Information transmission and information protection are two sides of the same tapestry, but with the information-protection side having more tangled and multitextured threads. At the core of the subject of information protection is the more specific subject of classical cryptography, which protects the content of a message from being understood by unauthorized receivers, but does not protect the message in other ways. Much of this book is concerned with cryptography in this classical sense, but treated in its modern sophisticated form. The modern subject of cryptography, and of information protection in general, is a fascinating mixture of mathematics, engineering, informatics, and computer science, and the same mixture is found in this book.

The subject of information protection is rapidly evolving into a subject that goes well beyond the classical notions of point-to-point cryptography. Now there is an intense need for secrecy and security in large public networks. Within this larger setting of public networked communication, many other issues are important, including issues of authorization, certification, and authentication, that bring many subtle considerations into the discussion. While the emphasis of the book is cryptography, it touches on these other topics as well. My goal, as in my other books, is to concentrate on the formal, and presumably timeless, aspects of the subject rather than on the details of systems in current use. Although this book is not designed to serve as a handbook describing the current standard cryptosystems, some topics are best described by discussing practical systems that are now in use.