Communication Theory of Secrecy Systems*

THE problems of cryptography and secrecy systems furnish an interesting application of communication theory. 1 In this paper a theory of secrecy systems is developed. The approach is on a theoretical level and is intended to complement the treatment found in standard works on cryptography. 2 There, a detailed study is made of the many standard types of codes and ciphers, and of the ways of breaking them. We will be more concerned with the general mathematical structure and properties of secrecy systems.