The Bletchley Park Codebreakers (24 page)

In their warren of cramped offices in the Munitions Building on the Washington Mall, the army codebreakers cast about for a way to regain the cryptanalytic initiative. Increased effort against Purple might result in a breakthrough, but the personnel in the Japanese section were already logging twelve- and fourteen-hour days. Additional personnel might advance operations, but there was no evidence that the notoriously parsimonious War Department was prepared to be generous. In a gesture that revealed the extent of its desperation, the SIS set aside inter-service rivalry and exchanged observations about Purple with its sister service in the United States Navy, OP-20-G. The Navy's help was welcome, but breakthroughs still eluded the Americans. Then, on 5 September 1940, a message arrived from London that promised an escape from the cryptanalytic wilderness.

After the fall of France, the Roosevelt administration had cast about for a way to support Britain within the limits set by an American public opposed to armed intervention and an American military establishment ill-equipped to support the security of its own country let alone the security of another. In July 1940 President Roosevelt, in consultation with Secretary of War Henry Stimson and Secretary of the Navy Frank Knox, accepted a British proposal that military representatives of the two governments convene a joint staff conference in London. Within a month, an ‘American Military Observer Mission' arrived in the British capital ostensibly to study ‘standardization of arms', but really to discuss co-operation between the armed services of the two countries.

On 31 August 1940, the army representative on the American observer mission, General George Strong, startled his hosts (as well as his American colleagues) by announcing that the US Army was working on Axis codes and ciphers and proposing that London and
Washington exchange information on their cryptanalytic operations. The British were particularly surprised by Strong's proposal, since the Royal Navy had been rather curtly rebuffed by the US Navy in October 1939 when it suggested sharing information on Japanese naval communications. Having had one door slammed in their face, the British were now pleasantly surprised to have another door opened even before they had knocked. Within days they accepted General Strong's invitation. On 5 September Strong sent the War Department the telegram that would electrify the Signal Intelligence Service: ‘Are you prepared to exchange full information on all German, Italian, and Japanese code[s] and cryptographic information therewith? Are you prepared to agree to a continuous exchange of important intercept in connection with the above? Please expedite reply.'

The Signal Intelligence Service was more than prepared to pursue any opportunity to advance its lagging cryptanalytic programme. Indeed, even before Strong's message reached Washington, William Friedman and Colonel Spencer Akin, the military commander of the service, may have already concluded that co-operation with the British promised a short cut to success. About the time that Strong was proposing cryptanalytic co-operation to his astonished British and American colleagues in London, Friedman and Akin were preparing a memorandum, ‘Proposed Exchange Basis with the British', that recommended the exchange of ‘any and all material that we have on a basis of complete reciprocity', and explicitly stated that the SIS was interested in information concerning specific foreign codes and ciphers.

It is easy to understand the SIS's enthusiasm for establishing links to GC&CS. When Akin and Friedman composed their memorandum, Japanese traffic was still America's only productive source of diplomatic signals intelligence. Unfortunately, even this source had declined significantly in value after Tokyo's introduction of the yet unsolved Purple cipher machine. The low-grade German and Italian traffic accessible to American codebreakers produced negligible intelligence and would continue to do so as long as Berlin's and Rome's high-grade ciphers remained impenetrable. The potentially useful communications of powers such as China, Russia and Vichy France had not even been studied let alone exploited. A connection to the British might provide short cuts to success against all these targets. Given the secrecy that surrounded GC&CS, American codebreakers knew nothing about operations at Bletchley Park, the wartime home
of the British codebreakers, but through Friedman they were vaguely aware of Britain's cryptanalytic achievements during the First World War and assumed that GC&CS was working hard to repeat those successes in the present war. Perhaps the British were already reading high-grade German and Italian ciphers. An exchange of ‘any and all material' might well produce just the information necessary for an American entry into Nazi and Fascist communications. With luck an exchange might even help the effort against Purple.

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.

* 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.'