For a one-semester undergraduate-level course in Cryptology, Mathematics, or Computer Science. Designed for either the intelligent freshman (good at math) or for a low-level junior year first course, Cryptology introduces a wide range of up-to-date cryptological concepts along with the mathematical ideas that are behind them. The new and old are organized around a historical framework. A variety of mathematical topics that are germane to cryptology (e.g., modular arithmetic, Boolean functions, complexity theory, etc.) are developed, but they do not overshadow the main focus of the text. Unlike other texts in this field, Cryptology brings students directly to concepts of classical substitutions and transpositions and issues in modern cryptographic methods.