F.L. Bauer 
Decrypted Secrets [PDF ebook] 
Methods and Maxims of Cryptology

Cryptology, for millennia a "secret science", is rapidly gaining in practical importance for the protection of communication channels, databases, and software. Beside its role in computerized information systems (public key systems), more and more applications inside computer systems and networks are appearing, which also extend to access rights and source file protection. The first part of this book treats secret codes and their uses — cryptography. The second part deals with the process of covertly decrypting a secret code — cryptanalysis — where in particular advice on assessing methods is given. The book presupposes only elementary mathematical knowledge. Spiced with a wealth of exciting, amusing, and sometimes personal stories from the history of cryptology, it will also interest general readers.