Originally published in the New Mathematical Library almost half a century ago, this charming book explains how to solve cryptograms based on elementary mathematical principles, starting with the Caesar cipher and building up to progressively more sophisticated substitution methods. Todd Feil has updated the book for the technological age by adding two new chapters covering RSA public-key cryptography, one-time pads, and pseudo-random-number generators. Exercises are given throughout the text that will help the reader understand the concepts and practice the techniques presented. Software to ease the drudgery of making the necessary calculations is made available. The book assumes minimal mathematical prerequisites and therefore explains from scratch such concepts as summation notation, matrix multiplication, and modular arithmetic. Even the mathematically sophisticated reader, however, will find some of the exercises challenging. (Answers to the exercises appear in an appendix.)