The American magic : codes, ciphers, and the defeat of Japan / Ronald Lewin.

Includes bibliographical references (pages 309-311) and index.

This book describes the impact of the American breaking of the Japanese codes and ciphers of WWII. The Japanese used both codes and ciphers for their messages. I was hoping for information about how these codes and ciphers were broken, but this book provides only a very general description; one that I suspect is wrong. According to the author, the text of a message sent by the Japanese Navy (the JN25 code) was first encoded using one of 100,000 numbers to replace each word or phrase of a message. The numbers were then scrambled (enciphered) using an enciphering machine. Budiansky, in Battle of Wits, describes the Japanese Navy using a somewhat different process, one that also used a number code book, but with the number disguised by adding another number, determined from a random selection from an additive table. However, both Lewin and Budainsky described the diplomatic code in terms of a machine scrambling the letters of the words of the message. Interestingly, Budainsky does not reference Lewin's book, even though it was written 18 years before his, but he does reference another of Lewin's books, Ultra Goes to War. Budainsky's reference for the breaking of JN25 are documents from the US Naval archives that he says describe the process in some detail, but he provides nothing from the general literature. Given the specific reference provided by Budiansky, and the fact that Lewin's references are mostly from books written for a general audience.