The Bletchley Park Codebreakers

by Michael Smith

  Denniston was scheduled to travel to Washington on 16 August. ‘Perhaps the most important purpose of the visit is to clear up the position concerning E traffic’, he wrote in a memorandum to ‘C’. Denniston proposed that he should explain to the Americans during his visit that GC&CS had received so far only 6 of the 36 bombes it had ordered and could not spare one. Allowing an American firm to build a bombe was out of the question on security grounds, he wrote; for the American cryptanalysts, the Enigma was at best a ‘new and very interesting problem’, but for Britain it was ‘almost life-blood to our effort’. But perhaps, Denniston suggested, it might be possible for the US codebreaking bureaux to send some ‘young mathematicians’ to Britain to work with GC&CS directly on the Enigma. ‘C’ shot down every one of these suggestions. He replied that he was ‘a little uneasy about the proposal for young mathematicians’ and that ‘I should feel inclined not to mention’ the bombe situation at all lest that might be used as an argument by the Americans to have them built in the United States, which the British were adamantly opposed to.

  On 1 October 1941, Denniston did apparently write Safford a letter supplying the information OP-20-G had been after, but the letter went astray. On 27 November, Admiral Leigh Noyes, the US Director of Naval Communication (under whose jurisdiction OP-20-G fell), sent a long complaint to the British alleging that they had failed to carry out their end of the bargain. The British reply, from ‘C, did little to mollify his concerns, for it argued rather jesuitically that on naval Enigma, ‘everything asked for has been supplied’, which did not exactly seem to the Americans to reflect a spirit of true co-operation. Finally Denniston realized that his letter to Safford had not been received and cleared the matter up; by 12 December the letter had arrived and Noyes stated that everyone was now satisfied.

  Nevertheless the British position was untenable in the long run, for it still insisted that neither the US Army nor the US Navy need participate directly in ‘exploiting’ German Enigma traffic – that is, in decrypting it and distributing the results on an operational basis. When it came to control of the product, that was a British prerogative, period. The Admiralty would ‘pass German naval intelligence to Navy Department when U.S.A. was affected’, Washington was informed. As for the US Army, they clearly were not affected by the ground war in Europe:

  1. No results are being passed out because they only affect operation in various localities in Europe and Africa in which U.S.A. has only academic interest.

  2. An undertaking was given to the War Department that our methods which may have led to partial success will be shared with them when they are really interested in the operational results.

  ‘Interested’ in this case was clearly meant in the strict sense of the word, that is, when the US Army had an interest at stake in the actual fighting. With the entry of the United States into the war, both OP-20-G and Arlington Hall (as the US Army SIS establishment came to be known after its move in 1942 to the former girls’ school of that name in Arlington, Virginia) renewed their demands for direct involvement in decrypting German Enigma traffic. And now British opposition began to assume a different complexion. Knowledge is power, and being able to control the distribution of such a valuable form of knowledge as Enigma intelligence gave the British a not inconsiderable point of leverage in the alliance. It was not so much that the British were out to directly manipulate the information or withhold it when military necessity dictated it be provided to field commanders, British or American. But many decisions in negotiations over military strategy between the new allies were tilted by arguments over the relative contribution each side was making to the war effort, and the Enigma was one bargaining chip: the British monopoly over this intelligence was one thing that made the Americans undeniably beholden to them. By the same token, the growth of a significant independent American capability would make the British – and British views – that much more dispensable.

  Denniston was alarmed by the new American demands for fuller participation in the Enigma work, and at once sent a message to the British liaison in Washington:

  In telegram from the War Department A.16 of 18th December, they raise the question of investigating the German Air–Army cypher. During my visit it was agreed that we should be responsible for this investigation and that when U.S.A. were in real need of this work we should invite their party to join ours.

  Could you find out if their views on this procedure have changed and if they wish to begin their own investigations now? It is devoutly hoped by all here that any such investigations will not interfere with their progress on Japanese work for which we count on them.

  For the US Navy, what finally broke the British claim to monopoly was GC&CS’s sudden failure to continue reading the Atlantic U-boat traffic. On 1 February 1942, the Atlantic U-boat networks changed from the three-wheel Enigma to a four-wheel machine. Running the bombe through all possible positions for one wheel order of a three-wheel machine typically took about fifteen minutes of machine time (the set-up time for each run added another ten or fifteen minutes, and additional time was required for testing the results of the run). The addition of a fourth wheel meant that the time required for each run on the three-wheel bombes was multiplied by a factor of twenty-six – the machine would have to be run through all possible wheel orders and at every possible position of the fourth wheel; testing all 336 wheel orders at all twenty-six positions of the fourth wheel could thus in theory take several thousand hours on a three-wheel bombe. Even with all sixteen bombes that were available at this time running simultaneously, and even exploiting methods that could reduce the number of wheel orders that needed to be tried, recovering each daily setting might take two days or more of effort. But there were other problems as well, notably a lack of suitable cribs. What was needed was a new, four-wheel bombe that could run at much higher speeds.

  Lieutenant-Colonel John Tiltman, one of GC&CS’s top cryptanalysts, visited the US Navy Department in April 1942 and at once assessed the situation with a clear eye. He began a tactful but straightforward lobbying effort with his new director, Edward Travis, who had replaced Denniston in February as the operational head of Bletchley Park. ‘In view of the fact that they are now at war and have a vital interest in submarine traffic they are entitled to results or a detailed statement as to why traffic cannot be read at present and what are prospects for future,’ Tiltman cabled to London. ‘Unless a rapid and satisfactory solution is found … the high command will insist on their naval cryptanalysts attempting to duplicate our work on E [Enigma].’ He noted that, as a riposte against the British excuse of concerns over poor American security, OP-20-G was already insisting that the US needed to form a ‘skeleton party’ with ‘some machinery’ as insurance, in case the Germans invaded England, or if the Luftwaffe bombed Bletchley Park.

  Travis cabled back, ‘hardly think necessary to form skeleton party as if real danger arose of present facilities being lost we would certainly send experts other side’. But he got the message. On 13 May 1942, Travis informed OP-20-G that ‘higher authority has agreed future policy regarding E solution … we will continue exploiting but will send you a machine for solution in August or September and lend you a mechanic to instruct in working. We will also give full instructions and try to spare some one to explain our method.’ Travis also agreed with Tiltman’s suggestion that several US Navy experts who had been working on methods for speeding up the bombes be sent over to Bletchley. Lieutenants Robert Ely and Joseph Eachus arrived on 1 July, and after a few more delays and excuses were finally given what the United States had been seeking since February 1941 – complete wiring diagrams and blueprints of the actual bombes.

  The British clearly hoped to assuage the US Navy by allowing it to participate more fully on the research end of things, and even to develop a small independent capability of breaking some of the traffic that could be intercepted from North America, while still keeping fundamental control of the intelligence output themselves. But that last ditch stan
d had to be abandoned finally in August when, having failed to make good on the promise of providing the US Navy with a bombe, GC&CS was suddenly confronted with a fait accompli by the Americans. It was clear the British designs for the four-wheel bombes were running into difficulties in particular in getting the high-speed rotors to make good electrical contact; the new head of OP-20-G, Captain Joseph Wenger, was also apparently convinced, though wrongly, that the British had achieved some success reading current U-boat traffic and were concealing that fact. On 3 September 1942, Wenger proposed to his superiors spending two million dollars – ‘it must be understood that it is a gamble’, he wrote – to build 360 four-wheel bombes. The British liaison to OP-20-G had tried to head off the American move, protesting that Tiltman promised only to provide results or ‘a detailed statement as to why traffic cannot be read’, and by doing the latter the British had fulfilled their obligations and the US Navy had no reason to complain. But the fact was that GC&CS only had about thirty bombes built by this point, and there was no doubt that they desperately needed the help. In July and August the demands of trying (still without success) to break the four-wheel naval Enigma keys had overloaded the available bombes, restricting work that could be done on some German Air Force keys. (Even six months later the situation remained almost desperate. On 5 January 1943, Gordon Welchman, who had played a key role in the original design of the bombe and in organizing the system at Bletchley Park for handling Enigma traffic, warned Travis: ‘An analysis of probable and possible requirements for bombes during 1943 is most alarming’. At the new year there were 49 machines in operation; Welchman calculated that as many as 120 three-wheel bombes would be needed for ‘urgent work’ on German Air Force and Army traffic, while breaking the U-boat and other naval keys might take as many as 134 high-speed, four-wheel bombes.)

  Faced with the inevitable, Travis and Frank Birch, head of GC&CS’s Naval Section, travelled to Washington in September 1942 and negotiated an agreement by which GC&CS and OP-20-G would establish ‘full collaboration’ on attacking the German naval Enigma – exchanging traffic, recovered settings, and the ‘cribs’ needed to run the bombes. The US bombes would be patterned on the general British design, but would use an electronic sensor instead of relays to detect when the rotors hit the correct position, an innovation that would allow the rotors to turn at a much faster speed. The Navy quickly contracted with National Cash Register (NCR) in Dayton, Ohio, to build the machines, and work began immediately.

  This pact, known informally as the ‘Holden Agreement’, was a breakthrough for American cryptanalysts; for the first time they had broken the British monopoly over Enigma. But one interesting aspect of the agreement was that the US Army was not consulted. Just because GC&CS had surrendered on one front it saw no reason to do so on the second front. As a GC&CS memorandum in late 1942 noted in relation to German Army and Air Force Enigma traffic, ‘it is not proposed to invite the Americans to take part in our work on “E” though the fruits are at present being handed to the Americans in the Mediterranean’. The US Army cryptanalysts were, however, permitted to study the British bombe blueprints that the US Navy had obtained, and within two weeks of the Navy’s decision to begin building bombes, the Army was proposing to build its own machine, too. The Navy agreed that NCR could build bombes under an Army contract as well, but the Army had what it thought was a better idea; in place of the rotating drums of the bombe it wanted to use telephone switching relays, and on 15 December 1942, a $530,000 contract with an ‘AAA’ priority rating was signed with AT&T to build a single huge relay bombe.

  The Army intended to produce a fait accompli of its own to present the British, but it was clear that Friedman and company had little notion of what was actually involved in creating a signals intelligence operation of the scale needed to handle Enigma traffic. The single Army bombe was the equivalent of 144 Enigmas, about four times the size of one standard British bombe. It was a highly innovative design, and incorporated some features that speeded up the operations considerably, including a system for automatically changing the wheel order; in the British bombes this required physically removing wheels and replacing them, a time-consuming procedure. But its huge cost (which eventually reached a million dollars, equal to the cost of fifty fighter aircraft at that time) made it totally impracticable for the sort of mass production of traffic required for a serious attack on Enigma intercepts. Moreover, without its own intercept capability in the proximity of the Continent, the US Army would be wholly dependent on traffic relayed from Britain in any event. It was inconceivable that the Army could truly create an independent capability of reading Enigma traffic, at least not soon.

  Undeterred, the US Army codebreakers pressed ahead, motivated if nothing else by a simple rivalry with the Navy. They had the perfect opportunity just a few weeks later to present their fait accompli to the British. Alan Turing had been despatched to America to visit the NCR plant in Dayton and make recommendations. (Turing went to Dayton on 21 December 1942, and did offer several suggestions, the most significant of which was that the American plan to build 336 bombes – one for each wheel order – was wasteful and ill-considered, given the methods available to reduce the number of possible wheel orders to be run for each test and the way the bombes were used on actual problems.)

  On 4 January 1943, Arlington Hall received permission to reveal to Turing and Tiltman, who was in America as a liaison officer at this point, ‘the fundamental principles and details of the equipment now in development’. Dale Marston, who was then directing the development of the rapid analytical machinery for Arlington Hall, later recalled that when Tiltman was briefed on the work, he immediately said that the Army and GC&CS ‘had better get together’. Turing was shown the actual prototype at Bell Laboratories in New York City on 5 February.

  The Army at once requested that the British send Enigma intercepts and cribs to Washington; Tiltman tactfully responded that GC&CS objections to doing so were ‘dictated entirely by considerations of security and their great fear that present exploitation of this traffic may be jeopardized by allowing such data to leave England’. Of course that was a disingenuous position, since GC&CS had already accepted the principle of sending such data to the US Navy in Washington, and was sending settings, cribs and traffic to OP-20-G on a daily basis at this very time. Cribs supplied by short weather signals had allowed GC&CS to break the four-wheel Enigma traffic in December 1942, and even before the first two US Navy bombes were completed on 3 May 1943 (they solved their first daily key on June 22), OP-20-G was involved in solving Enigma keys by hand methods and sending the results back to GC&CS.

  GC&CS tried other tacks: one was to emphasize the wasteful duplication of effort that would result if the US Army began ‘exploiting’ Enigma traffic on its own; much better and more efficient would be for the United States to concentrate on Japanese traffic. There was some sense to that but of course it was also self-serving; the British were proposing in effect full co-operation in Japanese traffic in exchange for a British monopoly on German traffic. The British had also never expressed concerns over the wasteful ‘duplication’ of effort on German diplomatic traffic, which was being worked on in both Washington and London.

  A better argument was the one that also happened to be true: that GC&CS by this point had more than two years’ experience in mastering the myriad subtleties not only of the daily Enigma keys and in using the bombes to their highest efficiency, but, even more important, in correctly translating and interpreting German military terminology and placing it in the broader intelligence context. Bletchley had accumulated thousands of points of reference – the meanings of abbreviations and German military terms and cross-indexes of the names of units and commanders – that Washington could not possibly expect to duplicate without years of work. If the labour were divided, that would mean that inexperienced American intelligence officers would be producing intelligence without supervision and they could easily make erroneous deductions.

  An inte
rnal GC&CS memorandum on 8 January 1943 laid out the situation. It noted that a ‘gentlemen’s agreement’ had been reached that the exploitation of intelligence would be left in British hands until America was actively engaged in military operations. True, two events had changed matters: American entry into North Africa, with the TORCH landings of 8 November 1942, and U-boat activity along the American coast. But the Admiralty was helping the US Navy meet the U-boat situation by telegraphing relevant decrypts as soon as they were available, and a special party of British intelligence officers, trained at Bletchley Park to handle Enigma intelligence, had been attached to Eisenhower’s command in North Africa. Since it was now ‘desired that the whole matter be placed on an official basis’, the thing to do would be to ask the Americans whether they would agree to leave the exploitation of Enigma in British hands, with results exploited by America only where operations were in proximity to her own seaboard.

  This was again an odd and rather Jesuitical position: Britain was still claiming a right of absolute and exclusive control over the breaking and distribution of Enigma traffic where joint US-British military operations were concerned; only where American interests alone were at stake – the East Coast of the United States – would they cede a right for America to become involved. Taking the same fallback position they had assumed with the Navy, the British would make it clear they welcomed co-operation ‘in the field of research’ and had no objection to machinery being built in America, using ‘results of British design and manufacture’, provided absolute secrecy was maintained. But they insisted that Britain could not cede its vital interests by sharing control over decryption and distribution of the resulting intelligence, and sent a draft formal agreement to that effect.