In today's extensively wired and unsafe world cryptology plays a vital role in guarding communication channels, databases, and software from unwanted 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 first part reviews secret codes and their uses – the foundations of cryptography. The second part deals with the process of covertly decrypting a secret code - cryptanalysis, and gives particular advice on assessing methods. The book presupposes only elementary mathematical knowledge. Spiced with a wealth of exciting, amusing, and sometimes personal accounts from the history of cryptology, it will also interest general readers.