The Bletchley Park Codebreakers (18 page)

The breakthrough was to provide one of the first bartering chips in the signals intelligence alliance between Britain and America that exists
to this day. A team of four US codebreakers, two from the army and two from the navy, travelled to Britain in January 1941, almost a year before America joined the war. One of the top-secret packages they brought with them to exchange for the British expertise in breaking the German Enigma messages was a Purple machine, designed to decipher the telegrams enciphered using the Type B machine. Two days after the American delegation arrived at Bletchley Park, Admiral John Godfrey, the British Director of Naval Intelligence, authorized a full exchange of Japanese signals intelligence between the Far East Combined Bureau and a US Navy codebreaking and intercept site, on the island of Corregidor in the Philippines. The Americans provided a ‘pinch' – a stolen version of the Japanese merchant shipping code, a naval personnel code, a new diplomatic cipher called ‘Hagi', and a ‘nearly empty' JN-25 codebook. The main British contribution was recoveries of the latest JN-25 super-enciphered code, which were greater in number than those available to the US station.

The Japanese had changed the JN-25 system on 1 December 1940, just two months earlier, introducing a new codebook which the Allies would designate JN-25B. But they had made a serious mistake in not changing the additives at the same time. As a result, the codebreakers had been able to break into the JN-25B immediately, recalled Neil Barham, one of the FECB JN-25 experts: ‘The Japanese introduced a new codebook but unfortunately for them, retained in use the current reciphering table and indicator system. These had already been solved in some positions and new codegroups were discovered immediately. But for this mistake on the part of the Japanese the form of the book might have taken a matter of months to discover.'

Harry Shaw gave the Americans a JN-25B codebook with 500 of the groups already recovered, plus 4,000 cipher additive groups and 290 indicator additive groups, all of which were from the old system in use before the codebook changed. Although the additive system had now changed as well, these groups covered the two-month window into the new system created by the Japanese mistake, allowing the codebreakers to use them to recover more groups. These in turn allowed sustained recovery of the new additive and further reconstruction of the JN-25B code-book. But the JN-25 exchange was very much a two-way street. Although the British were in the lead at this stage, Corregidor was better placed to intercept Japanese Navy messages than Singapore, which could only pick up the Combined
Fleet, based in Japanese home waters, at night. The two sites exchanged signals every three days, giving lists of the first three groups of every JN-25 message they had intercepted, and then sent on hard copies of the messages the other site had missed on the regular Pan-American Airlines flight between Manila and Singapore.

The increased co-operation allowed both stations to surge ahead with their recoveries. The Americans had developed a highly mechanized system, using punch-card tabulating machines to sort the code groups, and they soon began to catch up with their British counterparts. By April 1941, the combined effort had recovered 30 per cent of the new additive system. There was also collaboration on Japanese military systems between the British codebreakers and the US Army's ‘Station 6' intercept site at Fort McKinley, near Manila. Two US Army codebreakers were sent to Singapore, where Peter Marr-Johnson, the chief British Army cryptanalyst, handed them partial solutions of two Japanese Army codes, and Lt. Geoffrey Stevens, the FECB's other military codebreaker, was posted to Washington to liaise with the US Army codebreakers.

However, the British were not just co-operating with the Americans. They set up an exchange arrangement with a Dutch codebreaking unit, known as
Kamer 14
(Room 14), which was based at the Bandung Technical College in Java. The technical exchange was limited to information on diplomatic ciphers, but there was also a limited exchange of decrypts and intelligence on military and naval material.

Eric Nave had returned to Australia where he helped to set up a new ‘Special Intelligence Bureau'. The RAN had intercept sites at Canberra and Townsville and there was also a small Royal Australian Air Force site at Darwin. The Australians and the Royal New Zealand Navy, which had its own intercept unit, agreed to provide coverage of Japanese and Russian traffic to supplement the FECB's intercepts. Nave and Shaw also agreed an exchange of information on a number of codes and ciphers.

The decrypts from the Purple machine were not just providing details of Japanese intentions but also extremely good intelligence from inside Berlin, where the Japanese Ambassador Oshima Hiroshi was a close confidant of Hitler. As a result, the Purple messages provided some of the first evidence that the Germans were about to turn on their Russian allies and invade the Soviet Union. Confirmation came in Oshima's account of a meeting in April 1941 with Hermann
Göring, Hitler's deputy, who briefed the Japanese ambassador in detail on the number of aircraft and divisions that would be used in Operation Barbarossa. The code-breakers at Bletchley Park had been warning for some time that the Germans appeared to be withdrawing units from various parts of Europe and pushing them east towards the Soviet Union. But they were not believed in Whitehall until 10 June, twelve days before the invasion began, when the Japanese diplomatic section in Elmer's School translated two messages from Oshima. The first said that Hitler had told him personally that war with the Soviet Union was now inevitable. The other suggested to his bosses in Tokyo that ‘for the time being I think it would be a good idea for you, in some inconspicuous manner, to postpone the departure of Japanese citizens for Europe via Siberia. You will understand why.'

Hitler was anxious to draw the Japanese into a war with the Soviet Union in order to create a second front in the Far East that would drain resources from the war with Germany. But Japan was not prepared to be deflected from its main aim, which was to strike south, taking the European colonies in the Far East, including Hong Kong and Malaya. Confirmation of this came with the results of a full cabinet meeting attended by Emperor Hirohito on 2 July 1941. Two days later, the results of the meeting, which had been sent by telegram to Oshima, were deciphered by Bletchley Park's Japanese diplomatic section. The telegram made clear that Japan was intent on expanding its empire into south-east Asia. The first step was the occupation of the whole of French Indochina, by force if necessary, to provide bases that would allow it to launch attacks against Malaya and the Dutch East Indies. Should Britain or America attempt to interfere, the Japanese would ‘brush such interference aside', Oshima was told. A few weeks later, Bletchley Park read a Purple message confirming that Vichy France had agreed to allow the Japanese to occupy southern Indochina, ostensibly to protect it from a possible British attack.

The British codebreakers in Singapore began picking up almost daily intelligence reports from the Japanese Consul in Singapore on the situation in Malaya. By the end of October 1941, the FECB's naval intercepts had left no doubt that the Combined Fleet had been mobilized. Throughout November, a combination of traffic analysis, direction-finding and an improved capability against the JN-25B code enabled the FECB to keep track of a mass of Japanese ships heading south. Although the difficulty in breaking detailed operational
messages remained, the code-breakers had now recovered more than 3,000 code groups, and were able to produce ‘intelligence covering a wide field'.

The Japanese preparations for war were confirmed in the Purple messages. On 19 November, the Japanese embassy in London was told to await a coded weather message on Japanese overseas radio that would indicate the opening of conflict with Britain, America or Russia. The message, deciphered at Bletchley, read: ‘With America, the words:
higashi no kaze, ame
(easterly wind, rain). With Soviet, the words:
kita no kaze, kumori
(northerly wind, cloudy). With Britain, including invasion of Thailand, the words:
nishi no kaze, hare
(westerly wind, fine). On receipt of these code words all confidential books are to be burnt.'

On 1 December, the Japanese diplomats in London were ordered to destroy their codes and ciphers and began making preparations to leave. The Japanese Navy now changed both its call signs and the JN-25B additive. But the codebook remained in use, again limiting the damage. The codebreakers had recovered nearly 4,000 code groups, allowing them to get out additive on many of the most common messages and make further inroads into the codebook itself.

It was late on Sunday, 7 December, local time, when Hong Kong reported having heard the coded Winds message that was to precede the declaration of war. It had said:
‘higashi no kaze, ame; nishi no kaze, hare
(easterly wind, rain; westerly wind, fine)', indicating that war would be declared on both Britain and America. A few hours later, in the early hours of Monday, 8 December, Singapore time, the first Japanese troops began landing on Kota Bharu beach in northern Malaya. It was the first in a carefully co-ordinated series of attacks against Malaya, the Philippines, Hong Kong and, the only real surprise. Pearl Harbor. (Although the attack on Pearl Harbor, which brought America into the war, took place on Sunday, 7 December, the relative location of Hawaii and Malaya on either side of the international dateline meant that in ‘real time' the attack on Malaya occurred first.)

Persistent allegations that the British codebreakers knew of the Pearl Harbor attack and failed to warn America in order to drag her into the war are totally without substance. There were messages that might have indicated the existence of a ‘northern force', which had practised the use of torpedoes in shallow water such as would be found at Pearl Harbor. But given the limitations on the Allies'
ability to break JN-25B, these could not have been read. Nave, who was quoted as having backed the allegations in his memoirs, actually said the exact opposite, pointing out that the Japanese had used a sophisticated radio deception operation to prevent the Allies from realizing that Pearl Harbor would be attacked. Malcolm Kennedy, who was working in the Japanese diplomatic section at Bletchley, wrote in his diary that the attack came as ‘a complete surprise'.

As the Japanese advanced down the Malay peninsula, the Royal Navy codebreakers were evacuated from Singapore to Colombo in Ceylon, while their Army and RAF counterparts went to the newly formed Wireless Experimental Centre at Delhi in India. The small detachment left in Hong Kong was not so lucky. The British colony was captured on Christmas Day 1941 and the staff spent the next four years in Japanese prisoner-of-war camps.

The Royal Navy codebreakers moved into Pembroke College, an Indian boys' school about two miles from Colombo. But the deterioration in reception and the break in continuity had badly damaged their ability to break JN-25, which continued to evolve with new codebooks being introduced on a regular basis. It was to be another two years before they would come anywhere near catching up with the Americans, said John MacInnes, one of the GC&CS codebreakers attached to the Royal Navy unit:

The Corregidor unit was itself evacuated to Melbourne, where it effectively took over Nave's RAN unit. The latter's willingness to work with the Australian army units, which had been pulled back from the Middle East to work on Japanese material, led to major disagreements with the administrative head of the new US unit. Like many of his US Navy colleagues, Lieutenant Rudi Fabian regarded co-operation with anyone who was not in the US Navy or under its command as poor security. Not only did this cause problems for Nave, who in the end had
to be moved to the Australian army unit, but it would also cause much greater problems for the Royal Navy codebreakers in Colombo, who were now supposed to obtain any US results through Fabian's unit.

Despite the problems thrown up by the move to Colombo, the codebreakers were still able to break some messages as a result of the continued Japanese use of the ageing JN-25B codebook and the same additive tables introduced shortly before the outbreak of war. ‘As the life of the cipher table was extended, so more and more readable messages became available,' MacInnes said. The table remained in force for nearly six months. The book-building was delayed at first by much new jargon unknown in peacetime but, as regards units, was much helped by the possession of a library of messages going back to early 1941, so covering a period when the callsigns were well identified.'

However, in April 1942, the Royal Navy codebreakers were forced to withdraw to Kilindini, near Mombasa, in Kenya, after an attack by a Japanese task force which they had themselves predicted. George Curnock, one of the senior codebreakers, would later recall how amid confusion over the location of the target, a Japanese operator spelt it out in
kana
syllables ‘KO-RO-N-BO', sending an electric shock through what been until that moment a very relaxed office.

The move to Kilindini was a disaster for the Royal Navy code-breakers, according to MacInnes. It put their intercept operators out of reach of all but the strongest Japanese signals, cutting down on the depth of messages available to the codebreakers. ‘The moves from Singapore to Colombo, and Colombo to Kilindini, followed by the miserable volume of traffic which was intercepted there, caused an almost complete collapse in this field of work,' MacInnes said. ‘When efforts were resumed the leeway was too great. Signals frequently took up to a fortnight to be enciphered, transmitted and deciphered.'