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In World War II, Magic was the US codename for intelligence derived from the cryptanalysis of Purple, a Japanese foreign office cipher. 1 Information carried in Purple traffic
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.
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.
2 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).
3 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 WashingtonWashington, DC officially the District of Columbia (also known as DC Washington and, historically, the Federal City is the capital city and administrative district of the United States of America. Residents of the city and its surrounding suburbs refer to or HawaiiFor the 1959 novel and 1966 movie, see Hawaii (novel). Hawaii ( Hawaiian/ Hawaiian English: Hawai‘i with the ‘okina is the archipelago of the Hawaiian Islands in the Pacific Ocean. Hawaii constitutes the 50th state of the United States, and as of the 2000 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".
4 History
When Purple was broken by the US Army's Signals Intelligence Service (SIS), several problems arose for the Americans: who would get the decrypts, which decrypts, how often, under what circumstances, and crucially (given interservice rivalries) who would do the delivering? Both the US Navy and Army were insistent that they alone handle all decrypted traffic delivery, especially to highly placed policy makers in the US. Eventually, after much toing and froing, a compromise was reached: the Army would be responsible for the decrypts on one day, and the Navy the next.
The distribution list eventually included some—but not all—military intelligence leaders in Washington and elsewhere, some—but again not all—civilian policy leaders in Washington. The eventual routine for distribution included the following steps:
- the duty officer (Army or Navy, depending on the day) would decide which decrypts were significant or interesting enough to distribute
- they would be collected, locked into a briefcase, and turned over to a relatively junior officer (not always cleared to actually see the decrypts!) who would 'make the rounds' to the appropriate offices.
- no copies of any decrypts were left with anyone on the list. The receipient would be allowed to read the translated decrypt, in the presence of the distributing officer, and was required to return it immediately upon finishing. Before the beginning of the second week in December 1941, that was the last time anyone on the list saw that particular decrypt.
