Japanese Cryptology From The 1500s To Meiji - The State of Japanese Army Cryptology Around 1941

The State of Japanese Army Cryptology Around 1941

Hyakutake's two-letter, ten-chart system was exceedingly weak. It might have made a decent tactical field code --- it is simple to use, requires only the paper charts and a pencil, and is easily changed. As a code for military attachés around the globe, however, Hyakutake's system was much too weak. It was basically a slightly improved version of the Foreign Ministry's two-letter code that Yardley broke in 1919 and possibly not as strong as the four-letter code it replaced.

Kahn, Smith, and Budiansky all make it clear that superenciphering and using pseudo-random additives were nothing new even in the 1920s --- Kahn says that enciphered code was "the customary method for diplomatic communications." A system using random numbers to superencipher messages was not revolutionary in the 1930s.

Thus, Hara's system was not new and does not seem to have been any better than similar systems long in use in other countries. Nevertheless, devising and implementing the Army's system was an important accomplishment and it is possible that Hara was responsible for it. An interesting topic for further research would be why this system was chosen instead of machine ciphers. Was the random number system chosen for non-cryptological reasons? Were the Army cryptanalysts good enough to understand that random numbers were more secure, when used correctly, than cipher machines?

There were several books available that hint at ways to break cipher machines. William Friedman's The Index of Coincidence and Its Applications to Cryptography was revolutionary; the addition of advanced mathematical, especially statistical, methods to the cryptological toolkit made traditional cryptographic systems obsolete and machine systems breakable. So it is possible that the Japanese cryptanalysts knew that cipher machines were, in theory at least, breakable.

The Polish military realized early on that machine enciphering would change the science of cryptology and from 1929 employed mathematicians to work on cryptanalysis. However, as the goal of Japanese-Polish cryptographic cooperation was to train the Japanese side to break Russian codes, there would have been no need for the Polish cryptologists to reveal methods of breaking machines the Russians were not using. Teaching the Japanese the latest and greatest methods would not be of any use against Russian codes and would only risk the Germans finding out and changing their codes. The Poles thus had a strong incentive to teach the Japanese just as much as they needed to know.

The Japanese army was aware of machine systems; at the Hague in 1926, a Japanese military attaché saw a demonstration of the Model B1 cipher machine from Aktiebolaget Cryptograph. In fact, in the early 1930s, both the Japanese Navy and the Foreign Ministry switched to machine systems for their most secret messages. The fact that those systems seem to have been developed in Japan suggests that there were knowledgeable cryptographers in Japan. Which suggests that perhaps there were other, non-cryptographic reasons why the Army continued to use chart and book based systems. Perhaps further research into the cultural and institutional aspects of inter-war cryptology in Japan could uncover those reasons.