This self-contained introduction provides a focused tour of the central concepts of cryptography. It delineates cryptographic concepts in chronological order, developing the mathematics as needed. The text includes numerous examples and exercises, along with computer implementation sections that guide readers through the process of writing their own programs. Detailed solutions to many of the exercises can be found in the appendices while a solutions manual is available for qualifying instructors. A supporting website provides an extensive set of sample programs as well as downloadable platform-independent applet pages for some core programs and algorithms.