Multicasting is increasingly used as an efficient communication mechanism for group-oriented applications such as video conferencing, interactive group games, TV over Internet and broadcasting stock quotes. Nevertheless, the lack of security in the multicast paradigm obstructs the large scale deployment of such multi-party applications that have a large place in the information technology market. In this book we deal with the two keystone security issues of any secure multicast architecture: data origin authentication and confidentiality. For each service, we present a detailed analysis of the problem. Then, we present protocols and architectures that provide those security services and analyze their advantages and limitations. Finally, we discuss some new ideas and solutions dealing specifically with large scale and highly dynamic group communications. The analysis should help shed some light on security issues and solutions in multicast- based applications and services, and should be especially useful to professionals in group communication security, or anyone else who may be considering utilizing or deploying multicast-based communication solutions.