The general concept of security in message communications may be traced to the advent of human civilization. In contrast, however, security in automation and control is a recent phenomenon, originating with the computer age and is rapidly gaining importance with the proliferation of networks. With computer networks integrating the dual functions of (i) communications and (ii) automation and control, computer network security must address the security issues inherent in both communications and automation. Until recently, research and development in computer security was strongly linked with cryptography including encryption and decryption of electronic messages. However, as computer networks have started to proliferate into large, complex, realworld systems, such as electronic banking, the power grid, and the proposed intelligent vehicle highway system, the authors believe that computer network security has transcended the traditional definition and has migrated to a higher, logical level. In current and future networks, the information riding on the network may control parts of the network while the control, in turn, may ensure the correct propagation of information from the source to the intended destination. Thus, networks constitute complex, multidimensional entities that require security at different levels of both network hardware and network software.