*key distribution for access control, *source authentication of transmissions, and *non-repudiation of streams. This book discusses how to realize these operations both with high performance processors and resource constrained processors. It shows how to protect against adversaries who inject packets or eavesdrop. The focus is on functional descriptions rather than theoretical discussions. Prools are presented as basic building blocks that can be combined with each other and traditional security prools. The book illustrates these prools in practice by presenting a real implementation that provides security for an ad hoc sensor network. This book can serve as a textbook or supplementary reading in graduate level courses on security or networking, or can be used for self study.