Abstract: Cryptography can be used to provide data secrecy, data authentication, and originator authentication. Non-reversible transformation techniques provide only the last. Cryptographic check digits Provide both data and originator authentication, but no secrecy. Data secrecy, with or without data authentication, is provided by block encryption or data stream encryption techniques. Total systems security may be provided on a linkby-link, node-by-node, or end-to-end basis, depending upon the nature of the application.