A.1. Introduction

Cryptography, today, cannot bank solely on public-key (that is, asymmetric) algorithms. Secret-key (that is, symmetric) techniques also have important roles to play. This chapter is an attempt to introduce to the readers some rudimentary notions about symmetric cryptography. The sketchy account that follows lacks both the depth and the breadth of a comprehensive treatment. Given the focus of this book, Appendix A could have been omitted. Nonetheless, some attention to the symmetric technology is never irrelevant for any book on cryptology.

It remains debatable whether hash functions can be treated under the banner of this chapter—a hash function need not even use a key. If the reader is willing to accept symmetric as an abbreviation for not asymmetric, some justifications can perhaps be given. How does it matter anyway?