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
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.