[ARC5] FW: How the U.S. Cracked Japan's 'Purple Encryption Machine' at the Dawn of World War II

[Jim Haynes]

A good book on a lot of the Pacific action is "Joe Rochefort's War : the
odyssey of the codebreaker who outwitted Yamamoto at Midway" by Eliot
Carlson, Naval Institute Press, 2011.

As I understand it JN-25 was a book code, meaning that for each word or
phrase in plain text there were one or more numbers transmitted as the
encrypted text.  A complementary book translated from numbers back to
words and phrases.

What made it all the more difficult was that a second book called additive
tables contained a list of random numbers that were added, without carry,
to the numbers in the code book before transmission, and subtracted
from the received numbers before decoding.  So the communicators had to
have copies of the code book, plus the book of additives and information
about where in the latter to start using the additives.  In one sense a
code-book code is pretty archaic, but for a language like Japanese with
thousands of characters it is about the only way to go.  I believe JN-25
was only partially broken throughout the inter-war and war period.

Purple, on the other hand, is a mechanical cipher.  It makes use of a
Japanese rendering called Roma-Ji.  In this case there are syllables
spelled with Roman letters and the spoken Japanese is rendered into
those syllables, which then can be transmitted by any system such as
Morse or teleprinter code that can handle the Roman alphabet.  In the'
purple machine the consonants and vowels are encrypted by separate parts
of the mechanism.  Purple was used for the diplomatic service, partly
because it allowed transmission through commercial telegraph systems.
Machines equivalent to the purple machine were made by U.S. code breakers
and could pretty thoroughly decode the messages once the keying scheme
was worked out.

I have seen some articles by Navy officers highly critical of Washington
for sending purple machines to England while denying one to Pearl Harbor.
But this criticism ignores the fact that Pearl Harbor was told to work
on Japanese naval codes, while the diplomatic stuff was being handled in
Washington.  If Pearl Harbor had had a purple machine, and if they had
intercepted the Japanese messages, it would have given Pearl the warning
of war beginning some hours before the message decrypted in Washington
had been turned into a warning to Pearl.  But that's about the only time
a purple machine at Pearl would have had any effect on the Pacific war;
and it's a big IF whether Pearl was intercepting the diplomatic traffic
anyway.

Another reason, and I'll cut those Navy officers some slack because they
probably didn't know, not having a need to know.  The Japanese ambassador
to Berlin sent Tokyo voluminous reports of his observations and 
conversations with high Nazi officials, including Hitler himself.
These could be intercepted and decrypted in England and provided vital
information to the prosecution of the war in Europe.