The implications of adding security mechanisms to high-level network protocols operating in an open-system environment are analyzed. First the threats to security that may arise in such an environment are described, and then a set of goals for communications security measures is established. This is followed by a brief description of the two basic approaches to communications security, link-oriented measures and end-to-end measures, which concludes that end-to-end measures are more appropriate in an open-system environment. Next, relevant properties of data encryption--the fundamental technique on which all communications security mechanisms are based--are discussed. The remainder of the paper describes ho~ w end-to-end measures can be used to achieve each of the security goals previously established.