دانلود کتاب Secret and urgent - the story of codes and ciphers.

A combination history storybook and practical word puzzle manual. This book actually leads the reader step by step through the classic processes for breaking ciphers, from simple substitution up through Vigniere ciphers with disordered arrays with long keys. And he gives useful small examples of how to break codes. Someone who has read Pratt's book is in a much better position to understand what's hard and what's easy in trying to break such ciphers as Enigma, Fish and the M-209, and such codes as the various forms of "Purple".
In short: if you want to understand not only where cryptology has been but also where it's going, Pratt's little book (if the reader actually works through the examples) is a really good starting point. Very nice collection of tables in the Appendix full of basic useful frequency data for English and some other languages.
You can glean simple obfuscation tips on how to keep your private data out of the hands of the information conglomerates.