  In this work, we focus on the classical optical channel having Poissonian
statistical behavior and propose a novel secrecy coding-based physical layer
protocol. Our protocol is different but complementary to both (computationally
secure) quantum immune cryptographic protocols and (information theoretically
secure) quantum cryptographic protocols. Specifically, our (information
theoretical) secrecy coding protocol secures classical digital information bits
at photonic level exploiting the random nature of the Poisson channel.
  It is known that secrecy coding techniques for the Poisson channel based on
the classical one-way wiretap channel (introduced by Wyner in 1975) ensure
secret communication only if the mutual information to the eavesdropper is
smaller than that to the legitimate receiver. In order to overcome such a
strong limitation, we introduce a two-way protocol that always ensures secret
communication independently of the conditions of legitimate and eavesdropper
channels. We prove this claim showing rigorous comparative derivation and
analysis of the information theoretical secrecy capacity of the classical
one-way and of the proposed two-way protocols. We also show numerical
calculations that prove drastic gains and strong practical potential of our
proposed two-way protocol to secure information transmission over optical
channels.
