This book 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 book. Chapter topics cover origins, examples, and ideas in cryptology; classical cryptographic techniques; symmetric computer-based cryptology; public-key cryptography; and present practice, issues, and the future. For individuals seeking an up-close and accurate idea of how current-day cryptographic methods work.