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by Paul F. Whitman

Where was American Intelligence while the Japanese fleet had been crossing the huge expanse of the Pacific and creeping up on the American base? What form would the anger of the American people have taken if they had known the most closely guarded secret and realized that American experts had been reading the most confidential Japanese ciphers even before the attack, and that the Japanese war plans were no secret to American Intelligence?

Even prior to the opening of hostilities, the Corregidor unit had, together with the Singapore unit, commenced the attack and breakdown of JN25. The interesting question is how far had they gone down this path? What had they known, and at what point did 'pop' history stop knowing it?


There is a thicket of materials on the issue of code-breaking, Pearl Harbor, and the beginning of the war, and I admit I felt it difficult to see my way through until I had the opportunity to exchange correspondence with  Capt. Duane Whitlock U.S.N. (Ret'd)  concerning his article on cryptography and the role that Corregidor played in it entitled The Silent War Against the Japanese (and reproduced on this website here.)  Whitlock was a radioman on Corregidor, and it soon became evident to me that he never ceased to be interested (and current) in his views on the role of cryptography.

To bring you up to speed, I will quote from 'The Man Who Broke Purple' by Ronald W. Clark (Corgi 1997 0 552 10958 4)

"The first (PURPLE TYPE 97) replica was kept for use by the US Army while others were being built. Just how many others has never been stated but it seems that by the end of 1941 eight in all were available. Number One was retained in Washington and Numbers Two and Three were assigned to the mission to England, one going to Bletchley and the other to the Admiralty. The fourth was retained in Washington by the navy, and the fifth was sent to the Philippines at first to Cavite and finally to the famous communications tunnel on Corregidor. The sixth and seventh models were kept in Washington for use as spares while the eighth was sent to London in the early autumn of 1941. None, it was later pointed out, went to Pearl Harbor, the American naval base in Hawaii, and one of the most important American targets in the world; a lack of clear thinking at the top, rivalry between the army and the navy, and a dangerous assumption that no-one would really attack the United States, stand out as the main reasons for what, at least with hindsight, appears to have been a staggering omission. Conspiracy, the stand-by argument of the revisionists who claim that Roosevelt egged on the Japanese to attack, comes far down the list of possibilities."


Breaking Purple

by Paul F. Whitman




Speaking of the 'prior to the opening of hostilities situation', Ronald Lewin, in Ultra Goes To War (1978 Hutchison & Co. Ltd.,  London) gets straight to the point, and so will I:-

"And in no field - as intensive inquiries into Pearl Harbor and its preliminaries have demonstrated - was the American administration proved to have been more amateur than in its handling of the cryptographic intelligence which its agencies abundantly supplied."

So let's get a few working definitions settled before we go on.   ULTRA, in the context in which I am using it, was the overall name that was given to the product of code breaking.  We can talk about ULTRA now, because until 1974 it had remained one of the most protected secrets of WW2 that had never been allowed into the public domain. In 1974, Group Captain Frederick W.  Winterbotham, writing from memory and without access to official papers, released The Ultra Secret (Harper & Row 1974) and though later authors would criticize it for being neither encyclopedic or precise, it was the seminal work in the field.  In the US context, MAGIC was the same as ULTRA.

Codebreaking during WW2 was an operation so compartmentalised, and so enormous,  that no single individual could come to grips with the 'big picture' and what followed  Winterbotham were a number of books which represented partial experiences with vignettes of the 'big picture'.  

Purple was the name which the Americans gave to the Alphabetical Typewriter 97 machine utilized by the Japanese Foreign Office for encoding and decoding messages.  In time it would come almost synonymous with the product of the codebreaking of the Japanese diplomatic codes.   By 1937 (year 2597 of the Japanese calendar)  the Japanese  Foreign Office had acquired a commercial Enigma machine, and after modifying and improving it, it was adopted by the Imperial Navy for signal security, and then by the Foreign Office.  Indeed, it would be the use of the Type 97 cypher machine by the Foreign Office which would attract America's premier codebreaker of the time, William Friedman.

    From 1937, Purple would be  Friedman's preoccupation. By 1940, after his relentless effort in mastering the Purple cypher, he withdrew temporarily to a neuro-psychiatric ward to deal with the burn-out brought about by the prolonged stress.   But by cracking Purple, he ensured that the United States could read the Japanese Diplomatic traffic.

   Whereas the American government operated through its diplomats,  that was not necessarily the case in Japan, where the real people in power were the armed forces, not the civilian government and it's Diplomats. The Japanese, in fact , trusted their diplomats somewhat less than foreign governments trusted theirs, and not every important instruction was carried by Purple. Indeed, the manner in which the Japanese played their own diplomats in the negotiations leading towards Pearl Harbor reminds one of an expert magician - focus the audience's attention on the hand that is not performing the trick.

   In retrospect, it has appeared to many historians and writers (Van Der Rhoer, Ronald Lewin, James Rusbridger, Eric Nave, Clark, William Kahn and more recently Bruce Lee) that the access Purple gave the United States to sensitive communications prior to Pearl Harbor did as much harm as good.   The same breakthrough that made it possible to read Japan's most important and sensitive diplomatic communications - those passing back and forth between the minister of foreign affairs in Tokyo and the ambassadors in Washington, London, Berlin, Rome and Moscow - had an intoxicating effect on the American inner circle given access to the material.  Van der Rhoer goes so far as to say that it did more harm than good.

   Operational Orders, those that directed fleets, invasions, raids and the like, were not transmitted by  Purple.  The Imperial Army and the Navy each had their own operational codes.  (If one thinks of the Japanese  Imperial Army and  the Japanese Navy as huge corporations competing against each other for the military budget allocations to finance and sustain their own war, as opposed to their competitor's war,  then one can begin to comprehend why the Pacific War was fought the way it was, and even why it happened.)  The Japanese   Imperial Army's war had already been running for several years in China, and the Navy was a very minor player in it.  In the Pacific, where it was the Navy which had brought on the war against the Americans,  the Japanese Navy's Operational orders were transmitted in a code named JN-25.

  When one gets into any discussion of  codebreaking in the Pacific War  by far the big noise  which grabs almost all the public's attention is the story of Purple.  It's so far ahead in the mind of the US public (at least those of the public who care for such  things) that there's hardly any room for whatever comes second.  It's heroic, it's  the success story, and there's nothing an American loves more than a success story.  It's the one which gets the publicity and the spotlight, and what comes next is 'close, but no cigar'.  You have got to do a lot of reading of a lot of books before you start to question the accepted vision, and the suspicion  that the big story of the Pacific War is actually JN-25 and not Purple, comes as a thief in the night.

When you look more closely at the texts,  you recognize that it's the decryption of JN-25 which brought on the successes at Coral Sea and Midway, and that it's the reading of JN-25 which got Yamamoto shot down.  Where was Purple then?  Midway is the first, the classic  example of how cryptoanalysis was used to spring a surprise and achieve decisive results in the Pacific War, and  it was the reading of JN-25, not Purple that made it so.

Why would there be all the hoopla around Purple and not around JN-25? The answer is that mixed in with JN-25 there are some very serious areas of continuing sensitivity, even fifty years after the cessation of hostilities.  And there is no greater sensitivity of Pacific War cryptography that I know of than the issue of when it was that JN-25  first began to be read.  

This is an issue which still has some very powerful  sectional interest groups protecting their individual interests.  The British, one must never forget, were in the war some years prior to the US, and they were sufficiently concerned for the safety of their Far East Empire to have their  code-breakers working against Japan from not just from Bletchley Park, but from Hong Kong, Delhi,  Colombo and Singapore.  There was a Japanese section at Bletchley, staffed to a large degree by young men of ability recruited from Cambridge who had successfully mastered a six month crash course in Japanese. In Ultra Goes To War (1978 Hutchison & Co. Ltd., London) , author Ronald Lewin  comments of them "the exact function and achievement of this section has never been revealed."  Indications raise the probability that they were reading JN-25 well before the US, and that they were withholding from the US, then a neutral, the extent of their success against it. 

If the British were reading JN-25 before the Americans, and  that's where the smart money is, and one follows this line of thought through to it's logical conclusion, then the big question to be answered is what the British knew of the Japanese plans concerning Pearl Harbor before it occurred. 

The speculation of the revisionist historians is that Winston Churchill knew that the Japanese Navy was on it's way to attack Pearl Harbor, and withheld that information from the Americans.  Surely a good move from the British point of view, and perhaps a very good reason why American historians tend to treat him   without the veneration accorded him by the historians on the other side of the Atlantic.

The issue affects the US in a far different manner. The issue of when they first were able to read JN-25 raises three possibilities:

(a) They were concentrating their limited resources on reading the diplomatic intercepts, and that the operational orders, being a lesser priority, were being looked at only on an ad hoc basis.  They could only began to concentrate on  JN-25 when they discovered the error, and were given more resources.  The reason why there are no decrypts with decrypt dates prior to 7 December 1941 is that none (or not many)  were decrypted  before that date. The surprise at Pearl Harbor was brought about by negligence - the failure of the United States to recognize the importance of creating a system for obtaining and interpreting operational and not just diplomatic enemy intelligence and rendering both understandable to their respective users. There is no conspiracy.

(b) They were reading JN-25 prior to Pearl Harbor and through a combination of excessive compartmentalisation,  amateurish handling of the 'product' and 'yield',  that actual foreknowledge of the impending raid on Pearl Harbor was lost in the bureaucratic machinations of the system. The reason why there are no decrypts with decrypt dates prior to 7 December 1941 is that powerful persons within the  USN covered up their institutional embarrassment from the prying eyes of history, and removed them from the archives.  This being done, the US has prevailed upon the governments of Britain and Australia to ensure that no materials from their archives are released which are inconsistent with the official line that JN-25 was first cracked only after 7 December 1941. 

(c) The reason why there are no decrypts with decrypt dates prior to 7 December 1941 is that, in truth,  no one had been able to read them before that date, and that only a major effort spurred on by the disaster brought about their being cracked.

The variations of these possibilities is staggering.

One very telling book is "Deadly Magic" (1978 Charles Scribner's Sons, New York)  by Edward Van Der Rhoer.  The author is one of those people who was compartmentalised, and not graced with a 'need to know' the big picture. But from what he relates, it appears that he was working in a section that was dealing with the Japanese Operational (as distinct from diplomatic or Purple) codes. Though he does not refer to JN-25 anywhere in his book, as if it had been required of him not to mention it by name,  it appears that the compartment within which he worked at OP-20-GZ  was working on JN-25,  that is, when it wasn't working on reading and translating the encyphered Purple traffic.  The intercepted  traffic from the Japanese Consulate at Honolulu was considered 'low-level intelligence' and piled up in the In-basket.

Van Der Rhoer relates that on 6 December 1941, a colleague, Dorothy Edger, had picked out from the In-box a lengthy intercept that happened to be one of the Japanese Naval Intelligence officer's messages  from the consulate at Honolulu on 3 December 1941.  This  message had tweaked her interest, and she had worked on it throughout the afternoon, even after she was due to go home for the day.  It was about a set of signals which was to be used in sending information about the US fleet in Pearl Harbor.

When (Commander) Kramer finally appeared later in the afternoon, she spoke to him about it. By her account, he scanned the text rapidly, just standing there. He handed it back to her without a word.

"What should I do with it?" Dorothy asked.

"It can wait until Monday." 

Dorothy was not aware that Kramer had already immersed himself in the first parts of the fourteen-part message that was coming in over the PURPLE machine. He had seen enough to know that the message contained a diplomatic note that was obviously Japan's answer to the United States in the ongoing negotiations.   ...

Yet if he had not been so preoccupied with the fourteen-part message in PURPLE, Kramer would have certainly realized that the signal system described in Dorothy Edger's message was a visual one meant to be observed by Japanese submarines standing offshore at Oahu while the radio ads could be picked up by the submarines or even surface ships not far away from Hawaii.

Kramer's fate, Van Der  Rhoer goes on to relate,  was to live under a cloud for years, from where he would be called on to testify before naval boards of inquiry and congressional investigations for a number of years, even after the end of the war.  

So where is that likely to leave us, on this Corregidor website?   Even prior to the opening of hostilities, the Corregidor unit had, together with the Singapore unit, commenced the attack and breakdown of JN25.



1999 Paul Whitman



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