In early 1942 some half-dozen naval officers knowing German were summoned to Bletchley's Naval Section. Current gossip at the Park had it that there were two explanations for this. One was that the Section's head, observing that Hut 3 boasted several individuals in RAF and Army uniform, wished to have some navy-clad folk about the place to impress visiting VIPs. The other was that officers with first-hand experience of the naval war might be able to add a touch of verismo to the interpretation and analysis of the German naval Enigma decrypts then being done in Hut 4, mainly by men and women in civilian garb. We naval folk were soon to learn the supererogatory character of this second explanation. There was precious little that Hut 4's civilians did not know about how navies worked. Most of what I ever knew about the German navy came from those who had been working on the naval Enigma since it was first broken a year earlier.
I vividly remember the sense of shock produced on my first arrival at the Park by the grimness of its barbed-wire defences, by the cold and dinginess of its hutted accommodation, and by the clerk-work we were first set to do. But this was soon swept aside by the much greater shock of discovering the miracles that were being wrought at the Park. In Iceland I had been interrogating the survivors of the many merchant ships sunk in the, at first, highly successful offensive against the Atlantic convoys launched by the U-boats in March 1941. I had spent many hours trying to analyse their strength and tactics. I could have spared my pains. For I now discovered that all this, and everything else about the U-boats, was known with precision by those privy to the Enigma decrypts. Leafing through the files of past messages—for the dreaded Shark key for U-boats, that had been introduced a week before my arrival, was to defeat Hut 8's cryptographers for another ten months—I shivered at seeing the actual words of the signals passing between Admiral Dönitz and the boats under his command whose terrible work I had seen at first hand. . . .
I can place the date of my arrival in Hut 4 pretty exactly. It was shortly before 11 February 1942, the day of the famous "Channel Dash" of the Scharnhorst, Gneisenau, and Prinz Eugen. There was great commotion in the hut and cries of "Where's Harry? Harry will be furious!" Indeed he was. Harry Hinsley, whom we naval newcomers had already pin-pointed as perhaps the most knowledgeable of all those in Hut 4, flew in late in the afternoon scattering smiles, scarves, and stimulus in every direction, exclaiming, "It's happened again: whenever I take a day off something big blows up." There had, indeed, been good indications that the ships were about to move; but the Enigma settings of 10, 11, and 12 February, by a stroke of the ill luck which precipitated tragedy from time to time, were not solved until three days later.
Hinsley was a key figure in Hut 4. His uncanny ability to sense, from tiny clues in the decrypts or the externals of the radio traffic, that something unusual was afoot was already legendary in the Park. He was well versed in the ways of navies, having more than once visited the Home Fleet in Scapa Flow to explain the workings of the Enigma to the Commander-in-Chief. He was a popular figure there and was known, as I later discovered when I joined the Home Fleet, as "the Cardinal." He was the chief channel for the exchange of ideas between the Naval Section and the Admiralty's Operational Intelligence Centre (OIC). This capable—but not infallible—organization had already in 1940 rejected a suggestion by Hinsley which, if adopted, might have saved the aircraft carrier Glorious. On the other hand, during the "Channel Dash" it was smart work done in the OIC which contrived to use the decrypts to have the Scharnhorst and the Gneisenau mined off the Dutch coast with resounding strategic consequences. . . .
This episode coincided with the onset of Hut 8's ten-month-long inability to break into the newly introduced Shark. Severe losses of merchant ships were to follow, largely in consequence of this. But already by the end of 1941 a turning-point had been reached in the battle of the Atlantic. The evasive routing of convoys made possible by Hut 8's breaking of the naval Enigma in the spring of 1941 had, according to some historians' calculations, spared some three hundred merchant ships and so provided a cushion against the heavy losses yet to come. It also defeated Dönitz's offensive, which was intended to knock out Britain while the German armies disposed of Russia, so avoiding a two-front war. The six-months' long decline in sinkings also provided a crucial breathing-space during which the Allies could develop anti-submarine weapons and tactics, and get on with building more merchant ships. This victory, which saved Britain, was based entirely on the work of Bletchley Park. It was also responsible, though I did not know it at the time, for the great reduction in the number of survivors who passed through my hands in Iceland, reducing my work-load and making it possible for me to be released to Bletchley. . . .
We naval newcomers were at once impressed by the easy relations and lack of friction between those in, and out of, uniform. Despite the high tension of much of the work, a spirit of relaxation prevailed. Anyone of whatever rank or degree could approach anyone else, however venerable, with any idea or suggestion, however crazy. This was partly because those in uniform had mostly been selected from the same walks of life as the civilians—scholarship, journalism, publishing, linguistics, and so forth—and partly because these were the people who saw most clearly what stood to be lost by a Hitler victory. All at the Park were determined to give their all to see this did not happen. Service officers gladly served under civilians, and vice versa. Dons from Oxford and Cambridge worked smoothly together. . . .
. . . Hut 3 [was] responsible for translating and elucidating the German Air Force and Army decrypts from Hut 6, for supporting its cryptanalysts, and for signalling the gist of the decrypts to operational Commands in the Mediterranean and, later, in other theatres. . . . New, fascinating, and exciting work was found for us. [German General Erwin] Rommel, fighting in Africa, depended on shipments of fuel, ammunition, and other supplies sent across the Mediterranean in convoys controlled by the Italians. His fortunes waxed or waned with the adequacy, or inadequacy, of his supplies. His most notable victories came when his logistic position was good; and his defeats when he was weakened through want of supplies. They were adequate when the fortunes of war permitted his convoys to arrive safely and in sufficient number: but he faltered when the RAF and Navy contrived to prevent this. His final defeat owed much to the sustained sinking of his supply ships. Bletchley played a big part in bringing this about.
Hut 6's decrypts told us a great deal about these convoy movements. One source was the Air Force Enigma which throughout 1941 had been revealing, albeit somewhat spasmodically, the instructions for their air escort. This intelligence had, for example, resulted in considerable disruption of the transport to Africa of the first of Rommel's armoured formations. The second and more important source was the Italian administration machine cipher, C38m, broken by Hut 8 in the summer of 1941. This yielded, amongst other things, advance warning of the sailing dates, routes, and composition of virtually all trans-Mediterranean supply convoys. It also threw occasional light on Italian main-fleet movements. During 1941 the gist of the relevant Enigma decrypts had been signalled by Hut 3 to the Mediterranean authorities by the SCU/SLU channel, while that of the C38m decrypts had been sent by a part-naval, part-civilian processing watch in Hut 4 separately to Malta and elsewhere. An outstanding result of these messages had been a spate of sinkings in late 1941 which played a big part in Rommel's retreat to El Agheila at that time.
It was probably a coincidence that, at the time of the Hut 3 reorganization of early 1942, a twenty-four-hour watch of naval officers—one of them a regular—was set up and became an integral part of that hut. Called 3N, one of its jobs was to provide advice, hitherto lacking, to the watch on naval problems arising from the Army-Air Force decrypts. Its other and more important task was to co-ordinate the shipping intelligence from these decrypts with what came out of the C38m. To bring this about 3N and Hut 4's Italian watch became virtually a single team, the former being responsible for the final shape of the outgoing signals—and for taking on the chin any riposte from bewildered recipients at the other end (which was seldom). This development came at a bad time for the British in the Mediterranean. The Axis had greatly strengthened its convoy defences and Malta was all but immobilized by the attacks of the newly arrived Luftflotte 2. Axis convoys were getting through wholesale, and opportunities for attacking them were much reduced. Rommel's recovery in January 1942 was made possible by these developments. Every scrap of intelligence became doubly valuable. The co-ordination of the two sources came at the right moment. . . .
The Air Force Enigma, unlike the C38m, would sometimes indicate the importance—to the German Air Force, of course—of a given convoy, occasionally specifying that it carried urgent supplies of fuel or ammunition. Though these indications were made only in general terms, they were invaluable to the attack planners. From about August 1942 Hut 6 regularly broke the Chaffinch key of the German Army in Africa and this provided, as well as much else of the highest importance, precise details of cargoes. Ships carrying operationally urgent supplies could now be distinguished from those with routine shipments. This made selective attack possible and greatly increased the effectiveness of the anti-ship campaign. An example of its effectiveness may be found in the Allies' ability, which came as a surprise to some historians, to feed the 250,000 prisoners trapped in Cape Bon during the final phase of the war in Africa. Ships known from the decrypts to be carrying rations had been spared; while those with cargoes of tanks, fuel, and ammunition had been selected for attack.
The arrival of Chaffinch, supplementing the former sources, coincided with that of Rommel on the Egyptian frontier. It now became doubly urgent to deprive him of supplies. It also coincided, to our great good fortune, with the recovery of Malta as a base for anti-shipping operations, and with the breaking by Hut 8 of the naval Enigma key used by the Germans in the Mediterranean. Its yield made for much more efficient attacks on coastal supply shipping, on which Rommel, far from his supply bases and convoy terminals, now largely depended. All this sharply increased the pace of our work. Rommel clamoured for fuel, and attacks on tanker convoys were now given highest priority. Many were successful. Those during August 1942 were largely responsible for his failure at Alam Haifa. After the war I found my initials at the bottom of the signals giving details of the three supremely important tanker movements at the time of the October battle of El Alamein. Their sinking was largely responsible for Rommel's long and halting retreat westwards. I well remember the frustration that exploded from our Hut 3 colleagues at Montgomery's failure to overtake and destroy him. I had not seen such a demonstration since the Knightsbridge fighting six months earlier, when the Eighth Army advanced against an anti-tank trap at the so-called "Cauldron" position and lost heavily. Hut 3 believed that it had provided full details of Rommel's intentions on this occasion.
The Hut 4/3N routine continued until the final victory at Cape Bon. It seemed scarcely credible that the Axis could have sufficient ships left unsunk to succour the fighting in Tunisia. But they did. And the pressure continued right up to the end.
Edward Thomas, "A Naval Officer in Hut 3." Reprinted in F. H. Hinsley and A. Stripp, eds. Codebreakers: The Inside Story of Bletchley Park (Oxford, UK: Oxford University Press, 1993), pp. 42–49. Reprinted by permission of Oxford University Press. .