Cipher Machines





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Japanese Purple Cipher Machine

Purple Cipher
Remnant of the Japanese Purple cipher recovered from their
bombed-out embassy in Berlin

The Japanese Purple Cipher pictured here is a remnant from the bombed-out Japanese embassy in Berlin, Germany and is now in the National Cryptologic Museum in Laurel, Maryland. Unfortunately, none are known to survive the war. The US broke the code for this cipher even before the attack at Pearl Harbor. The Germans had good evidence of this and informed the Japanese, who refused to believe their code could be broken. The Japanese only accepted this fact after the war, when the US had a Congressional investigation and public hearings into who knew of the upcoming attack on Pearl Harbor.

The Japanese diplomat and Imperial Army General in Berlin who used this specific cipher machine was also an aficionado of war strategy and armaments. His name was Baron Hiroshi Oshima and he is shown shaking hands with Hitler in the picture. He followed very closely the German advance in Europe and the latest technology in the German war machines. He sent very detailed reports to his superiors in Tokyo, using the purple cipher machine, which the US was able to intercept and read.

Hiroshi Oshima

During his time in Germany, Oshima became a confidant of Hitler, so was aware of the most sensitive plans from the Nazis. In recognition of this value to Japan, he was promoted in a few short years from Colonel to Lt. General. US General George Marshall said that Oshima was, "our main basis of information regarding Hitler's intentions in Europe". This was especially useful in the preparations for D-Day, since this gave the US confidence that the Germans had taken the US deception and were preparing for the landing at the wrong beaches.

The Japanese Purple machine is based on the rotor technology of the Enigma, and the Japanese called it "97-shiki O-bun In-ji-ki", which means Alphabetical Typewriter '97. The 97 means the Japanese year 2597 which is 1937 to the Western world, the year it was developed. The Japanese commonly referred to it as "the machine" or "J", it was the US codebreakers that dubbed it "Purple", after the color of the folders that held the decryptions, to distinguish it from other Japanese ciphers. It used two Underwood typewriters, one for input and one for output. This helped reduce errors and manpower but came at the cost of being more cumbersome on the battlefield. The rotors were electro-mechanically rotated by "stepping switches", rather than by mechanically pressing a key. Also the typewriters were electrical, so the combined cipher system with the power needed would have been much less portable than the Enigma, or especially the M-209. Functionally, it operated like the Enigma with four rotors.

Purple Cipher Replica
US functional replica of the Japanese Purple cipher

The Japanese purple cipher was first and partially broken in August, 1940. Without the benefit of ever seeing the device, William Friedman and members of the U.S. Signal Intelligence Service then built a functional replica (pictured on the left) which imitated the operations of this cipher machine. It was a marvel of mathematical and engineering insight and ingenuity to accomplish this feat. This is the same William Friedman who will go on to play a major role in cryptology, with significant impact on world events for the rest of the century. Friedman was the person who negotiated the deal with Boris Hagelin to allow the US NSA to have a backdoor access into the Hagelin cipher machines. You can read more about this amazing story in the description of the Hagelin pin and lug cipher machines.

Purple was the highest security code used by the Japanese during WW2, but they also used 3 lower levels of cipher. The most important of these was the Japanese Fleet Code, which the US called JN-25. This was the tactical code used by the Japanese Navy. The US also broke this code, which was a significant factor in the naval victory in the Battle of the Midway, the turning point in the war with Japan. It also contributed greatly to the elimination of most of the Japanese naval air power.

Purple Cipher Internals
Internal view of Purple with stepping switches removed

It was a JN-25 decrypt that alerted the US of Admiral Isoroku Yamamoto's planned visit to the Solomon Islands in 5 days, on April 18, 1943. This was to be a morale and inspection visit by the Commander of the Combined Fleet who was the military leader behind the attack on Pearl Harbor. He was an aggressive and strong leader who inspired his men as well as the entire Japanese population. This was to be his closest approach to the combat zone and the US intercepted his minute-by-minute itinerary for the one-day visit.

The island Yamamoto was visiting was still a great distance from the nearest US landing strip and even with extended fuel tanks there was only enough fuel for our planes to make a quick strike and return home. Luckily, we also knew Yamamoto was compulsively punctual, so the US could calculate his exact location and planned to strike just 10 minutes before his scheduled landing. The US also set up a ruse that Australian coast watchers got a tip from some friendly Solomon Islanders about Yamamoto's flight, so that the Japanese would not suspect their cipher had been broken.

The US was able to shoot down his plane, but his death was not revealed to the Japanese people for more than a month later. This had a devastating effect on the population, but was probably more important for the military. His successor was much less aggressive and innovative, making this ambush of a military leader a significant gain by the intelligence from a deciphered message.

Copyright © 2016 by Ralph Simpson - Creative Commons Attribution 4.0 International License