When you are developing security systems, new penetration techniques seem to appear as responses to new security measures but in general the flow is the other way around: security exists and evolves because of the evolution of threats. Beginning with the rise of radio in the 20th Century attacks on communication networks have shown two forms: those that go for the big kill --- such as the breaking of Enigma --- and those that assemble small seemingly innocuous leaks of information into a comprehensive understanding of the target's behavior. We will analyze the way in which these trends interact with others to create a situation in which what is possible in security and even the meaning of security in communication networks needs reexamination.

The development of public-key cryptography is described, and its principles are elucidated. The discussion covers exponential key exchange, the trap-door knapsack public-key cryptosystem, the Rivest-Shamir-Adleman (RSA) system, and the breaking of the knapsack cryptosystem. Early responses to public-key systems and the problem of key management are examined. Applications and implementations are described. Significant development in multiplying, factoring, and finding prime numbers which have resulted from public-key research are sketched. Directions in public-key research are discussed.>

Two kinds of contemporary developments in cryptography are examined. Widening applications of teleprocessing have given rise to a need for new types of cryptographic systems, which minimize the need for secure key distribution channels and supply the equivalent of a written signature. This paper suggests ways to solve these currently open problems. It also discusses how the theories of communication and computation are beginning to provide the tools to solve cryptographic problems of long standing.