Electronic transmission systems are evolving with the expansion of broadcast satellite, cable, wireless (cellular, local area networks, and LMDS (local multipoint distribution service)), and satellite radio. While the systems are designed for transport of different types of media, convergence is changing this aspect of the systems. More importantly, these systems share the common feature of being multiple-access systems. These transmission systems allow new applications ranging from very small amounts of information directed at a single user to incredibly large amounts of information directed at a large but selective group of individuals.

With these capabilities comes the responsibility to provide security, protecting the property of the content providers and service providers as well as maintaining the privacy demanded by the individual subscribers or users. The conditional access of information involves the three A's: auditing (identifying what has been accessed by whom), authorization (addressing the information only to those entitled to receive it), and authentication (guaranteeing the identity of all the parties involved). This article addresses the threats to modern electronic transmission systems as well as the mechanisms used to prevent these threats. While each system has its unique problems, the generic issues associated with most of the systems will be treated here.