An Introduction to Secrecy Capacity

This paper introduces the reader to several information theoretic aspects of covert communications. In particular, it discusses fundamental limits on the amount of information that can be reliably communicated in such a way that a malevolent third-party observer cannot decode the messages. This situation is considered first for a system with a single intended receiver, and then extensions of secrecy capacity to more general multi-terminal settings are presented. We begin in Section 2 with a discussion of the wiretap channel introduced by Wyner in [1], along with the classical generalization due to Csiszár1 and Körner [2]. Next, Section 3 ties the older work in with topics receiving recent attention through a discussion of network secrecy capacity and common randomness. The presentation of newer results centers around Csiszár and Narayan’s contemporary work relating secrecy generation to multiterminal source coding [3], which in turn relies on some interesting results from Ahlswede [4, 5].