|
Purple was the
name used by the US military to identify the most secure diplomatic
cryptographic system used by the
Japanese Foreign Office during, and just before,
World War II. It was not a
code, but an electromechanical cypher. The color name referred to
binders used by US cryptanalyists for material in this cypher; there had
been a Red 'code' (also a cypher) used by the Japanese Foreign Office and
purple was the next available color. The Japanese also used the Coral and
Jade stepping switch cyphers; it's not clear whether whoever named them kept
to the binder color system. The Purple machine was a successor to, and
improvement on, both the Red machine and what the Americans called the M
machine (used in some embassies and consulates by attaches). All were
designed by a Japanese Navy captain. The information gained from decryptions
was eventually code-named Magic within the US government.
In operation, the encrypting machine accepted typewritten
input (in Latin letters) and produced cyphertext output. The result was a
potentially excellent crypto system. In fact, operational errors, chiefly in
choosing keys, made the system much less secure than it could have been. The
Japanese believed it to be effectively unbreakable throughout the War. It
was broken by a team from the US Army Signals Intelligence Service, then
directed by
William Friedman. The team was led by Frank Rowlett.
The Purple machine itself was first used by Japan in
1939, but US and British cryptographers had broken some of its messages well
before the
attack on Pearl Harbor. US cryptographers decrypted and translated the
14-part Japanese diplomatic message declaring war against the States before
the Japanese Embassy in Washington could. The US never found any hint of the
attack on Pearl Harbor in the Purple traffic at the time, nor could they
have as the Japanese were very careful to not discuss the planned attack in
Foreign Office communications. In fact, no detailed information about the
planned attack was even available to the Japanese Foreign Office; it was
regarded by the military, particularly the more nationalistic military, as
insufficiently 'reliable'. US access to private Japanese diplomatic
communications (even the most secret ones) was less useful than it might
otherwise have been because policy in Japan in the pre-War period was
controlled largely by military groups (eg, in China and Manchuria), not by
the Foreign Office. And, the Foreign Office itself deliberately kept from
its embassies and consulates much of the information it did have, so the
ability to read Purple transmissions was less than definitive regarding
Japanese tactical or strategic military intentions.
Even so, the diplomatic information discovered was of
even more limited value to the US because of its lack of dissemination
within the US Government. Magic traffic was distributed in such a way that
many policy makers who should have access to it to do their jobs knew
nothing of it, and those to whom it actually was distributed (at least
before Pearl Harbor) saw each message (only briefly, as the courier stood by
to take it back) in isolation from all others (no copies or notes were
permitted). Before Pearl Harbor, in any case, they saw only those decrypts
thought 'important enough' by the distributing Army or Navy officers.
Nonetheless, being able to read Purple messages gave the Allies a great
advantage in the war; for instance, the Japanese ambassador to Germany
produced long reports for Tokyo which were encrypted with the Purple
machine. They included reports on discussions with Hitler, a report on a
tour of the invasion defenses in Northern France, ....
The break into the Purple traffic, and into Japanese
messages generally, was the subject of acrimonious hearings in Congress
after WWII in connection with an attempt to decide who, if anyone, had
allowed the disaster at Pearl Harbor to happen and who therefore should be
blamed. During those hearings the Japanese learned, for the first time, that
the Purple cypher machine had been broken. They had been continuing to use
it, even after the War, with the encouragement of the American Occupation.
Much confusion over who in Washington or Hawaii knew what and when,
especially as 'we were decrypting their messages' has led some to conclude
that 'someone in Washington' knew about the Pearl Harbor attack before it
happened.
In fact, Purple was an enticing, but quite tactically
limited, window into Japanese planning and policy because of the peculiar
nature of Japanese policy making prior to the War (see above). A better
tactical window was the Japanese Fleet Code (an encoded cypher actually),
called JN-25 by US Navy cryptographers. Breaking into the version in use in
the months after December 7 provided enough information to lead to
U.S. naval victories in the battles of the
Coral Sea and
Midway, crushing the main air offensive power of the Japanese fleet at
the latter and stopping the Japanese advance south in the former. Later,
broken JN-25 traffic also provided the schedule and routing of the plane
Admiral
Yamamoto Isoroku would be flying in during an inspection tour in the SW
Pacific, giving the US Navy the chance to assassinate the tactician who had
devised Pearl Harbour.
Public notice had actually been served that Japanese
cryptography was inadequate by the Chicago Tribune, which published a series
of stories just after Midway in 1942 directly claiming -- correctly, of
course -- that the victory was due in large part to US breaks into Japanese
crypto systems (in this case, the JN-25 cypher, though which system(s) had
been broken was not mentioned). Fortunately, neither the Japanese nor anyone
who might have told them, seem to have noticed either the Tribune or stories
based on the Tribune account published in other US papers. Nor did they
notice announcements made on the floor of the US Congress to the same
effect. There were no changes in Japanese cryptography which can, then or
now, be connected with those newspaper accounts or Congressional
disclosures.
An excellent (and somewhat brief) account of the WWII
crypto struggle is Battle of Wits, by S. Budiansky. Combined
Fleet Decoded by J. Prados has, in somewhat scattered form, a
complementary and fuller account of Japanese cryptography, mostly from the
Japanese side. Both are recent enough to reflect much of the release of
information that had been kept secret since the War.
The United States obtained portions of a Purple machine
from the Japanese Embassy in Germany following Germany's defeat in 1945 and
discovered that the Japanese had used the exact same stepping switch in its
construction as Leo Rosen of SIS had chosen when building a 'duplicate' in
Washington in 1939/1940.
In the book Sword and Shield, by C. Andrew,
based on the Mitrokhin archive smuggled out of Russia in the early '90s by a
KGB archivist, the claim is made that the Soviets independently broke
into Japanese Purple traffic (as well as the Red predecessor machine) and
that decrypted Purple messages contributed to the decision by Stalin to move
troops from Far Eastern Asia to the area around Moscow for the counterattack
in Dec of '41. Those messages are said to have been credible enough to
convince the Soviets there would not be a Japanese attack on them.
The
German Enigma machine and Purple
The German
Enigma was unrelated to the Purple machine, though there have been
published claims that Purple was 'merely' an Enigma copy of some sort. In
fact, the Purple machine was a Japanese development, the last of a series
designed by a Japanese Navy captain, though there seems to have been some
assistance by at least one Polish officer prior to the 1930s.
|
