Our society has become highly dependent on its modern communications systems, e.g. the radio, television, telephone, telex, facsimile, high-speed data links etc. A high proportion of the information disseminated over these communications circuits is confidential in nature. In these situations the communicants should take steps to conceal the content of a message from not only the casual listener but also the determined interceptor. There is little doubt that this problem of protecting and securing communications will continue to grow at an increasing rate over the coming years, not only in its traditional military role but also in the public and commercial domains. The paper considers the type of cryptographic systems available today. As well as discussing the mathematical and statistical requirements of such systems, it explores how, with careful design, most of these needs can be met while the complexity of the hardware implementation is kept to a minimum.