The electronic communications systems that are proliferating throughout modern society offer speed, accuracy and ever diminishing cost. They also present serious problems of security. As the ordinary transactions cond ucted in person, on the telephone or by written co�respondence have come increasingly to be conducted by new kinds of electronic systems the suscepti­ bility of organizations and individuals to eavesdropping and forgery has grown dramatically. One way to prevent tam­ pering with the new electronic systems and to protect the vast quantities of pri­ vate information such as the credit recby Martin E. Hellman