GCHQ. Richard Aldrich

John Croft, who worked at the GC&CS diplomatic code-breaking centre at Berkeley Street in London, was one of those who soldiered on with Soviet material. Croft was engaged on wartime Comintern traffic in Europe, known as ‘Iscot’, which could be read. Although circulated only to a very select group of individuals within Whitehall, this material mostly revealed a dutiful Soviet struggle against their shared enemy, Nazi Germany. There is no indication that this material was exchanged with Washington.³⁰

      Early British cooperation with the American code-breakers was also tentative. Again, the obstacle was obsessive security. Security problems existed on several different levels. The British and the Americans had cooperated on sigint during the First World War, but this had bequeathed a legacy of doubt and anxiety, even distrust. In November 1940, when reviving sigint cooperation with the Americans was discussed, Alastair Denniston was quick to point out that after the First World War the ‘notorious’ American code-breaker Herbert O. Yardley had published a tell-all book about his experiences. The very name ‘Yardley’ caused a shudder in British code-breaking circles. Yardley was now working for the Canadians, and GC&CS insisted that they sack him summarily before they were allowed to join the wartime sigint club.³¹ Indeed, the Canadians were told that other agencies ‘would not touch Yardley with a ten foot pole’.³²

      The British, and especially Sir Stewart Menzies, the Chief of SIS, were frosty towards the Americans, and regarded them as fundamentally insecure. By contrast, the Americans generously opted to share the secret of their spectacular code-breaking success against the Japanese ‘Magic’ diplomatic cypher with the British as early as January 1941, even handing over precious examples of their copies of the Japanese cypher machine. The British were ‘flabbergasted’. They did not expect the Americans to ‘simply walk in and plonk down their most secret cryptanalytical machine’.³³ Yet the British remained reticent, and did not initially reciprocate fully with their knowledge of Enigma. The jibe about American insecurity had a certain irony, since the British chose to send one of the priceless American copies of the ‘Purple’ machine out to their naval base at Singapore shortly before it fell to the Japanese. The machine was delivered by ship just as the Japanese invasion of Malaya began, and disappeared into the chaos of battle. To this day its fate is unknown.³⁴

      Collaboration with Washington was also hard because American sigint was a house divided. Although William Friedman, the US Army’s best cryptologist, was busy advocating sigint cooperation with the British in early 1940, the US Navy’s chief code-breaker, Commander Laurance Safford, was adamantly set against working with allies. But after pressure from President Franklin D. Roosevelt the Navy had been won round, and the Americans sent a team of technical experts to Britain in early 1941. Known as the ‘Sinkov Mission’, they spent several weeks touring Bletchley Park and visiting outlying intercept stations. The British were willing to receive them because they knew the main focus of American attention was Japan. At this stage the British were keen to keep discussions focused on Japan, because this allowed them to hide the extent of their knowledge of the German Enigma system. Both Sir Stewart Menzies and Sir Alexander Cadogan were adamant that the Ultra secret would not be shared with the Americans.³⁵ Laurance Safford later represented the first Anglo-American exchanges of late 1940 and early 1941 as a one-way street in which the Americans handed over their precious ‘Magic’ material on Japan but got nothing in return. In fact this is far from the case. Prescott Currier, one of the Americans who came to Bletchley in early 1941, recalled: ‘All of us were permitted to come and go freely and to visit and talk with anyone in any area that interested us.’³⁶ Later that year, a select circle of American code-breakers were also given more details about Enigma.³⁷

      The hottest issue was the distribution of sigint to the policy-makers. In late May 1941, Brigadier Raymond Lee, the American Military Attaché in London, conveyed an American request for comprehensive intelligence exchange in the Far East. There followed painfully slow and complex discussions about who would get sigint with what levels of security: ‘The whole thing has been so tangled up,’ he complained.³⁸ Sigint was also very confused in Washington. Unlike Britain’s GC&CS, American signals intelligence was less centrally organised, resulting in great rivalry between the armed services.³⁹ Because the American wartime sigint organisation was divided between the Army and the Navy, one of the great problems for the British was cooperating with one without upsetting the other. Famously, the Americans solved the tussle over who would decrypt Japanese codes by agreeing that the Army would decode the material on the even days of the month, and the Navy on the odd days. A more ludicrous system for the division of labour would have been hard to devise.⁴⁰

      GC&CS might have been more centralised than the Americans, but it had less money. Expanded cooperation with America on Japan allowed GC&CS to shed some difficult code-breaking tasks. High-grade Japanese Army cyphers had proved impenetrable for a decade. By 1941, Bletchley Park was too busy with the European war, while its Far Eastern code-breakers were struggling to cope with the mass of material on Japanese espionage derived from low-level consular intercepts in South-East Asia. On 22 August 1941, Anglo-American cooperation lifted this task from their shoulders. During talks in Washington, Alastair Denniston persuaded the US Army that it should ‘take over investigation of Japanese main army cipher soon as priority commitment’. Shortly after, Captain Geoffrey Stevens from Singapore travelled to Washington carrying all the British material on the Japanese main army cypher.⁴¹ The British were glad to see the back of it. At the end of the war approximately 2,500 Americans would still be working on this one Japanese cypher to no avail.

      All the while, Britain was also decyphering some American traffic. Amongst the decrypts selected for the personal perusal of Winston Churchill were those of many Allied and neutral countries. GC&CS was clearly working successfully on the American diplomatic code ‘Grey’ until December 1941.⁴² Remarkably, there was no embarrassment about this. In June 1941, while discussing comprehensive sharing of Far East intelligence, the British asked the US Military Attaché, General Raymond Lee, for his opinion on the security of American cyphers. This was the conduit through which sigint would pass between London and Washington. Lee replied tartly that the GC&CS already knew a great deal about this matter. He recorded in his diary:

       The talk then turned again on the question of security. They wanted to know whether my despatches went by radio or cable and were relieved to hear that they went by cable, and were further relieved to hear that we have a direct wire straight into the War Department. However, I pointed out that this wire was subject to interception by their people here in England [GC&CS] and I had no doubt they had taken our messages and attempted to decipher them.

      He added that it was now very much in the interests of GC&CS to be honest about the security of American cypher systems, ‘because the stuff that is going over it is more vital to them than to us’. Lee’s frank exchange with the British underlines one of the hidden benefits of cooperation between the Allied code-breakers. Once they began to share their most precious assets, ‘Magic’ and then eventually Ultra, improved communications security became paramount. London and Washington now had a vested interest in the impenetrability of each other’s messages. After all, if GC&CS could break American codes, then so, perhaps, could the Germans.⁴³

      Churchill eventually wrote to Roosevelt and owned up to British work on American diplomatic codes. ‘From the moment we became allies,’ he explained, ‘I gave instructions that this work should cease. However, danger of our enemies having achieved a measure of success cannot, I am advised, be dismissed.’