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ISSUE 1930, Wednesday 6 September 2000

How we achieved a cracking victory

By Michael Smith


interceptIllustration added by this website (from David Irving: Churchill's War, vol. ii): a page of British intercept at Pearl Harbor time.

IN the Hollywood film U-571, the Americans, with no assistance from the Brits, were portrayed as having won the codebreaking war by snatching an Enigma machine from under the noses of the Germans.

Even respectable American historians, although they do at least accept that Bletchley Park broke the Nazi's Enigma cipher, have always argued that it was American codebreakers who broke the Japanese codes and ciphers. But only the Japanese diplomatic machine cipher Purple was broken by American codebreakers alone and that was in the summer of 1940, when the Battle of Britain was concentrating the minds of the Bletchley Park codebreakers on the Enigma machine.

To find the true heroes of the Allies' battle to crack the Japanese codes and ciphers we have to go back to 1925 when Eric Nave, an Australian naval officer, was seconded to the Royal Navy and sent to the Far East to set up a codebreaking operation. Initially, it appeared that his new mission was headed for failure.

The advent of the telegraph had brought problems for the Japanese whose written language was based on kanji, pictorial characters originally borrowed from the Chinese, and around 70 phonetic syllables call kana. This was not easily adapted to the Morse code. The Japanese language has a large number of different words which, while having distinctive written forms, sound exactly the same (much like "principal" and "principle" in English). So a system of transliteration, known as romaji, was developed, allowing the kana syllables to be spelt out in Roman letters.

The Japanese created their own Morse code which contained all the kana syllables plus the romaji letters and was totally different from the standard international Morse code. Early attempts by Royal Navy operators to take down Japanese messages in Morse proved unintelligible. But eventually they intercepted a practice message in which the operator had run through the entire Japanese Morse code symbol by symbol. Nave and his assistants now had the start they needed.

They were assisted in their task by the Japanese belief that their codes were impenetrable. When the Emperor Yoshihito died at the end of 1926, the official report of his death and the succession speech of his son Hirohito were relayed to every Japanese diplomatic, naval and military outpost around the world. Nave knew that the Japanese love of ceremony and obsession with predictable courtesies would ensure that every message was exactly the same. It was a simple task to follow it through the various codes, breaking each in turn.

Another of the codebreakers helping to decipher the Japanese diplomatic and naval messages was Hugh Foss, an eccentric 6ft 5in Scot, who wore a long straggly red beard, a kilt and sandals. Foss was a brilliant, but highly eccentric, codebreaker. His first major success came in 1934 when he broke a new machine cipher used by Japanese naval attachés in their embassies. British codebreakers would later play a key role in the development of the world's first programmable electronic computer. But Foss's efforts to construct a device to read the Japanese machine cipher did not have the same degree of sophistication.

Nave recalled: "The first trial was made in the office using a brown foolscap file cover with a collar stud, a piece of string and slots cut in the cover for the letters." But the device worked. It was not until 15 months later that the Americans broke this system, and then apparently only with the aid of a "pinch", or theft, of information, possibly even a machine itself, from the Washington flat of the Japanese naval attaché. Around the same time, the Japanese foreign ministry introduced its own machine cipher.

Foss also broke that, beating the Americans by two years and ensuring that the British were able to keep watch on Japan's increasingly close links with Nazi Germany. Nave and Foss were responsible for breaking most of the Japanese naval codes and ciphers in the pre-war years.

The military codes were the chief responsibility of a man who was by any standards one of the greatest cryptographers who ever lived, but whose name remains virtually unknown.


John Tiltman was born in London on May 24, 1894. At the remarkably young age of 13, he was offered a place at Oxford. He served with the King's Own Scottish Borderers in France during the First World War, winning the Military Cross, and was seconded to MI1b, the military intelligence department dealing with codebreaking. After the war, he was sent to India where he helped the authorities to set up their own codebreaking operations. But he then returned to London to work in the recently formed British codebreaking organisation, the Government Code and Cipher School, where he began breaking the main Japanese army codes.

The Royal Navy had its own team of cryptographers attacking the Japanese codes and ciphers working at the Far East Combined Bureau, initially in Hong Kong and, from late 1939, in Singapore. They intercepted both naval and military messages. Anything that could not be broken was sent back to Bletchley Park, to Tiltman. As war loomed, Japan adopted a wholly new code system, known as the superenciphered code. It was based on a codebook containing a large number of commonly used words and phrases, each of which was allocated a five-figure code group. The operator, or cipher clerk, encoded the message to produce a series of five figure groups. He then took a second book containing row upon row of randomly produced five-figure groups.

The operator selected one group from any of the pages in this second book. He then used the subsequent stream of figures to encipher his already encoded message. Each group was placed in turn under each of the encoded groups. Each figure was then added to the one above it, using non-carrying arithmetic, so that 7 and 5, for instance, produced 2 rather than 12. The result was a seemingly random series of figures which appeared impossible to unravel.

Yet without any indication even of what system was in use, yet alone the luxury of having the books, Tiltman managed not only to work out what was going on, but also to begin breaking some of the messages. Perhaps the best known of these new Japanese superenciphered codes was the main Japanese Navy code, generally known as JN25. It first appeared in June 1939 and within weeks Tiltman had broken it. The Americans later claimed to have broken JN25. They did, but not until many months later.

At the end of the war, details of how the US army broke the Purple cipher were swiftly made public, horrifying British codebreakers. News also leaked out of how the US navy had read a JN25 message that allowed its aircraft to shoot down the head of the Japanese navy, Adml Yamamoto Isoruku. This publicity gave the lasting impression that the Americans had broken the Japanese codes. By contrast, the British clamped down on any mention of the remarkable achievements of their own codebreakers so that they could continue to intercept the communications of other countries with impunity.

The official files, still in the possession of Bletchley Park's successor, GCHQ, did not begin filtering into the archives of the Public Record Office until the Nineties and those on the British codebreaking efforts against the Japanese were among the last to be released. Only now has evidence begun to emerge of how much work on Japanese codes and ciphers was done by British and Australian codebreakers.

Related story:

US 'stole credit for cracking Pacific war code'


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