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Seventy Years Ago This Month at Bletchley Park August 1940The Battle for Britain. Britain awaits the invasion as the battle rages in the sky above. Gradually as the days pass with every day bringing news (however exaggerated) of fresh triumphs in the air, British morale rises. Perhaps they won’t dare to come? The Intelligence Services struggle to determine which side is winning, whether they can build the fighters faster than we can, whether the supply of pilots will last out. Churchill orders an air raid on Berlin, and Hitler reacts by making the crucial mistake of redirecting his air armada to attack London, thus relieving pressure on the airfields and radar sites. Our friends begin to rally round as the US signs the “Lease-Lend” treaty and Commonwealth troops arrive in strength. The Battle of Britain. The air assault on Britain had been mounting for some weeks, in the first place largely aimed at coastal shipping, harbour installations, and aircraft factories. Then from 10th August the assault on the radar stations and airfields of the RAF starts to build up. 13th August, “Eagle Day”, marks the start of the all-out campaign in German eyes. That day Germany loses 45 pilots, killed or captured, but Britain only loses 7 pilots. So despite the hundred British aircraft destroyed on their airfields in the first ten days, in terms of the losses of the scarce experienced pilots the battle is going in favour of the British, however exaggerated the announcements about the air-kill figures were eventually found to be. (British claims of 1,112 Luftwaffe aircraft positively destroyed between 9th August and 2nd October were actually found to amount to 635 destroyed). By 14th August the Inter-Service Intelligence Committee has pronounced that the Germans will delay a decision on the invasion until this struggle for air superiority has been resolved. So it is not only Air Chief Marshall Sir Hugh Dowding who recognises that he must try to preserve his fighting force at all costs, even if that means avoiding all-out battles that would drain our meagre resources of airmen. Even so on 16th August at one time every single squadron of the crucial group defending the south of England is in the air with nothing in reserve. That day we lose 60 aircraft on the ground, in addition to the losses in the air fighting. But the German losses are running at more than they can sustain, and so on 17th August they are forced to cut back the level of their attack, and withdraw the vulnerable Stukas from the battle. One further major assault follows on 18th August but again the German losses of 71 aircraft to only 27 British losses in the air are unsustainable. There is no German attack on Britain on 19th August; the writing is in the sky! Though there is to be a last major daylight assault at the end of the month, on the night of 24-25th August the Luftwaffe goes over to widespread night bombing. On the 23rd August one flight of 12 German bombers had dropped their loads on London. So, on the evening of 25th August, British bombers attack armament factories in Berlin. On the 26th August in daylight Germany attacks British airfields once again, but this time all but one of the German formations are intercepted. The Germans now change their plans and divert a part of their remaining daylight assaults from airfields to the London docks. With this false step the battle is won for Britain, even if it does not feel like it to the Londoners. It would be nice to think that the bombing of Berlin demanded by Churchill had triggered this fatal German error of switching the attack to attempt to break British morale by bombing the cities. The Value of Intelligence Information during the Battle of Britain. While there can be no doubt that the accumulation of information about the German air force from the flow of “Red” key decrypts is of very considerable background importance, the work of BP could not be of much direct help in the air battle. BP had completed its first attempt to produce a detailed Luftwaffe order-of-battle on 5th August, and had brought about a scaling down of the official, vastly exaggerated, estimates of the likely number of bombers available to the Germans. But the crucial question was “Could the RAF outlast the Luftwaffe?” and here the Enigma decrypts could be of little help. The British air intelligence teams were suspicious of the official kill figures, but continued to over-estimate the Luftwaffe strength, and did not realise that the Germans were suffering from the low serviceability of their aircraft. Because the Germans were largely using landlines for orders on strategic policy, during the battle advanced warnings of policy changes were not obtained. But in the day-to-day fighting, the Enigma decrypts provided an ever-increasing amount of intelligence information, though short notice changes in the Luftwaffe plans could invalidate the Enigma-based forecasts. When BP began to produce tactical information from high grade Enigma decrypts that information went only to the Air Intelligence section, not to the operational commands. Lower-grade information from the Y service, especially plain language radio traffic, was of some considerable value but at first tended to report the situation as it emerged rather than providing a forecast. Information from the Home Defence Units round the coast, using German speaking WAAF and WRNS staff on high frequency radio intercepts from the pilots and their ground controllers, was sent direct to the local RAF commands as well as to HQ Fighter Command at Stanmore, where it could be correlated with the invaluable radar information and that from the Observer Corps. The simple code for these transmissions had been broken by BP and was read by the RAF Y station control centre at Cheadle. With growing regularity and accuracy as the battle proceeded, the organisations exploiting the Luftwaffe signals traffic are able to give advanced information about the purpose, type and scale of the enemy’s attacks. The War at Sea. By now German submarines are well established in their new bases on the French Atlantic coast. This saved them the long and perilous journey from the Baltic to the happy hunting grounds of the North Atlantic. At first they were delayed in mounting a renewed assault because a problem with the firing mechanism of their torpedoes had to be rectified. But the next few months marked the period when the submarines were free to operate independently against the stragglers, the loners, and the fast ships that did not take part in the convoy system across the North Atlantic. The British Admiralty had withdrawn many of its ships from the Atlantic to deal with the invasion threat, and they were slow to redeploy them when the threat diminished at the end of September. The submariners looked back on this period as the “Happy Time” when kills were readily available and orders from Admiral Dönitz had not yet grown into the close control that arose as the “Wolf-Pack” attacks on the convoys were developed. But in August Britain at last changed their book-code system that had been compromised at the time of the Abyssinian crisis in 1936. The new system depended upon the use of numeric groups from a book code modified by a subtractor drawn from a long table; long but not long enough for the heavy traffic that developed soon enabled the Germans to once again start reading some of our naval cyphers. It was not until a stencil subtractor system devised by BP’s Chief Army Cryptographer, John Tiltman, was introduced in July 1943 that our codes used in the North Atlantic at last became secure. Listening In on the German Naval Traffic. Experience by the RAF Y service during the fighting in Europe had shown that the enemy was using VHF transmissions for their air to ground and air-to-air communications, and that these could often be read by Y station staff if they were fluent in the German language. So the Royal Navy now set up a Y station on the North Foreland especially to tackle the equivalent German naval traffic, in particular those VHF transmissions used by the German E-boats. At first the Navy had to rely on the Army for the linguists needed but soon specially trained WRNS took over. The Navy was now building up its Direction Finding stations which became a major weapon in the Battle of the Atlantic, crude as the bearings obtained were in the early days. Naval Enigma. The inability of Hut 8 to emulate their colleagues in Hut 6 who were busy reading the Luftwaffe Enigma signals began to cause friction in the British Naval intelligence world both within BP and between BP and the Admiralty. Alan Turing had long ago uncovered the methods that the German Navy was using to make extremely difficult the determining of their Enigma setting up positions. Rather than leaving the operator to select his own message settings, the German naval methods involved looking up a book list and a settings list for the day. Alan knew that the capture of a book list or key sheet would be necessary before they could hope to break into these keys. So despite the good news that his code-breaking machine, called a bombe, is becoming productive on the Luftwaffe keys (when used with the added Gordon Welchman “Diagonal relay board” to cut down the false stops) no naval Enigma has yet been read with the exception of a few days worth of traffic following a key sheet capture last May. The second bombe, “Agnes” arrives on 8th August and is installed in the first “bombe hut”, Hut 1. (Actually though it has gone down in history as “Agnes” the girls called it “Aggi” and Alan Turing had named it on his drawings “Agnus Dei”). At this time the Hut 8 naval cryptography team has had to give up most of their bombe time to the Hut 6 team, whose Luftwaffe decrypts are desperately need to help with the air battle and to monitor the progress of the build-up for the invasion of Britain. It is in August 1940 that the Hut 8 team at last obtains an example of the eighth rotor wheel used by the German navy, possibly recognising its existence for the first time. The first 5 rotors were identical to those used by the other German Services and well known to BP, at first from the work of the Polish code-breakers. Rotors 6 and 7, used only by the German navy, had been recovered from the U-33 in February 1940. That this eighth wheel was obtained by “capture at sea” is known, but no details seem to be recorded; perhaps divers obtained it from the U-13 that had been sunk in relatively shallow water off the Norfolk coast on 1st June. Because the German navy had eight rotors available from which to draw the three rotors for use in their Enigma machine, the bombes had to check through up to 336 rotor combinations rather than the 60 available for the other Services who used only the five rotors. This took typically 80 hours of bombe time, compared with some 14 to run through the possible combinations for the Luftwaffe keys. Alan Turing had devised a method called Banburismus for reducing the number of rotor orders that needed to be tested by ascertaining the rotors in the Enigma’s right-hand and middle slots. But this could only be used once the indicator message settings were known, which needed the code-book by capture or reconstruction. (A new code-book had been introduced by the Germans on 1st July so the slowly growing reconstruction of the old book was of no use). Frank Birch, the head of Naval Section at BP, complains in a memo of 21st August to the Deputy Director, Edward Travis, that “Turing and Twinn are brilliant, but like many brilliant people they are not practical”. He claims that they have altered the cribs provided by his section, notably by Harry Hinsley. And the code-breakers complained about “Hinsley’s certain cribs” which did not work. And the Admiralty Operational Intelligence Centre complained that BP is not putting enough effort into naval Enigma. But it was to be for a further year before the naval Enigma messages could be read regularly, after the capture of material from the weather ships and the U-110. Meanwhile, Dilly Knox and his “harem” in the Cottage No.3 are having more success in reading the simpler Italian naval Enigma. The Bletchley Park Trust welcomes the preparation of these notes, but the authors are responsible for the statement and the views expressed. |
