The Bletchley Park Codebreakers

by Michael Smith

  The Americans were correct in assuming that in the early autumn of 1940 the Government Code and Cypher School (which admittedly had been playing the game longer and harder than its American counterpart) was significantly ahead of the Signal Intelligence Service in most areas of diplomatic signals intelligence; indeed the SIS was not even playing in the same league.

  Figure 10.1 Diplomatic Crypto-systems Read by GC&CS and SIS, 1940

  Systems GC&CS SIS

  Japanese 19* 15*

  German 1**1** 1**

  Italian 7 2

  French (Vichy) 10 0

  Chinese 6 0

  Russian 0 0

  Latin American 47*** 40†

  Balkan 12†† 0

  Near Eastern 65††† 0

  * Includes Red but not Purple

  ** DESAB

  *** Includes Argentina, Bolivia, Brazil, Chile, Colombia, Ecuador, Mexico

  † Mexico only

  †† Bulgaria, Romania, and Yugoslavia

  ††† Iran, Iraq, Saudi Arabia, and Turkey

  Sources: Untitled table of foreign codes and ciphers, PRO HW 14/11; ‘The Status of the Cryptanalysis of Japanese, German, Italian, and Mexican Systems, August 1940’, NACP HCC box 587.

  In 1940, when the SIS was covering only four targets, GC&CS was working the diplomatic communications of twenty-six countries.

  Figure 10.1 compares the records of GC&CS and SIS against some of those countries. British codebreakers, for example, had solved high-grade Italian ciphers while the Americans were struggling to master low-grade versions. GC&CS was reading several Vichy French diplomatic ciphers at a time when the SIS was hoping that the next staff increase might free up one or two officers to open a French section. GC&CS routinely solved Balkan and Near Eastern systems that did not even appear on the cryptologic horizon of the SIS. Even in the matter of Japanese codes and ciphers (an American speciality) the British were reading more systems (including Red) than the Americans, although GC&CS had had no more luck with Purple than their cousins across the Atlantic. Only in the area of German and Russian communications were the prospective partners equal. In their efforts against Berlin’s diplomatic systems neither had been able to advance beyond the reconstruction of the DESAB code. As for Russian systems, neither was studying Moscow’s diplomatic ciphers, although GC&CS was reading a few Red Army and Comintern systems.

  There can be little doubt that the United States stood to benefit significantly from any collaboration with Britain. Of course, General Strong’s proposal called for an exchange; the British would certainly expect some return for any secrets they shared with the Americans. It wasn’t clear, however, that the SIS had anything GC&CS might need. The principal product that the SIS could bring to the exchange was information on American progress against Japanese diplomatic ciphers. What if GC&CS had already solved these ciphers? In the midst of a life-and-death struggle against the Axis would the British share the secrets of German and Italian cryptography in return for a copy of the cipher used by the Mexican finance ministry? SIS stood to benefit disproportionately from collaboration, but it needed something big to bring to the table. By the end of September it had something big.

  For eighteen months the battle to solve the Purple machine had consumed the SIS. Under the direction of Frank Rowlett, the Japanese section routinely logged fourteen-hour workdays as they struggled under great pressure to find a weak spot in the cryptographic armour that protected Tokyo’s most sensitive diplomatic communications. On 20 September 1940 the perseverance and hard work paid off. On that day, Genevieve Grotjan, a studious young statistician who had come to the SIS from the Railroad Retirement Board in 1939, noticed certain patterns in the cipher alphabets so far reconstructed by the American team. It was the breakthrough. One week later Frank Rowlett handed William Friedman the first two decrypted Purple messages.

  The solution of Purple reopened access to Tokyo’s high-grade diplomatic traffic and significantly improved America’s bargaining position in any exchanges with Britain. The Americans had no way of knowing if GC&CS had broken Purple for itself, but even if it had it would immediately understand that the solution of the Japanese machine was an impressive accomplishment that established the United States as a potentially valuable partner in the signals war. The SIS believed that Purple would purchase any number of British secrets.

  On 11 September, almost two weeks before the solution of Purple, the Army Chief of Staff, General George Marshall, had approved the exchange of cryptanalytic information with Britain. The US Navy cryptanalysts, however, suspected British intentions and managed to block the plan to pass American Sigint secrets to London until December when the combined political power of the President of the United States, the Secretary of War and the Secretary of the Navy finally quashed their opposition. To open the collaboration the Army and Navy signals intelligence services each selected two of their officers to form a joint mission to Britain. Still a reluctant participant, the Navy gave its representatives, Lieutenant Prescott Currier and Ensign Robert Weeks, no guidance beyond a general injunction ‘to get whatever you think we should get and have a look around’. As their contribution to the exchange the naval officers carried the latest version of the Imperial Japanese Navy’s fleet code to the extent that it had been reconstructed by OP-20-G. The latest version of this system, JN-25B, had been in service less than two months and naval cryptanalysts had as yet recovered so few code values that the codebook carried by Currier and Weeks was ‘almost empty’. For its part, the Army, always more enthusiastic about connections with the British, stripped bare its cryptanalytic cupboard. The most precious gift in the baggage of the military representatives, Abraham Sinkov and Leo Rosen, was an analogue of the Purple cipher machine designed and built by Frank Rowlett’s Japanese section. Lacking any information concerning GC&CS operations and successes, Sinkov and Rosen packed anything else that might help or impress the British: a copy of Japan’s Red cipher machine, several additional Japanese diplomatic systems solved by the SIS in the 1930s, a complete copy of DESAB as confiscated by the Army from a clandestine German courier transiting the Panama Canal, the Italian systems X and TR as reconstructed by the SIS, and four Mexican diplomatic systems.

  The American mission reached Bletchley Park on 7 February 1941 and over the next several weeks they toured GC&CS facilities, visited intercept stations, examined equipment, and generally talked shop with their British hosts. Much of the controversy surrounding the ‘Sinkov Mission’ has revolved around the question of whether the British reciprocated the gift of a Purple machine by revealing to their guests their success against the German Enigma cipher machine (they did). In the debate over what the British told the Americans about Enigma, it is easy to lose sight of what the hosts told their guests about a range of other cryptologic subjects.

  A broader perspective seems especially appropriate in view of the fact that in early 1941 Enigma was not a priority for the Signal Intelligence Service. Germany used the famous cipher machine for army, navy, and air force traffic. At the time of the American mission, however, the SIS was not even intercepting, let alone studying, the service traffic of Germany or any other country. It focused exclusively on diplomatic ciphers, and no foreign ministry except the Swiss used Enigma. Of course, any insight into the German machine would have been welcomed by Sinkov and Rosen (who were collecting insights into everything), but the success of their mission did not turn on how much Enigma material they could cram into their return baggage. In any event, that luggage was already full. It is often difficult to weigh the relative value of contributions in a collaborative relationship. This is especially the case in intelligence history. Still, one might plausibly argue that, given the current priorities of the SIS, the Sinkov mission departed Bletchley Park with more than they brought.

  The American analogue of the Purple machine allowed Britain access to Tokyo’s high-grade diplomatic traffic for the first time since the spring of 1939, and this valuable gift was most certainly welcome
at GC&CS. The (very) partially reconstructed JN-25B codebook from OP-20-G may also have been useful, though GC&CS’s outstation at Singapore, the Far East Combined Bureau, had been working this system and may well have progressed as far, if not further, than the US Navy cryptanalysts. The Sinkov mission’s other offerings were less impressive. Bletchley’s German section was already familiar with the DESAB code. Similarly, the low-grade Italian diplomatic systems were already read at Bletchley as were probably the Mexican systems, which in any event were hardly a priority for British signals intelligence. In contrast, the items that the American mission carried back to Washington contained much that was new to American codebreakers: insights into German, Italian and Russian systems, a new Mexican cipher, a complete Brazilian codebook and partially reconstructed Argentine and Chilean books. Evaluating the importance of these items, Sinkov concluded, ‘The material … will result in a saving of several years of labor on the part of a fairly large staff.’

  The impact was most dramatic in the Italian section. Before the Sinkov mission, the section was reading two Italian diplomatic systems, neither of which produced significant intelligence. At Bletchley, Sinkov acquired information (including reconstructed codebooks) concerning two high-grade diplomatic systems hitherto unknown to the Americans, as well as details concerning a new version of TR, one of the systems already under study at the SIS. This information so accelerated the work against Rome’s communications that in the Munitions Building six analysts were added to the four already committed to the Italian problem. By the summer of 1941 they were decrypting messages in IMPERO, the first high-grade Italian cipher read by the SIS, and the number of decrypted Italian messages deemed worthy of circulation began to increase.

  Some of the material contributed by GC&CS (e.g. Red Army codes) could not be immediately exploited by the Americans because the SIS lacked the necessary staff or could not intercept traffic. Other material further illuminated difficult problems even if it did not provide a key to solutions. Although GC&CS had not cracked high-grade German diplomatic ciphers, the British had determined the nature of these systems. They confirmed for the Americans that the Reich foreign ministry used four different systems and that all used the same codebook (DESAB), each differing only in the encipherment applied to that code. At the SIS, Solomon Kullback’s German section had reconstructed the codebook, but for lack of traffic was struggling with the enciphered version known as Spalierverfahren. The British also lacked sufficient traffic to make much progress against Spalierverfahren, but they provided information about the encipherment and actually donated a set of cipher tables recovered from the German consulate in Reykjavik during the British occupation of Iceland. From his new friends at Bletchley Park, Kullback also learned that Berlin’s high-grade diplomatic system enciphered DESAB with a one-time pad, while the next system in importance (dubbed Floradora by the British) enciphered with a long additive. GC&CS told the Americans that, for the moment, it had abandoned work on these formidable ciphers, but at least the Americans now knew what they were facing.

  As both London and Washington intended, the Sinkov mission launched a programme of cryptanalytic exchanges. For its part, the SIS wasted little time in exploiting the new relationship. Sinkov and Rosen had hardly unpacked their bags before the SIS was asking GC&CS for information regarding Vichy French diplomatic ciphers, especially those used on the circuits connecting Vichy and French territories in the western hemisphere. In return, the SIS began sending to Bletchley Park cryptanalytic observations regarding German, Italian and Japanese systems. These contributions, while earnest, were not always relevant to the problem at hand. An early consignment of German material included American observations relating to ciphers used by Germany in the First World War.

  In August 1941, Commander Alastair Denniston, the operational head of GC&CS, crossed the Atlantic to observe American cryptanalytic operations and discuss further collaboration. At the SIS (he also visited OP-20-G) Denniston was pleased to note that collaboration on German diplomatic ciphers was making progress towards solving a problem that had seemed intractable. He was less sanguine about the new Vichy French and Latin American sections that the SIS had opened with the assistance of material from GC&CS. Here, too, collaboration was promising, but Denniston hoped that the Americans would concentrate their resources on Japan and not be distracted by forays against secondary targets that the British could cover well on their own.

  By the end of 1941, collaboration against diplomatic targets was sufficiently established that the Japanese attack on Pearl Harbor on 7 December had little impact on SIS-GC&CS relations beyond accelerating the pace of co-operation. For example, by March 1942, when Lieutenant-Colonel John Tiltman, chief of the military section at Bletchley Park, arrived in Washington for a four-week visit to American signals intelligence facilities, GC&CS had added to its list of items passed to the SIS all the Vichy French codes and ciphers it was reading, as well as the basic codebook used by the Spanish foreign ministry and a description of its encipherment for high-grade traffic. Tiltman brought additional gifts. Instructed by Alastair Denniston to effect a ‘complete interchange of all technical knowledge available and in particular to hand over to [the Americans] all our technical documents’, Tiltman arrived with a ‘considerable quantity’ of material from the various sections at GC&CS, including three more Vichy codebooks, microfilms of Floradora material generated by GC&CS and ‘descriptions of the methods used for the solution of 3 or 4 different complex ciphers by our Research Section’. Tiltman also offered to produce upon request Brazilian and Portuguese codebooks.

  In May 1942 the Americans further exploited British cryptanalytic experience when the Signal Intelligence Service despatched another mission to the Government Code and Cypher School. The American team, Major Solomon Kullback (senior SIS civilians had been given military ranks for the duration of the war) and Captain Harold Brown, spent most of their time at Bletchley Park, but they also visited the centre for diplomatic cryptanalysis recently established by GC&CS in Berkeley Street, London. At Berkeley Street, Kullback and Brown visited the sections working German, Japanese, Italian, French, Spanish, Chinese, Swedish, Near Eastern and Latin American diplomatic systems. Upon the mission’s return to the United States in July, Kullback could assure his colleagues, ‘I found the British most helpful and co-operative … They were completely frank, open and aboveboard with me and kept no detail of their operation, procedures, techniques, or results from me.’

  By the end of 1942 the Signal Intelligence Service (now ensconced in new headquarters at Arlington Hall, a former girls’ school outside Washington) had significantly expanded the range of its diplomatic operations. Much of this expansion can, of course, be attributed to a dramatic increase in staff and resources after Pearl Harbor. The British connection, however, was a crucial factor behind the increasingly impressive performance of the SIS. Indeed, it would not be an exaggeration to say that in the period 1941–2, GC&CS fuelled American diplomatic cryptanalysis by generously sharing its experience and material. In many ways, the collaboration that emerged in that period was one-sided. With the exception of the Japanese problem (and that is an important exception), the American contribution to the common effort remained relatively modest throughout 1942, mainly because the American cryptanalytic record against targets other than Japan was relatively modest. In the early stages of the joint effort against important targets – Germany, Italy, France, the neutrals – GC&CS simply had more to contribute.

  It is perhaps indicative of the early history of co-operation that when Tiltman visited Washington in the spring of 1942 he brought a bag full of cryptanalytic gifts, but when he returned to Bletchley Park he arrived empty-handed. On the other hand, through the missions of Sinkov and Kullback, Denniston and Tiltman, GC&CS certainly obtained from its ally useful insights into particularly recalcitrant problems, such as the high-grade German diplomatic ciphers. On at least one occasion the British codebreakers frankly acknowledged that liaison with the Ameri
cans on the German systems, particularly Floradora, had been especially fruitful. Still, the British may be forgiven if they sometimes felt in this early period that they were contributing to the relationship more than they were receiving. Often the Americans were so unrevealing about their own work that GC&CS was not sure which foreign ciphers the SIS was reading. In the spring of 1942, for example, GC&CS complained that it required such information in order to determine what French, Italian, Spanish and Latin American material should be passed to the SIS. As late as December 1942 Alastair Denniston felt compelled to ask his liaison officer in Washington: ‘Strictly between ourselves, are the Americans making a massive library of foreign government systems for filing purposes, or do they actually work on the stuff which we send them? We hear so little about Spanish and even French results that sometimes we wonder if they are actually deeply interested.’