From TheBestLinks.com
A fragment of an actual Purple machine found in Berlin at the end of WWII
Purple (sometimes capitalized as 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. Purple was an electromechanical (stepping switch) cypher. The color referred to binders used by US cryptanalysts 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.
Weaknesses
In operation, the encrypting machine accepted typewritten input (in Latin letters) and produced cyphertext output, and vice versa when decyphering messages. The result was a potentially excellent crypto system. In fact, operational errors, chiefly in key choice, made the system much less secure than it could have been. The Japanese believed it to be effectively unbreakable throughout, and somewhat after, 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 United States obtained portions of a Purple machine from the Japanese Embassy in Germany following Germany's defeat in 1945 (see image above) and discovered that the Japanese had used precisely the same stepping switch in its construction as Leo Rosen of SIS had chosen when building a "duplicate" in Washington in 1939 and 1940.
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 breaking off relations (ominously) with the United States at 1PM Washington time on 7 December 1941 before the Japanese Embassy in Washington could do so. Difficulties at the Embassy were a major reason the note was delivered late.
Information carried in Purple traffic
The US found no 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. 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.
Handling of Magic prior to Pearl Harbor
Even so, the diplomatic information discovered was of even more limited value to the US because of its dissemination pattern 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, and 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 personal discussions with Hitler and a report on a tour of the invasion defenses in Northern France (including the D-Day invasion beaches).
Post-war debates
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, and, since Pearl Harbor was not expecting to be attacked, the "failure to warn Hawaii one was coming must have been deliberate, since it could hardly have been mere oversight".
Other Japanese ciphers
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). Early on, 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, 1941 provided enough information to lead to US naval victories in the battles of the Coral Sea and Midway, eliminating most of air power of the Japanese fleet at the latter and stopping the Japanese advance south with a 'draw' at 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 US Army Air Corps pilots a chance to assassinate the officer who had devised the Pearl Harbor attack plan. And still later, access to Japanese Army messages from decrypts of Army communications traffic assisted in planning the island hopping campaign to the Philippines and beyond.
Public notice had actually been served that Japanese cryptography was dangerously 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 in the newspaper stories). Fortunately, neither the Japanese nor anyone who might have told them, seem to have noticed either the Tribune coverage, or the 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 could, then or now, be connected with those newspaper accounts or Congressional disclosures.
Other claimed breaks into Purple
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 December 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 rotor cypher was unrelated to the Purple cypher 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, one 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. There is some evidence that the Germans shipped several military Enigma machines to Japan by submarine late in the War; several sources suggest that they never arrived.
Further reading
An account of the WWII cryptographic struggle is Battle of Wits, by S. Budiansky, which is not too overwhelmingly long or technical. Combined Fleet Decoded by J. Prados has, in somewhat dispersed form, a complementary and fuller account of Japanese cryptography specifically, much of it from sources on the Japanese side. Both are recent enough to reflect much of the release of information that had been kept secret since the War.
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