The Bletchley Park Codebreakers

by Michael Smith

  The Central Bureau was also facing difficulties in its relationship with FRUMEL. It originally attempted to set up an exchange relationship with the US Navy site, but Fabian was not interested and was backed in his reluctance to exchange material by Admiral Joe Redman, the Director of Naval Communications. This was perhaps unsurprising. For much of the war, the US Navy codebreakers and their US Army counterparts were barely on speaking terms. But Fabian insisted that the Central Bureau could give FRUMEL nothing it needed and it had nothing that could help them. The US Navy unit was ‘concerned solely with information on Japanese naval circuits’, Fabian said, where ‘the Central Bureau was not’. It was not that simple. Few wars had seen more need for complete co-operation between the army and navy. The Japanese Army was forced by the very nature of the campaigns it was fighting, cut off from its home bases by thousands of miles of ocean, to pass messages on naval communications circuits, often in naval codes and ciphers. Messages would be translated from one system to the other, providing a wealth of potential ‘cribs’ if only they could have been followed through the system.

  It is impossible to say how much help this might have given the army codebreakers. In the event, the US Army codebreakers at the Central Bureau and the two main British outposts converged on the solution to the first mainline army code at the same time in March 1943. Some credit Wilfred Noyce, a classicist from King’s College, Cambridge, and a prominent mountaineer, who was working at the WEC in Delhi, with having broken the Japanese Army Water Transport Code, senpaku angosho 2, assisted by Maurice Allen, an Oxford don. The difficulties of communication between the various codebreaking bases make it difficult to tell for sure whether they were first. They may well have been beaten by Warrant Officer Joe Richard, a US Army codebreaker at the Central Bureau, Brisbane, who also broke it at around the same time. There is even a possibility that Brian Townend, who remarkably was working on it in his spare time at Kilindini, may have broken it first.

  Bletchley Park was not in any way involved in what was perhaps the most controversial use of the ability to break the high-level Japanese codes – the shooting down, in April 1943, of an aircraft carrying the Japanese Navy’s Commander-in-Chief Admiral Yamamoto Isoruku. A JN-25 message giving the itinerary of a tour by Yamamoto of the Solomon Islands was deciphered by the US Navy codebreakers in Hawaii, allowing US fighters to intercept his aircraft and shoot it down, killing all those on board. The Japanese ordered an investigation but fortunately details of the visit had also been sent on low-level nets and it was assumed that the Allies had intercepted these rather than having been able to decipher the original JN-25 message.

  Meanwhile, the Japanese military section, where John Tiltman was leading the research, broke the Army Air Force General Purpose Code, koku angoo-sho 3, designated 3366, and began to expand to cope with the increased intelligence expected as a result of the recent successes. It acquired its own Army Air subsection in May 1943, presumably as a direct result of the breaking of the 3366 code, and some weeks later, a military intelligence section was added, reporting direct to MI2, the War Office intelligence section covering the Far East.

  The breaking of the first two mainline army codes led Tiltman to call a conference at Bletchley Park in July 1943 to allocate coverage among the various Allied outposts. It was agreed that Bletchley Park and Delhi should concentrate their cryptographic resources on the codes and ciphers of the Japanese Army Air Force. Arlington Hall (headquarters of the SIS) would deal with the high-level systems used by the Japanese ground forces, leaving the Central Bureau to concentrate on the low-level material produced by their forward field units and the Army Water Transport code.

  A separate Japanese air intelligence section was set up at Bletchley Park in October 1943, with Leonard ‘Joe’ Hooper, a future head of the postwar GCHQ, in charge. So many Japanese military messages were now being sent over Japanese naval circuits, and in a number of cases actually using JN-25 and other naval codes and ciphers, that it was decided to co-locate all the Japanese sections. They were moved into Block F, the largest of a series of brick-built blocks designed to cope with the expansion of Bletchley Park. This consisted of a number of wings jutting off a long, central corridor which, as a result of its length and the proliferation of various Japanese subsections in the block, became known as ‘the Burma Road’.

  The summer of 1943 saw a marked improvement in the relations between the British and the US Navy codebreakers. Redman attended a number of conferences at Bletchley and the Admiralty during which he promised to ensure that the Royal Navy codebreakers would receive all urgent Japanese Navy material and all the American JN-25 recoveries. The Royal Navy codebreakers were to return to Colombo in September to improve their coverage and Redman agreed that the new station would have a direct cipher link to Melbourne.

  The new Colombo site was on the Anderson Golf Club. The Royal Navy codebreakers were soon swamped with Japanese naval messages to work on. But despite Redman’s promises the exchanges with Melbourne did not improve. Commander Malcolm Saunders, a former head of Bletchley Park’s Hut 3 intelligence-reporting section, toured Allied naval codebreaking sites in the autumn of 1943. He was very impressed with the FRUPAC centre in Hawaii but less so with FRUMEL, where Fabian continued to block co-operation. ‘The liaison with Colombo is not nearly as good as it should be,’ Saunders said. ‘This is partly due to bad communications and insufficient staff at each end, but also due to the present lack of productivity of the Colombo unit, and to lack of a clear-cut statement of policy from Washington in this regard. The security aspect is constantly in mind and there is a constant suspicion of “leakage” to the American military authorities at Brisbane.’

  Harry Hinsley, the leading naval intelligence analyst at Bletchley Park, was sent to Washington in late 1943 in an attempt to improve co-operation with the US Navy. The result was an agreement, in January 1944, to set up a comprehensive exchange circuit between the main stations tackling Japanese Navy material, including Bletchley Park and the Royal Navy site at Anderson, ‘as early as practicable’.

  Perhaps the best example of the new spirit of co-operation came in the breaking of Coral, the successor to the Japanese naval attaché machine cipher broken by Foss and Strachey. With the US Navy now bearing the brunt of the attacks on the German U-boat Enigma, Hugh Alexander, the head of the German naval Enigma section Hut 8, began to examine the Coral cipher. Since the permutations effected by its banks of stepping switches were broadly similar to those made by Enigma, many of the processes used were similar. At the end of September 1943, the British began to make real headway and Alexander produced a report, which according to the official US history, ‘marks the birth of the successful attack on the Coral’.

  Alexander flew across to America in early 1944 to help in the final attack on Coral. Since Foss, who had broken Coral’s predecessor, had also just arrived in Washington as liaison officer, he may have been involved too in the discussions with Lieutenant-Commander Frank Raven, who had been leading the US Navy team that solved Jade, a different Japanese navy machine. By 11 March, the codebreakers had solved the wiring of the Coral machine and read a few messages. The British codebreakers ‘contributed heavily’ to what was a joint US-British success, the US history records. Coral, together with the Japanese military attaché code and the Purple diplomatic cipher, would also contribute to the Allies’ knowledge of the German defences in Normandy and thus to the success of the D-Day landings.

  As the Allies pushed forward on a number of fronts in the Far East, the codebreakers benefited from an increasing number of ‘pinches’ of Japanese codebooks. One of the most important, and certainly the largest, occurred in early 1944 when the 9th Australian Division overran the positions of the 20th Japanese Division at Sio in northern New Guinea. The division’s chief signals officer should have burned its codebooks. Instead he dumped them into a water-filled pit inside a metal container. When it was retrieved it was found to contain the current code-books for six different mainlin
e systems, allowing the Allies to read the Japanese military codes for the next two months and to keep on top of most of the main military systems after new code-books were introduced. The Central Bureau was able to provide General Douglas MacArthur, the Allied commander, with full details of the Japanese order of battle in New Guinea, greatly aiding his advance.

  Bletchley Park now began to reinforce the Central Bureau with a number of graduates of the Bedford course. Hugh Melinsky, who had been recruited from Christ’s College, Cambridge, arrived in April 1944 as one of a dozen or more British reinforcements. He was put to work on the naval air desk, part of Eric Nave’s air-ground section. ‘What Captain Nave did not know about code-breaking was not worth knowing,’ Melinsky said. ‘He had a sixth sense which enabled him to sniff out a meaning in what looked to me like a jumble of letters or numbers.’ Melinsky and other British codebreakers were sent forward with Australian mobile wireless units to provide MacArthur with up-to-date tactical intelligence as the Allies pushed northwards.

  The British and Indian advances in Burma during the second half of 1944 and into 1945 were also aided by mobile signals intelligence units that pushed forward with the Allied troops. But one of the most significant pieces of assistance they received from Sigint was a result of Bletchley Park’s breaking of the three-figure super-enciphered code used by the Japanese Army Air Force for its communications with the ground, known as kuuchi renraku kanji-hyoo 2-goo to the Japanese and BULBUL to the Allies. The code was broken in the new Japanese air section, which was rapidly expanding, with around 250 people working on army air alone in September 1944, but was exploited in India to great effect.

  Michael Kerry was one of the codebreakers assigned to work on the BULBUL code in Comilla. ‘The Japanese bombers used to be kept safely down in Bangkok and then when there was a full moon, they were moved forward to the Mingaladon air base in Rangoon. On one occasion we got wind in advance that a raid was going to take place and passed the information on. Most of the time, we had no way of knowing if what we did was a pennyworth of use but in this particular instance the nightfighters got the lot and all night we could hear Mingaladon air base calling for its lost children.’

  The US codebreakers’ achievement in breaking the Purple cipher, and the fact that they were reading the main Japanese Navy code JN-25, was revealed almost immediately the war ended through a series of leaks and the congressional inquiry into the attack on Pearl Harbor. But the part played by the British codebreakers in the war in the Far East remained secret until the late 1990s when GCHQ began to release the files that recorded the achievements of men like Tiltman, Foss and Nave. As a result, most of the existing literature credits the Americans with having led the way in the breaking of the Japanese codes and ciphers. Only now are the British codebreakers beginning to receive the recognition they deserve.

  9

  SOLVING JN-25 AT BLETCHLEY PARK: 1943–5

  EDWARD SIMPSON

  Introduction

  One of the most important of the Japanese codes in use during the Second World War was the Imperial Japanese Navy’s General Operational Code, dubbed by the allied codebreakers JN-25. This was first used in June 1939 and, quite remarkably, broken by John Tiltman at Bletchley Park within a few months of its introduction. Tiltman had made the initial breaks into the main Japanese military code in late 1938. This was typical of the main Japanese systems during the Second World in that it was a super-enciphered code, the messages being first encoded using numerical groups signifying different concepts or Japanese kana or syllables and taken from a codebook, which in the case of this initial military system used four-figure groups. These were then enciphered by the addition of randomly produced streams of numerical groups taken from an additive book (See Appendix V). Tiltman recognized JN-25 as a similar type of system, albeit using five-figure instead of four-figure groups. The main UK effort against JN-25 was then carried out by British codebreakers working at the Far East Combined Bureau in Singapore, and for the Americans by US codebreakers based at Corregidor in the Philippines. But the Japanese sweep across south-east Asia, in late 1941 and early 1942, led the British to move first to Colombo, in what is now Sri Lanka, and then to Mombasa in British East Africa (Kenya), while the Americans based at Corregidor moved to Melbourne. The main attack on JN-25 was then led by the US codebreakers in Hawaii, greatly assisting the US Navy’s victory in the Battle of Midway in June 1942. The British codebreakers at Mombasa were hampered in their own efforts by their distance from the main Japanese naval activity and a shortage of the ‘punch-card’ tabulating machinery used by the Americans. Although there were a number of sections at Bletchley Park working on the Japanese ciphers, including the Japanese naval section Hut 7, the main focus was necessarily on the war in Europe and it was not until the surrender of Italy in the summer of 1943 that the Japanese effort in general, and the work on JN-25 in particular, was expanded. In this chapter Edward Simpson gives perhaps the most detailed description yet by one of those taking part in the work on JN-25 at Bletchley Park in the latter part of the war.

  MS

  When Italy surrendered in August 1943 I was a mathematician cryptanalyst TJAO (Temporary Junior Administrative Officer) in Italian Naval Section, in Block B, which was headed by Wilfred Bodsworth (‘Boddy’). He in turn reported to Frank Birch, Head of Naval Section.

  Mostly I worked on the main Italian Naval subtractor Cifrario Libia. It was not being read currently, so there was less sense of urgency than elsewhere. By contrast, for the first few days of each month I joined the Italian Naval Hagelin team under Colin Thompson (later Director of the National Galleries of Scotland). The Hagelin C-38 cipher was read currently and was of major importance for the harrying of Axis shipping carrying supplies to North Africa. Its keys changed monthly and the team then borrowed additional mathematicians. We worked intensively on three shifts until the new month’s keys were broken.

  Elsewhere in Italian Naval there were many small ciphers on the go as well as the big ones. One simply used the Calendar of Saints for a daily key; we called it the Holy Indicator. We all looked over each others’ shoulders a lot and chipped in with comments and suggestions. Boddy’s management style was that of the senior member of a common room rather than that of a director.

  So when I was made TSAO (Temporary Senior Administrative Officer) and told to head the new team on the cryptanalysis of JN-25, this was a novel concept. Having gone to Bletchley Park in October 1942, directly after graduating at age 19, I had neither managed nor been managed. I now reported to Commander J. P. McIntyre RN, a Japanese linguist with experience of JN-25 in Singapore.

  Those who moved from Italian to JN-25 with me were mainly the younger ones. The aristocrats and the senior academics (June Capel, Elizabeth Wyndham, Philip Hall of King’s and later Sadleirian Professor at Cambridge, Professor (of Classics) T.B.L. Webster and his equally Hellenic wife Margery Dale, Patrick Wilkinson of King’s, Cambridge, and his wife-to-be Sydney Eason, Professor (of Italian) E. R. Vincent, Professor (of German) Walter Bruford who had worked in Room 40, Willy King (from Ceramics at the British Museum) all dispersed elsewhere.

  Of our group, Tony (A. J.) Phelps (an Oxford classicist) and Gladys Gibson (an English graduate and now my wife Rebecca Simpson) and I immediately went on John Lloyd’s first short Japanese Language Course within the Park for the month of September. Mathematicians who joined me were Ian (J. W. S.) Cassels, later Fellow of Trinity and Sadleirian Professor at Cambridge, and Jimmy (J. V.) Whitworth. Both were newcomers to Bletchley Park. Also from elsewhere came Lieutenant-Commander Belfield RN, grey-haired and courteous but without any known Christian name, who had intelligence experience in the 1914–18 war and had worked on Italian ciphers in Alexandria in this one.

  At the beginning of October our team numbered 20. The four men TJAOs had a total – not an average – cryptanalytical experience of 23 months. Of the more junior staff, all civilians, I can name: José Cassar, Gill Cave, Adelaide Dickens, David Foxon, Mary Fraser
, June Jarratt, Christine Kendall, Jimmy Love, Mary Manners, Margaret Pyke, Frances Sidebotham, Olive Thorogood, Joyce White, Petronella Wise. They varied in rank from unskilled Grade III to junior cryptographer TA and in experience from three months to four years. Ability to do the JN-25 job was not well correlated with rank and experience; a problem which was exacerbated with the arrival later of Wrens whose quickness more than compensated for their inexperience.

  We acquired a third Japanese linguist at the end of March 1944: following the Italian surrender Henry Reed had gone from Italian Naval to the six-month language course. After a year in our party he took over the direction of the one-month course. Although he was older than most of us – all of thirty – and in poor health, Henry was not exempt from serving in the Bletchley Park Company of the Home Guard. He was not a very military person but his Home Guard service has left us a memorable experience. His poems ‘The Naming of Parts’, described as ‘the best-loved and most anthologised poem of the Second World War’, and ‘Judging Distances’ beautifully and laconically balance the unforgettable teaching style of our two regular Army Sergeant-Instructors with what caught his eye when his attention wandered.*

  For six months from January to July 1944 Japanese Naval was joined by Marshall Hall, already a noted academic mathematician at Yale, and thinly disguised as a Lieutenant, US Navy. He was for that period OP-20-G’s Liaison Officer on Japanese matters. He was not attached to any one party but moved around them all, including ours, discussing and advising in a very wry and acceptable way. He introduced to us ‘Hall’s Weights’, a statistical approach which I describe (but inadequately) in Appendix VII.