Dolphin was broken throughout the war but the cipher was not used by U-boats after Oct 5, 1941.

The British politician the 2nd Viscount Hailsham once said that ‘The one case in which I think I can see the finger of God in contemporary history is Churchill’s arrival at the premiership at that precise moment in 1940.’2 Another candidate for the intervention of the Almighty in the Second World War might be the Allied cracking of the German Enigma codes, producing a stream of decrypts known by their British special security classification, Ultra. This allowed the Allies for much of the war to read many of the communications sent and received by the OKW, OKH, Wehrmacht, Luftwaffe, Kriegsmarine, Abwehr, SS and Reichsbahn (railways), amounting in total to several million items of intelligence.3 From the correspondence of the Führer himself right down to that of the harbour-master of Olbia in Sardinia, messages were routinely decoded by the Allies. It made the Second World War, as Michael Howard has put it, ‘like playing poker with marked cards, albeit against an opponent with a consistently better hand than you’. Its importance can be gauged from the jokey acronym ‘BBR’ that the Americans gave to Ultra, which stood for Burn Before Reading.

For those who would prefer other explanations to divine intervention, the story of the cracking of the Enigma machine is also full of secular miracles. The design was patented by a Dutchman, H. A. Koch, in 1919, and by 1929 had been bought by the German Army and Navy (which used different versions of it). Looking like a normal typewriter but with three, four or five twenty-six-spoke rotor wheels attached, as well as lights and plugs that resembled a telephonist’s board, the machine had the ability to transform a typed message into a code so complicated that the Germans assumed it could never be broken. ‘To give an idea of how secure these machines are,’ General Franco’s intelligence officer Commander Antonio Sarmiento wrote in a 1936 report, when the Nationalists were buying ten Enigma machines from the Germans at the start of the Spanish Civil War, ‘suffice to say that the number of combinations is a remarkable 1,252,962,387,456.’

The technical side of the Enigma story is ferociously complex, and involves specialist terms such as the Banburismus procedure, Caesar reflector, Dolphin, Porpoise, Shark and Triton nets (that is, sub-codes), the Eins catalogue, Cillis, the Herivel Tip, codes-within-the-code, Gamma wheels, perforated sheets and plugboard connections, rodding, Bigram tables, bombes, cross-ruffing, straight-cribs and a related code entitled Geheimschreiber (secret writer). The cracking of Enigma and its related codes – such as the Japanese diplomatic cipher Purple which was transformed into decrypts codenamed Magic – was a genuine Allied operation, involving the secret services of Poland, France, Britain, Australia and the United States. It began as early as 8 November 1931, when a traitor working in the German Cipher Office called Hans Thilo Schmidt allowed the French Deuxième Bureau (secret service) to photograph the Enigma operating manuals, which he had momentarily spirited out of a safe in the War Ministry. The French told the British, who subsequently told the Poles about the machine, but none could crack the code without building a replica of the machine itself. This was achieved by the Polish cryptographer Marian Rejewski in December 1932, although the Poles did not initially inform the French and British that this had happened. From that point on, the Poles could read Wehrmacht and Kriegsmarine radio traffic, although when in 1937 the latter changed its Enigma indicating key (the setting on a vital cog) the naval side fell silent, and was to remain so for the next three crucial years. Changes in the machines instituted by the Germans in December 1938 (installing an extra two rotor wheels, bringing the total to five) and January 1939 (doubling the number of plugboard sockets) also plunged the Poles into darkness. In late July 1939 they finally told the French and British secret services that they had been reading the German traffic until late 1938.

Ultra was not the sole means by which the Allies gathered intelligence, of course. Prisoners were captured and interrogated; simpler signals intelligence (sigint) codes used by front-line communications were eavesdropped upon and decoded by a British organization known as the Y Department; aerial photo-reconnaissance was interpreted at Medmenham on the Thames; resistance groups in Occupied Europe passed on information; SIS produced human intelligence (humint) from its own sources, although many were compromised early on in the war during the disastrous incident when in November 1939 two SIS officers, Captains Payne and Best, were kidnapped at Venlo on the Dutch–German border by Gestapo agents posing as Resistance figures; German generals in British captivity were eavesdropped upon when they discussed important subjects such as rocketry. Nonetheless, Ultra was by far the most important intelligence source and, because of its direct nature, the least corruptible in analysis. The Bletchley code-breakers were, as Churchill put it, ‘the geese who laid the golden eggs’ and who, just as importantly, ‘never cackled’. They were also almost all amateurs, recruited from civilian life, although their contribution was far to outweigh that of the career intelligence officers of the day.

After the invasion of Poland in September 1939, several senior Polish cryptographers escaped with their replica Enigma machine and were installed by the Deuxième Bureau in a château near Paris, where they began – with British and French help – to decode messages, although at the time it took them two months to do so, meaning that the information they divulged had usually been long superseded by events. On 12 February 1940, however, the German submarine U-33 was attacked off the west coast of Scotland and two of the extra rotor wheels used by the naval Enigma were captured. Five weeks later, a brilliant, eccentric, accident-prone, homosexual Cambridge mathematics don called Alan Turing installed something known as a bombe machine, an electro-mechanical device which made hundreds of computations every minute, at the Government Code and Cypher School at Bletchley Park in Buckinghamshire, 40 miles north-west of London. Other heroes of Bletchley were to include the mathematicians Stewart Milner-Barry and Alfred Dilwyn (‘Dilly’) Knox. In modern computing parlance, while the Poles provided the Enigma hardware, the assorted civilian geniuses stationed at Bletchley provided the software that produced Ultra.

Far from being a school, Bletchley was a department of SIS, operating from a Victorian mansion that housed 150 workers in 1939, before expanding into huts in the grounds to fit 3,500 people by 1942 and no fewer than 10,000 by the end of the war. (Several of the huts can still be seen today, including the ones where the most important work was done, along with captured Enigma machines and the bombe predecessors of the computer.) Huts 6 and 3 deciphered, translated, annotated and passed on Wehrmacht and Luftwaffe signals, while Huts 4 and 8 (run by Turing and subsequently the chess champion Hugh Alexander) did much the same thing for the Kriegsmarine, sending reports to the Naval Intelligence Division of the Admiralty. Hut 4 also analysed sudden increases and decreases of signals traffic volume, which could suggest possible enemy intentions. On 4 April 1940, five weeks before Hitler unleashed Blitzkrieg on the West, same-day decoding of the German Army codes first became possible, but on 1 May the British at Bletchley and Poles in France were ‘blinded’ for three weeks when the Germans altered their indicating systems. Overall, however, Wehrmacht and Luftwaffe signals were decoded between three and six hours after they were sent, and naval signals during the battle of the Atlantic could be read as swiftly as one hour after transmission.

Before May 1940 the cracking of the codes depended upon chance factors such as the transmission of flaws and errors, as with one German unit reporting every morning the same phrase, Verlauf ruhig (situation unchanged), thus giving the Cambridge mathematics don in Hut 6, Gordon Welchman, who had improved Turing’s bombe machine in 1940, a vital clue about several letters. The major expansion of the Luftwaffe before the war meant that its signallers were generally less well trained and disciplined, and more sloppy, than their Army and Navy counterparts. The fact that there were only twenty-six letters in the alphabet, the key flaw in the machine that no letter could represent itself in the code, and the absence of number keys meaning that every number had to be spelt out, also encouraging replication, were the major aids to decryption. The vast number of permutations – just under 1,253 trillion – that the Enigma code depended upon could therefore be narrowed down considerably by Turing’s and Welchman’s bombes.

It was not until the beginning of April 1941 that the German naval Enigma codes were broken – except for a very brief period in April 1940 – although there had been no shortage of plans to try to obtain German codebooks to help the process along, with the scheme of the intelligence officer and future Bond author Ian Fleming to crash a captured aircraft into the English Channel, and then ambush the rescue boat, being the most hare-brained. It turned out to be the capture off Norway of the German trawler Krebs that yielded up the vital settings list that Bletchley needed to operate Turing’s Banburismus procedure for decryption. Although all German skippers were under strict instructions to destroy or throw overboard all codebooks at all costs, with the capture by HMS Bulldog and HMS Broadway of soaking codebooks from Julius Lemp’s U-110 on 9 May 1941 by Sub-Lieutenant David Balme – they were dried over a stove on a British destroyer by Lieutenant Allon Bacon from the Naval Intelligence Division – Bletchley was able to discover future settings, the so-called Offizier procedure. This meant it could pick up announcements of future settings changes. By the autumn of 1941, evasive routing of convoys due to Ultra meant that the U-boats were sinking far fewer merchantmen; and as one historian has put it, ‘Bletchley Park had gone from being stymied by maddening cryptanalytic obstacles to being overwhelmed by its own success.’ Yet it was not to last long.

Although the Abwehr set up regular investigations into the security of Enigma, and the commander of the U-boat branch of the German Navy, Karl Dönitz, had himself questioned whether it could have been broken, the Germans only continued to refine the existing machine settings rather than institute a brand-new communications system. Geheimschreiber, for example, was a non-Morse cipher that had up to ten rotary wheels, against the Enigma’s maximum of five. Its product was codenamed Fish at Bletchley and was far harder to crack, but it was not universally employed. Had a suspicious Reich turned to it instead of relying upon Enigma, the story of the Second World War might have been very different. Sir Harry Hinsley, the historian of British wartime intelligence, calculated that without Ultra the Normandy landings could not have been launched until 1946 at the earliest.

Although the Allies could not be seen to rely on it too much, for fear that the Germans would realize it had been compromised, information gleaned from Ultra was used to great advantage at many key moments of the war – for example, it brought about the battle off Cape Matapan, enabled the sinkings of the Bismarck and Scharnhorst, disclosed Rommel’s weaknesses and shortages prior to El Alamein, simplified Montgomery’s advance into Tunisia in March 1943, made the planning for the invasions of Sicily and southern France much easier, exposed the whereabouts of German divisions before D-Day and revealed Hitler’s orders for a counter-attack at Falaise in August 1944. (The day before the Mediterranean battle of Cape Matapan, Admiral Cunningham strode ashore at Alexandria carrying his golf clubs, so as to lull the suspicions of the Japanese consul-general there. The next day, 28 March 1941, he sank three Italian destroyers and two cruisers whose whereabouts and intentions he knew from the Ultra decrypts he had received.) Yet it was undoubtedly in the battle of the Atlantic that Ultra was put to greatest use. Hut 8 of Bletchley Park succeeded in decoding about 1.12 million of the 1.55 million Kriegsmarine signals that were intercepted during the Second World War.

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