The Japanese had obtained an Enigma machine from Germany, and decided to use the same principle to encode their messages. As a result, the device used by Japanese diplomats in World War II was
called Purple.  In Japan, Purple was titled 97-shiki O-bun In-ji-ki, which means Alphabetical Typewriter '97.  The number 97 came from the Japanese calendar year 2597 (equivalent to 1937). Informally the Americans called the machine "J." the diplomatic "Purple" and the intercepts were known as 'Magic".

Rather than using rotors operated by key presses from the keyboard, the '97 employed electro-mechanical "stepping switches". An electromagnet, acting through a pawl and ratchet mechanism, caused rotating contacts to pass over banks of electrical contacts. The overall machine, although constructed differently, was equivalent to a four rotor Enigma with electric typewriters on each side. A message was entered on one typewriter, and printed out, encoded, on the second. Although this eliminated some errors in copying an encode from illuminated light bulbs, the weight of the stepping switches and typewriters made it far less portable than the German field Enigma.
The Alphabetical typewriter '97 had a polyalphabetic foundation. It could encipher English letters and created substitutions numbering in hundreds of thousands.  This capability presented an immense

Having previously uncovered Tokyo's naval conference codes, American analysts were familiar with specific salutations and closings.  Military radio stations around the Pacific constantly monitored radio telegraph transmissions.  Every possible clue was sought.  Frequencies and patterns slowly began to emerge.  Blanks were to be filled by such lucky breaks as Japanese cipher senders making mistakes and then repeating dispatches to make corrections.  The American teams began to piece together the obscure permutations.  In August, 1940 they had their first awkward but readable solution.
Cryptanalysts working for the Signals Intelligence Service' (SIS) of the U.S. Army knew how crucial it was to decipher and read Japanese secret messages.

But this new code, "Purple," wasn't breaking easily. For eighteen months the team struggled with this difficult Japanese diplomatic code. Then, one day in September 1940, Genevieve Grotian made a discovery that would change the course of history. By analyzing and studying the intercepted coded messages, she found a correlation that no one else had yet detected. This breakthrough enabled other cryptanalysts to find similar links. Then, William Friedman and members of the S.I.S. (U.S. signal
Intelligence Service) actually built a crude but serviceable model that was a remarkable imitation of Purple.  Soon this product of American engineering and mathematical insight was help in reproducing the most guarded Purple communications.  So impressed was one USN Rear Admiral that he called the process Magic, and the nickname stuck.

Magic was in operation the night of December 6, 1941.  Japanese embassy dispatches were being picked up by Navy radio stations and sent on to the Navy Department in Washington D.C.  By the morning of December 7, thirteen parts of a Japanese government reply regarding negotiations had been deciphered.  The fourteenth segment of the message was Tokyo's decision to break negotiations with the United States by 1 p.m. that same day.  OP-20-G (the American Navy counterpart) and S.I.S. cryptanalysts knew this by 7:30 a.m. Washington time.  It was not  yet dawn in Hawaii, and with dawn it was too late to stop the attack.

Volumes have been written about the attack, the excuses made, and the blame often hastily placed at the time.  Questions about everything from late warnings to mistaken radar settings. The answers continue to be confused, often speculative at best. One answer is certain.  The American cryptanalysts of Purple did not fail.  They broke every important message from the Japanese.  After all, no Japanese message saying "attack Pearl Harbor" ever existed to decipher.  Still, American cryptography staffs had done their work as quickly as the methods and governmental limitations of that time.

This is the PURPLE Analog No. 1 cipher machine used by American code breakers to crack the Purple Japanese code. (Photo by Jerry Proc)

Almost sixty years later, Frank Rowlett, a cryptologic pioneer and head of the "Purple" team, remembered that historic day when the code broke. "What [Genevieve] Grotjan did was a big step forward and was very significant in the solution of Purple." Her discovery, and the work of other team members, allowed the United States to read secret Japanese diplomatic messages and to continue reading them throughout World War II.  Genevieve Grotjan's contribution to the Allied victory cannot be measured.  Nor can the contributions of the thousands of women serving their country through the field of cryptography. Like Genevieve, many women working in cryptology during World War II were civilians; thousands of others were in the military.

This is the largest of three surviving pieces of the famous Japanese diplomatic cipher machine. It was recovered from the wreckage of the Japanese Embassy in Berlin in 1945. This assembly is on display in the National Cryptologic Museum. (Photo by Jerry Proc)
In this larger view, the three stepping relays of the Purple assembly have been removed in order to provide a better view of the internal parts. (Photo by Gregg Vicik, The Mariners' Museum, Newport News, VA).

1) Sharing the Burden: Women in Cryptology During WWII, an NCM pamphlet.
2) "Purple" by Bill Momsen
3) "Purple" by Karen Kurzeja  http://raphael.math.uic.edu/~jeremy/crypt/contrib/kurzeja.html
4) Cracking The Purple Code http://io9.gizmodo.com/how-the-u-s-cracked-japans-purple-encryption-machine-458385664

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July 7/17