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April 1944 : Preparations for D-day

The Allied deception campaign to confuse the Germans about the site and date of the invasion, Operation Fortitude, is now in full swing, directly fed from BP by all decrypts that might have a bearing on that work. While it is accepted that it is impossible to disguise that the main operation is to be in North-West Europe, an intrinsic part of this complex deception plan is to tie down German forces elsewhere, and if possible keep them tied down for at least fourteen days after D-day. An imaginary army in Scotland threatens an invasion of Norway, and another one in Kent aims at the obvious landing site in the Pas de Calais. Radio transmissions, dummy vehicles and aircraft, and double agent reports add to the welter of confusing reports the Germans are receiving. As Churchill put it to Stalin ‘In wartime, truth is so precious that she should always be attended by a bodyguard of lies’. After considerable argument, the Russians agree to play their part in the deception campaign, assembling shipping in the Baltic to resemble a task force preparing to undertake an invasion of northern Finland. The launch of the main Russian summer offensive will be timed to prevent the Germans moving troops from the Eastern front to repel the actual landings. The Allied Command is very conscious of the disaster that is likely to ensue if the Germans learn the true place and date planned for Overlord, the invasion. Churchill orders all civilian travel between the UK and Eire to stop, because the German diplomats there are known to be a fertile centre of espionage. And then on 16th April a coastal belt, ten miles deep, from the Wash to Land’s End is closed to visitors and unprecedented restrictions are placed on foreign diplomats. One problem is that the Foreign Office has demanded that these bans be lifted as soon as the landings have taken place, but the deception plan requires the Germans to believe that Normandy is only a feint with the real landings in the Pas de Calais yet to follow. [In the event this dilemma was resolved when Eisenhower found, after D-day, that this aspect of the deception plan was working, and so insisted that the restrictions on foreign diplomats be retained. The bans were not lifted until 25th August, well after the break out]. By April the redeployment of Allied forces from the Mediterranean to the UK has taken place, remarkably without the Germans getting wind of it. Assuming that the Germans would soon learn that certain famous regiments were back in the UK, the names of some of them are released to the Press, but not before the double agents had planted the information with the Abwehr, thus greatly adding to their credibility. In all this deception work the British are much aided by the knowledge of what the Germans are thinking, in particular through the reading of the Abwehr codes by ISK & ISOS, now in Block G at BP. It seems that the Germans are quite unimpressed with the visual and only a little by the radio deception work, in spite of the dummy landing craft, etc, but they are being influenced by the double agents. They remain convinced the landings are coming across the Channel, but are still uncertain about the date and place. However, the discovery by air recce of considerable concentrations of shipping along the south-west coast of England causes Field Marshal von Rundstedt, C-in-C West, to warn Erwin Rommel, as Commander of the reserve counter-attack Army Group in the area, that the twin peninsulas of Normandy and Brittany are likely landing areas. At the end of April Hitler takes the same position: ‘The Führer attaches exceptional importance to Normandy. Measures are to be improvised to strengthen Normandy against attack, especially by air landing troops’. This did not make for happy reading in Blocks D & G and Hut 3 at BP and then in Whitehall. SHAEF continued to insist that Overlord would have small chance of success if the enemy obtained so much as 48 hours notice of the plans, and that a longer period of notice would spell ‘certain defeat’. To further increase the tension several decrypts in the first weeks of April, probably from the Fish link to Paris, Jellyfish, confirm that, amongst other divisions moving into the area, the above-normal-strength 21st Panzer division has been transferred to Normandy and is now established in a broad area round Rennes on the borders of Normandy and Brittany.

The Disaster off Slapton Sands. On the 27th April General Montgomery and Admiral Ramsey attend the final exercise off Slapton Sands in Devon for a US force training for landing on Utah beach on D-day. It proves far from satisfactory as, due to a signals muddle, only two companies land after the warships had completed the bombardment. Worse was to follow for in the early hours of the next morning a ‘swarm of German E-boats’ succeed in getting amongst the landing craft now fifteen miles off Portland Bill, sinking two of the large tank-landing ships (LSD) and damaging a third. A signal warning that the nine E-boats are leaving Cherbourg and heading westwards is not decrypted in Hut 8 until too late. [The 749 casualties that night would prove to be more than would be lost on Utah beach on D-day]. In view of the shortage of landing craft this is a serious blow to the Allies, and a tragic loss to the US forces; many of the men lost are not easily replaceable specialist engineers. The loss is compounded by the fear that some of the ten lost individuals who had been ‘bigoted’, that is had been told the D-day secrets, have fallen into German hands. So a detailed search for the bodies has to be mounted, and it was a considerable relief when those ten bodies were recovered. This incident spurred the successful effort to persuade the security officials to allow Naval Section at BP to put up temporary radio aerials in the Park, so that the time delay in intercepting and decoding some of the German reactions on D-day would be minimised. It is said that Churchill himself had to make the disputed decision to allow the aerials to be installed. [In practice this worked very well, enabling the navy to learn, on D-day and during the following days, when and where the E-boats and destroyers were going to assemble, with the result that they were harassed whenever they ventured out, and most of them were finally destroyed in a daylight raid by Bomber Command on Le Havre on the evening of 14th June].

War at Sea. Early in the New Year the U-boats had concentrated in the Western Approaches and the Luftwaffe had tried to provide close air-support. But now that the position of the U-boats tended to be known from Shark decrypts, only 54 Allied ships had been lost in three months, but 62 U-boats had been sunk. In April an analysis of U-boat movements confirmed that now they were being held back from venturing into the Atlantic in order to form an anti-invasion reserve. This was not a surprise as back in January it had been learnt from decrypts that, following an invasion scare when the Luftwaffe had reported seeing 200 to 300 landing craft off the Gironde Estuary, U-boats from as far away as Rockall had been ordered to proceed to Biscay at full speed ‘regardless of the danger from Allied aircraft and mines’. The knowledge that the U-boat menace to the Atlantic and Artic convoys was temporally reduced was valuable, as it enabled the Admiralty to reorganise safely its dispositions for D-day. Now it is clear that the invasion force are going to have to face strong attempts to disrupt the landings and follow-up supply ships from U-boats and E-boats, and perhaps from the few remaining German large surface vessels. It is hoped that some of the surface ships will be held in the Baltic to meet the threat from the planned Russian offensive in the north, but the attempts to destroy the Tirpitz in her Norwegian Kaafjiord are renewed. It is known that she has nearly completed repairs to the damage caused by the midget submarines in September 1943. On 30th March major warships of the Home Fleet, led by Admiral Fraser in Duke of York, left Scapa Flow to join six carriers, already at sea. The force headed north at speed as secrecy was essential to success. By great good fortune an Enigma decrypt on the afternoon of 1st April discloses that the Tirpitz intends to sail for trials at first light on the 3rd. So the raid is brought forward and the force splits, Vice Admiral Moore in Anson commanding the air attack. Early on the 3rd April, two waves of 21 Barracuda fly off the carriers, each wave escorted by some 40 fighters. Tirpitz, which is weighing anchor, is taken completely by surprise as the first wave of bombers dive to attack just before 5.30 a.m. They succeed in landing nine bombs on her, and though the second wave an hour later has to cope with a thick smoke screen and a thoroughly alerted defence, it achieves a further five hits. She does not suffer fatal damage as had at first been hoped, perhaps for the perverse reason that the bombers have released their bombs too low (under 1,000 feet and some as low as 600 feet) in order to improve their aim, but she is put out of action for about three months. By the 6th the fleet is back in Scapa, having fulfilled the subsidiary aim of strengthening the deception plan by creating the impression that the task force is a preparation for a landing in Norway. Further raids were planned for April and May but poor weather caused cancellation on three occasions.

A Major Problem hits Hut 6. On 1st April turmoil hits Hut 6 when the Luftwaffe abruptly changes its method of allocating codenames to keys. They had used the same codebook of call-signs since early in the war, called by BP the ‘Bird book’. Back on 1st September 1943 the German Army, followed on 1st November by the Luftwaffe, had abandoned putting a ‘discriminant’ at the start of each Enigma message to identify to which net or key it belonged. In theory, this meant that the Y stations were sending into Hut 6 some 3,000 intercepts each day with no way of determining from the message to which of up to 90 keys a particular intercept belonged. But in practice the Y stations recorded the call-sign and frequency of the radio channel of each message intercepted, and, by keeping a check on these, the Hut 6 Technical Information Section could allocate the vast majority of the messages to one or other of the known keys, so that appropriate cribs could be applied to break the settings for that key for the day. However now Hut 6 abruptly lost the ability to use the call signs and frequencies to identify the key. The change had not come as a total surprise, and a way of coping with the crisis, when it arrived, had been planned. On the first day chaos reigned and the system largely broke down. The head of Hut, Stuart Milner-Barry records that staff were working up to two continuous shifts (16 hours) in the attempt to work out what was going on, recording his admiration for people who worked ‘in conditions as unconducive to calmness and clarity of thought as it would be possible to imagine’. They knew the Luftwaffe were using the newly introduced ‘F call-sign book’, but at first were unable to work out how the call-signs were allocated. Gradually, with the help of the Y station girls, who could sometimes identify ‘their’ German cypher clerks from their morse keying ‘fist’, and Sixta, the radio network traffic analysis experts who monitored the frequencies and direction of the transmitters, some order was restored. There had been planned to be a ‘Giant Foss’, an automatic mechanism designed to correlate frequencies and identify alternatives for known frequencies. (Presumably this concept, which worked well in normal times, had been first devised by the great veteran codebreaker and Japanese expert, Hugh Foss). But at first there were so many staff trying to fill in the cards that those who were supposed to be analysing them could not get near them. However by the morning of the fourth day it became evident that the system had been brought under control, though full service would not be restored until the wizards at call-sign prediction, Spencer & Grant had broken the new system of call-sign allocation. Milner-Barry tells the Director that, almost unbelievably, ‘this remarkable feat was actually achieved in little over a week’. However he acknowledges that ‘the shock to the whole system throughout Hut 6 was profound; the crisis was even sharper than had been anticipated’. He praises the performance of his staff, and particularly singles out the unexpectedly good help they received from Sixta. Now he records that his Research Section, who had begun to wonder helplessly if their unusually difficult Luftwaffe traffic would ever be identified again, was happily back ‘looking round for Cockroaches and Daffodils to break’ (both difficult but important Luftwaffe keys used in Germany which were getting special attention at this time as D-day approached). He acknowledges the strain on the girls who worked on the Enigma machines in the Decoding Room who had had to make ‘innumerable ‘tries’, all too few of which came out’. Stuart tells of one Luftwaffe expert in the Hut who broke an Army key in use in Russia ‘under the impression it was Skunk’, a key used by the Luftwaffe. Despite the chaos Milner-Barry was able to report that the Watch had carried on with the most important keys as if nothing had happened, breaking on average ‘the Red at 1525, Primrose at 1945 and Puma at 2150 - all average if not better than average. In fact the only real casualty is the Zoo; the disappearance of this cryptographic oddity has not affected our hold on Snowdrop, but has meant we have lost touch with Jaguar’. (Snowdrop and Jaguar were also Luftwaffe keys used in the West but BP had only read Jaguar recently. The Zoo was probably a collection of the keys which could only occasionally could be broken, out of which a particular key would sometimes emerge when good cribs were found). From some of the messages they did break they learnt that the steckerboards on the Luftwaffe Enigma were going to be changed three times a day from 1st May, and it was obvious that this was going to cause yet more work at just the time when the Hut needed to prepare for the flood of work from D-day. The head of Hut 6 summarises this eventful first week of the month: ‘Of many memorable weeks in our strange and eventful history, this was the most memorable - and the most inspiring’.

Order for Colossus. The first motor runs were successfully made on Colossus. The specification for the MKII Colossus was agreed with Tommy Flowers towards the end of April. The date of 1st June for the hand-over of the first machine to the Newmanry was confirmed, with delivery of the rest of the twelve, now ordered, to be at the rate of roughly one a month.

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'Can anyone identify 'Spencer & Grant', the remarkable duo in Hut 6 who succeed in working out how the new F codebook works within a week of its introduction? What were their Christian names, and how long had they been working on code-signs in Hut 6?'

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