Electronic computers have evolved from exiguous experimental exterprises in the 1940s to prolific practical data processing system in the 1980s. As we have come to rely on these systems to process and store data, we have also come to wonder about their ability to protect valuable data. Data security is the science and study of methods of protecting data in computer and communication systems from unauthorized disclosure and modification. The goal of this book is to introduce the mathematical principles of data security and show how these principle apply to operating systems, database systems, and computer networks. The book is for students and processionals seeking an introduction to these principles. There are many references for those who would like to study specific topics further.