus increasing secrecy. We
collect 50,000 intermediate speckle patterns and analyze their correlation
distribution using Kolmogorov-Smirnov (K-S) test. We search for further
correlations using the K-Means and Hierarchical unsupervised classification
algorithms. We find no correlation between the intermediate speckle and the
message, suggesting a person-in-the-middle attack is not possible. This method
is compatible with any digital encryption method and is applicable for
codifications in optical wireless communication (OWC).
