ABSTRACT
The Imperial Japanese Army developed an extremely complex yet user-friendly code system that was, in theory, unbreakable by outsiders. A combination of Japanese systemic errors in transmitting enciphered messages and poor physical security when handling code materials enabled the Allies to break what was one of the most difficult book-based cryptosystems of World War II. Recently available declassified American documents amplify and clarify the Imperial Army’s use and abuse of its code systems in the Southwest Pacific Theater with specific wartime examples that illustrate how the Japanese army codes unraveled.
