In today's extensively wired world, cryptology is vital for guarding communication channels, databases, and software from intruders. Increased processing and communications speed, rapidly broadening access and multiplying storage capacity tend to make systems less secure over time, and security becomes a race against the relentless creativity of the unscrupulous. The revised and extended third edition of this classic reference work on cryptology offers a wealth of new technical and biographical details. The book presupposes only elementary mathematical knowledge. Spiced with exciting, amusing, and sometimes personal accounts from the history of cryptology, it will interest general a broad readership.