The Bletchley Park Codebreakers

by Michael Smith

  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 original work on this had all been British but from the start of co-operation with Corregidor in 1941, the burden was carried more and more on the broad shoulders of the US stations. Work on the British side was badly dislocated by the move to Colombo. The loss of depth on the cypher table caused by the break in interception during the move, and the subsequent reduction in volume, greatly hampered stripping in bulk. It was at this time that the US Navy first took the lead in cryptanalysis.

  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 coll
eagues, 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.’

  Meanwhile, the Americans were able to push ahead. The US Navy codebreaking unit at Pearl Harbor, under the brilliant Commander Joe Rochefort, enjoyed good reception of the Japanese Navy messages and was breaking JN-25 with ease. As a result, the Americans were able to achieve the extraordinary feat of deciphering the complete Japanese operational orders for Admiral Yamamoto’s attempt to draw the US Pacific Fleet into an ambush off the island of Midway, 1,000 miles west of Hawaii. Having obtained the full details of its opponent’s plans in advance, the US Navy inflicted a crushing defeat on the Japanese, destroying four irreplaceable aircraft carriers and putting them on the defensive for the rest of the war.

  On the other hand, the British codebreakers working on Japanese Navy material were now at their lowest point of the war. The belligerent Fabian blamed security concerns over Nave for his reluctance to exchange material with the British codebreakers at Kilindini. But they were too far away from the action to intercept anything apart from the loudest signals that FRUMEL, the US Navy’s Melbourne site, and FRUPAC, the US site in Hawaii, were already able to receive without assistance. Fabian was also completely open over his belief that his unit had no reason to exchange material with anyone who could not give him anything in return. One senior British officer who visited FRUMEL said that ‘the most notable feature was the inability of the Americans to appreciate the full meaning of the word “co-operation”. The atmosphere was “What is yours is mine, and what is mine is my own”.’ Having given the Americans the help they needed before the start of the war, the British were now denied the technical assistance they desperately needed to get back on top of JN-25. With Admiral Sir James Somerville, Commander-in-Chief of the Royal Navy’s Eastern Fleet, complaining that Kilindini’s problems, and the US Navy’s concentration on the Pacific at the expense of the Indian Ocean, meant he was not receiving sufficient intelligence, the Admiralty considered the most drastic of measures.

  ‘The lack of US intelligence supply to C-in-C Eastern Fleet led the British to consider ditching the Americans on the Japanese side,’ said Frank Birch, the head of the Bletchley Park Naval Section. ‘Admiralty was not willing to be dependent on such small scraps as US were willing to provide and the only alternative to sharing all available intelligence between the two countries was for this country to build up independently an organization big enough to provide, without American help, as much intelligence as could be got with American help.’

  As a result, the British compromised on their previous ‘Europe first’ approach and began to expand the Japanese naval section, Hut 7, as well as sending more codebreakers out to Kilindini. Despite their difficulties, the Kilindini codebreakers did have some significant successes, most notably with the Japanese Merchant Shipping Code, dubbed JN-40. This was believed to be a super-enciphered code similar to JN-25. But in September 1942 a textbook error by the Japanese gave John MacInnes and Brian Townend, another of the civilian codebreakers sent to Kilindini by GC&CS, the way in. The Japanese operator omitted a ship’s position from a detailed message and instead of sending it separately in a different message, re-enciphered the original with the same keys, this time including the longitude and latitude that had previously been missing. A comparison of the two messages made it immediately clear that JN-40 was not a code at all. It was in fact a transposition cipher. It was based on a daily changing substitution table, containing 100 two-figure groups or dinomes, each representing a kana syllable, a romaji letter, a figure, or a punctuation mark. The operator wrote out the message in kana syllables and then substituted the relevant dinomes. This produced a long sequence of figures, which was written into a 10 by 10 square horizontally and then taken out vertically, thereby splitting up the dinomes and making it more difficult to break. “Within weeks, MacInnes and Townend had not only broken it but were able to read all previous traffic and were confident of breaking each message in real time, allowing enemy supplies to be tracked and attacked at will by Allied submarines. What was more, since it was a cipher, there were no code groups to recover and therefore no gaps in any of the messages. Over the next fortnight, they broke two more systems. The first was the previously impenetrable JN-167, another merchant shipping cipher. The second was JN-152, a simple transposition and substitution cipher used for broadcasting navigation warnings.

  There had also been a notable British success on a Japanese military code in 1942. The mainline Japanese Army codes had so far evaded the efforts of the Allied codebreakers. But John Tiltman had spent some of the early part of 1942 trying to break the Japanese military attaché (JMA) code. He discovered that it was a digraph code in which the basic kana syllables stood for themselves and other two-letter groups stood for certain words or phrases commonly used in military communications, e.g. AB stood for ‘west’ and AV for ‘message continued’. The two-letter groups were then set out in a square grid in adjacent squares, sometimes horizontally and sometimes diagonally, and the letters were read off vertically to form the basis for the encrypted text. They were then enciphered using a prearranged ‘literal additive’, a series of letters that would be notionally ‘added’ to the letters taken out of the grid on the basis of a pattern laid down in advance on a separate table. Reading off the enciphered letter along the relevant horizontal line and the ‘additive’ letter down the appropriate vertical column would produce a super-enciphered letter, which would be transmitted by the operator. ‘By March 1942, I and my section had partially recovered the indicating system and had diagnosed the cipher as a literal additive system with indicators which gave the starting and ending points for messages,’ Tiltman recalled. ‘The normal practice was to tail successive messages rigorously through the additive tables, i.e. to start reciphering each message with the additive group following the last group of the preceding message.’

  Tiltman set to work on a large number of JMA messages from one particular embassy, where the cipher clerk had used the additive table again and again, giving a large ‘depth’ to attack. ‘It was clear from the indicators that the sender had
tailed right round his additive table five times and it was this depth that I set myself to resolve.’ The solution took a lot of work, but with a depth of five on the cipher additive, he eventually managed to break the system. One of the first JMA messages deciphered revealed the Japanese intentions to construct a Burma Railroad. It was not until several months later that it became clear from another Japanese military attaché decrypt that British prisoners-of-war would be used as slave labour to build the railway.

  Tiltman set up a small Japanese military section at Bletchley Park in June 1942. It comprised a codebreaking sub-section and a traffic analysis team, but its main purpose was to handle the JMA material. The staff came mainly from a Japanese course set up by Tiltman in Bedford, which confounded the experts by turning out fluent Japanese linguists within the space of six months.

  By the beginning of 1943, there were a number of British sites around the world intercepting or decoding Japanese messages. At Bletchley Park, there was the Japanese naval section Hut 7, under the control of Hugh Foss, and Tiltman’s military section. Apart from Kilindini, there were a number of Bletchley Park outposts covering Japanese armed forces traffic. The main outpost was a joint RAF and Army intercept and codebreaking operation at the Wireless Experimental Centre (WEC) just outside Delhi. There was also a Wireless Experimental Depot at Abbottabad on the North West Frontier, but this mainly covered diplomatic links. The WEC had two main outposts: the Western Wireless Signal Centre at Bangalore in south-western India, which was merely an intercept site; and the Eastern Wireless Signal Centre at Barrackpore, near Calcutta. The latter site had its own code-breakers and traffic analysts as well as its own forward intercept and codebreaking outpost at Comilla, covering tactical air force communications nets. There was also a British officer. Major Norman Webb, based at the Central Bureau, a joint US and Australian Army codebreaking operation in Brisbane, where Nave was also now working, following his disagreement with Fabian.