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
challenge.
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.
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