In this handbook, we have attempted to emphasize the interplay between communications and the field of information security. Arguably, this is the first time in the security literature that this duality has been recognized in such an integral and explicit manner. It is important to realize that information security is a large topic – far too large to cover exhaustively within a single volume. Consequently, we cannot claim to provide a complete view of the subject. Instead, we have chosen to include several surveys of some of the most important, interesting, and timely topics, along with a significant number of research-oriented papers. Many of the research papers are very much on the cutting edge of the field.