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Seventy Years Ago This Month at Bletchley Park September 1941Bleak News from Russia. Leningrad is now cut off by land, and the daily bombardments commence on 15th September. In the south the German assault culminates in the Kiev pocket closing and the city falling on 19th September. It is said that the Russians lost a further 500,000 men in this encirclement. But in the Centre the Russians achieve their first substantial success with their counterattack freeing Yelnya. Hitler now reverses his decision of August to halt the drive on Moscow and orders a renewed offensive to capture the city before the weather makes fighting impossible. The first snows fall on 12th September, and the onset of the rainy season slows the German armies, so those who know their history begin to wonder if the Germans have overstretched themselves. So far the campaign has cost the Germans 500,000 casualties; far fewer than the Russians but a serious loss, as these are some of their best-trained men. Half their motor transport was lost. Africa Corps Enigma Cypher Broken for first time. In general the German Army used their Enigma machines much more securely than did the Luftwaffe. BP had first broken an Army key when they broke Vulture on the Russian front in June, and Hut 6 was still reading it in September, though it became unreadable in mid-December largely because land-lines had been installed by then. But except for an occasional day, BP had failed to break an Army Enigma key in the Western Desert until on 17th September they succeed with Chaffinch II. They manage to break it with some regularity until 19th October, though at this time with great effort and often taking a week or more. And then they broke Chaffinch again from 2nd November until 6th December, helped by the capture of the key sheet for November during the Crusader offensive. The keys were then changed and not read again until April 1942. Chaffinch turns out to consist of three related keys, two being used between the supply bases in Africa, Rome and Salonika, and the third for special messages from Africa back to Rome and Berlin, such as the daily situation report. Taken together with the Luftwaffe Enigma key, Light Blue, used in the Mediterranean, the British are now quite well informed on the plans and state of supply of Rommel’s Africa Corps. While the British are slowly building up their forces to start an offensive to throw Rommel back from the Egyptian frontier and relieve Tobruk, Rommel is suffering severe supply shortages due to the prior demands of the German armies in Russia and the loss of his shipping due to the well informed British. But this does not stop him from making a swift lunge into Egypt on 14th September, with 21st Panzer Division advancing some 50 miles across the desert to a British fuel supply depot which turns out to be empty, having been abandoned some weeks before. The German tanks begin to run out of fuel, and they return whence they came that same evening, bombed by the RAF with Rommel himself only just managing to escape. It was eventually disclosed that this otherwise inexplicable operation was the result of a sting by the British who persuaded a double agent, “Cheese”, to tell Rommel that the British are going to attack on the 15th September. Rommel’s attempt to disrupt this phantom attack perhaps helped to persuade him that all was quiet on the desert front for he was away sightseeing with his wife in Rome when General Auchinleck was finally ready to launch Crusader on 18th November 1941. Admiral Dönitz suspects his U-boat codes are being read. Hut 8 at BP are now reading the main German naval Enigma key, Dolphin, regularly and with ever reducing delay. During September they are breaking Dolphin within 36 hours on the first of the paired days, and within a few hours further work for the second day when the wheel order would be unchanged. Admiral Karl Dönitz was always fearful that his submarine cyphers might he broken, so he had been planning to give them a separate key, and since April 1941 he had done this but in the first place with a trivial change in that if Dolphin had a ground state, say ABC, then the submarine key, called Shark at BP, would be the reverse, CBA, which created more work but not a significant problem for BP. At the Admiralty’s Operational Intelligence Centre, the Submarine Tracking room under Rodger Winn soon became skilled at using the Intelligence material for routing the convoys round the known positions of the submarines whenever that was feasible, so the convoy losses declined and the submarine sinkings rose. On 23rd September a message is read confirming that U-111 is to meet up with U-68 and U-67 in a bay on one of the Cape Verde Islands off West Africa to exchange supplies. So the Admiralty, rashly risking the secret of the source of its information, instructs a submarine, HMS Clyde, to ambush these U-boats. Due to what some might kindly describe as a chapter of accidents, HMS Clyde failed to sink any of the U-boats but was badly damaged by U-67 crashing into her. Admiral Dönitz writes in his war diary for 28th September “The most likely explanation is that our cypher has been compromised”. Dönitz immediately orders his submarines to adopt, from the beginning of October, an Enigma key with a completely different ground state from Dolphin. Once BP recognised what had happened they could handle this change, though at the price of considerably more work. (At last the number of bombes at BP is growing fast, with 8 in operation in early September rising to 16 by the end of the year. So Hut 8 can now afford to use more bombe time and was reading the new key by 5th October). Dönitz also set up an official investigation by the head of the German Navy’s Communications Service, but when Vice-Admiral Erhard Maertens reported on 24th October he concluded that the incompetence of the attack suggested that the British submarine had not been forewarned, so “Our cypher does not appear to have been broken”. His cypher experts had assured him that their Naval Enigma was “one of the most secure systems for enciphering messages in the world”. Never-the-less Dönitz pressed ahead with the development & deployment of a new cypher machine to be carried by his U-boats, a much more serious change which would result in BP being unable to break the U-boat key for nine terrible months after the new machines were introduced on 1st February 1942. But BP continued to break Dolphin, now used by most of the other German naval ships in “Home Waters”, most days until the end of the war. Unfortunately in September the German naval Intelligence service B-Dienst, the equivalent of the naval part of BP, succeeded in reading our Admiralty Naval Cypher No 2. They had been reading our Naval Cypher No 1 until it was replaced by No 2 on 20th August 1940. For what the Official Historian describes as “some unexplained reason”, in September 1941 the Admiralty abandoned an indicator procedure that had been causing B-Dienst real difficulty for one which was very much weaker. This enabled the Germans to read a good deal of our naval traffic for the four months until 1st January 1942 when No 2 was replaced by No 4. Luckily and surprisingly, good naval discipline ensured that they obtained no hard evidence that we were reading their naval Enigma from our messages in Cypher No 2 that they read. The First Break into the Fish Codes. The first detection that the Germans were experimenting with a new kind of enciphering teleprinter is said to have been made by the Southampton police in the summer of 1940, probably when they were searching for the radio transmissions from the inept set of German spies sent into the UK at the time when preparations for the invasion of the UK were being made. A Metropolitan Police unit at Denmark Hill under H. C. Kenworthy (by now their “Y” work was under control of BP) started to investigate these non-Morse transmissions and to intercept them systematically after the RAF Y station at Chapel-Ferne, near Dover, picked up similar signals in mid-1941. A small team under Denmark Hill was set up to do this at Abbotscliffe House adjacent to the Chapel-Ferne station. Use was made of undulator recording, dragging a recording pen over a moving sheet creating a picture of the incoming supersonic waveform which could then be “read” at slower speed to give the pattern of the “noughts and ones” that formed the five bit characters. It was probably there, or possibly at the Sandridge Y station near St Albans, manned by ex-GPO staff but part of BP since the start of the war, that the first of these teleprinter messages to be broken by BP was intercepted on 30th August 1941. It was a long message of nearly 4,000 characters, but when it had been transmitted the receiving cypher clerk sent back a message asking for it to be repeated; some bright Y station operator spotted this and the intercept station managed to take the two versions of the message. The German cypher clerks made the bad mistake of resetting their machines to the same initial settings that they had used for the first transmission. Had this been the only mistake they made all that BP would have obtained would have been two identical copies of the same enciphered message, but the sending cypher clerk then compounded this error by shortening the odd word throughout the message – so the signals were not identical because the code wheels on the original were now an ever increasing number of notches behind the repeat. It was the veteran and brilliant code-breaker, Colonel John Tiltman at BP who guessed that the Germans were using a form of the international teleprinter code encrypted by a stream of added bits generated by a complex series of code-wheels. So by the ‘in depth’ process of subtracting the first message sequence from the second he was soon able to reveal the message. Colonel Tiltman had started the war as a Major and ended it as a Brigadier, and throughout he was head of the Military Section at BP; somehow he always found time to practise his cryptographic skills, standing up at a specially constructed tall desk. There were few people at BP, even amongst that remarkable collection of geniuses that had his sixth-sense for the art. (He also happened to be one of the nicest of people; Ronald Whelan of the Hut 7 & Block C Hollerith team records that “He made a point of returning to us following the successful outcome of his labours so that he could personally thank the operators. He always made light of his break-in into the cypher, sometimes saying he didn’t quite know how he had done it; that he had just had a hunch and found it worked. It was always a pleasure working for him”). BP soon learnt that these Fish messages, as they called all these transmissions that used what turned out to be the Lorenz SZ 40 (later the SZ42) machine, started with a sequence of twelve German names, of which they guessed the first letters of the names in some way gave the initial settings of the code wheels. The Fish messages were clearly of the utmost importance, covering the very top-level army communications. Tiltman gave to a new young member of the Research Section, Bill Tutte, a chemistry graduate just down from Cambridge, this one broken message and many unbroken ones, asking him to see if he could reconstruct the details of the unknown cypher machine that was being used. “That the Research Section was able to achieve this feat within a matter of a few months was one of the outstanding successes of the war” said Nigel de Grey who became Deputy Director of BP and so could recognise the mixture of brilliant intuition and mind-numbingly persistent work that was needed to do this. Though far fewer than the Enigma decrypts, the Fish messages proved to be invaluable. Churchill Visits Bletchley Park. On Saturday 6th September Churchill pays a visit to Bletchley Park. He is taken into the Huts, and Alan Turing is asked to tell him about the great breakthrough, but the hopelessly shy Alan is unable to say anything so Gordon Welchman takes over and explains that there are three points they wish to make to the P.M. When he finally gets to the end of the second point the Director, Alastair Denniston, interrupts to suggest that Churchill might wish to be moving on; Gordon describes “…whereupon Winston, who was enjoying himself, gave me a grand schoolboy wink and said ´I think there was a third point, Welchman’”. He goes on to visit the machine room, Hut 7, where Ronald Whelan noted that as he passed the sentry on-guard at the entrance “his bodyguards attempted to follow but in a voice which would have done credit to that of a huge bear, he rasped out ´Not You!` causing them to stop dead in their tracks”. The Head of that Hollerith machine installation in Hut 7, Freddie Freeborn, was a great showman. “On entering the Machine Room the visitor was presented with a scene of intense activity. There were 45 machine operators in action at as many machines. Then all the machines were halted at the same instant, and in the complete silence that followed Mr Freeborn gave an introductory explanation….At the conclusion of the demonstrations all machines were brought back into action as the visitor was conducted to the exit, but all brought to rest as Churchill paused on the threshold to make his farewells”. Finally standing outside Hut 6 on a pile of builders rubble, Churchill addresses some of the Codebreakers and starts with the somewhat inauspicious words “You all look very innocent; one would not think you knew anything secret”. But he made up for this by calling them “The Geese that lay the golden eggs – and never cackle!” John Herivel remembers “In just a few words, with deep emotion, he said how grateful he was to us for all the great work we were doing. So that was our finest hour”. But it would seem that Churchill was not entirely reassured by the sight of these clearly ill-disciplined and eccentric folk, more reminiscent of an old University Common Room than of the key battleground of the Intelligence war. When he drove off it is said that Churchill wound down the window of his car, and said to the Director “About that recruitment - I know I told you not to leave a stone unturned but I did not mean you to take me seriously”. The Bletchley Park Trust welcomes the preparation of these notes, but the authors are responsible for the statements and the views expressed. |
