Seventy Years Ago This Month at Bletchley Park   

July 1940

Britain Stands Alone.

The French Armistice with Germany came into force on 25th June, so Britain now  stands alone, awaiting the seemingly inevitable German onslaught.  There is an almost perceptible sense of relief in Britain that she no longer has to deal with difficult allies, expressed by the King in the words “Personally I feel happier now we have no allies to be polite to”.  From Enigma Luftwaffe decrypts (the “Red” key) Britain learns on 28th June that most of the German bombers would have completed their refitting by 8th July, following their work in France in close support of their armies. But for the moment Hitler had no plans for his next move and it was not until 2nd July that he instructs his staff to prepare detailed plans for the invasion of Britain “provided air superiority can be attained”. The loss of so many of his destroyers in the Norwegian campaign means that he knows his invasion fleet will be subjected to intense attack from both the sea and air unless the RAF is first rendered ineffective. Early in July the Germans land in the Channel Islands unopposed, this occupation of British territory being hardly noticed in the UK.

The British Attacks on the French Fleet. 

Determined to prevent the French fleet falling into German hands, Churchill orders the seizure or neutralisation of all French warships. The British ultimatum of 3rd July gives them the choice of joining the British, scuttling their ships, or sailing to the French West Indies to hand them over to the USA. When these alternatives are rejected, the British navy opens fire at Mers-el-Kebir, killing more than 1,250 French sailors.  With a five-minute bombardment, two battleships and a battle cruiser are sunk, but several French ships, including an aircraft carrier, escape to Toulon.  Churchill defends his action to Parliament with the words “I leave it to the nation, to the world and to history”.   It would seem that it was this brutal action to a country which had been Britain’s ally only two weeks before that convinced Roosevelt that Britain would continue to fight on, alone.  And it is now suggested that the action finally persuaded Hitler that he was wrong in his belief that Britain would make peace once isolated. It is now known that the Pétain government, in passing on the alternatives to the French navy, suppressed the proffered alternative of sailing to the neutral USA.  From 1st July BP is reading the French naval cyphers, with the help of the documents from the commander of a French submarine who had sided with the British in Malta. A decrypted message from the French Admiral shows that he has ignored the option to sail to the USA, but that decrypt reached the Admiralty from BP 15 minutes after the bombardment had started.  BP continued to decypher French naval signals until the Allies landed in north-west Africa in November 1942, when the Germans occupied all France, changing the cyphers, though the French fleet at Toulon had scuttled.

Intelligence and the Invasion Threat.

The air campaign in France during June has cost Britain a quarter of her remaining fighter strength.  Only 600 serviceable fighter aircraft are left to oppose the enemy air-raids, which start on
10th July with attacks on convoys in the Channel and raids on dockyards in South Wales. But at least the reading of the Enigma “Red” Luftwaffe messages enables Air Intelligence to reduce their estimate of the number of German bombers that now threaten the UK from 2,500 to 1,250.  (The Air Ministry mounts a campaign to collect aluminium pots and pans, ostensibly to increase aircraft production).  On 16th July Hitler issues to his staff the first plans for “Sea Lion”, the invasion of the UK, naming 5th August as the date for the commencement of the major air offensive to destroy the capability of the RAF to make any significant attack on the German invasion armada.  But, unknown to the British, following the Russian annexation of the Baltic States on 21st July Hitler gives instructions for plans for an invasion of Russia to be prepared.  At that staff meeting he spoke with a noticeable lack of enthusiasm about the plans for the invasion of the UK, warning that air superiority over the RAF must be obtained by the beginning of September if the landings in the UK are to be made before the weather worsens. He names 15th September for the invasion, provided a prior week-long bombing attack on southern England had done substantial damage to the Royal Navy and the RAF; otherwise the invasion would be postponed until May 1941.  Air raids have already become a daily feature of life in the UK, and they steadily build up throughout July.  Even before the battle is joined in earnest, Britain has lost 526 pilots during the air fighting in June and July.

UK Intelligence staffs had been very conscious of the possibility of an invasion for some months, and on 29th May they warned that an invasion could be expected at any moment.  This alert state was cancelled a few days later when Enigma decrypts established that the German first priority was to knock out France.  On 5th July the UK Intelligence authorities again warned, precipitated this time by the first indirect references to German invasion planning in the Enigma decrypts, that large-scale raids might be expected at any time, though not a full scale invasion before mid-July. The Official Historian of the Intelligence War (Harry Hinsley) dryly records: “We might wish to attribute the views of the Military Intelligence to the fact that, its staff being soldiers, they were ignorant of the difficulties involved in organising a seaborne expedition against powerful naval and sea defences. Unfortunately for this argument, the views of Military Intelligence were shared by Naval Intelligence”. It is nice to record that our senior leaders were wiser than their Intelligence Staff; for Churchill paid no attention to these warnings and the Chiefs of Staff refused to order an invasion alert on the grounds that they expected an invasion attempt to be preceded by a major air battle. 

Hut 6 and the Coming Battle for Britain. 

After the end of the fighting in France the number of Enigma messages falls off drastically as the Luftwaffe reverts to the use of landlines, but the remaining Red decrypts do give a clear warning of the approach of the Battle of Britain even if in very general terms.  On 28th June Air Intelligence reported that the majority of German bombers would have completed refitting by 8th July; the Battle of Britain is normally considered to have started on 10th July.  The massive amount of detailed information that has been gathered from the Enigma decrypts over the last two months now bears fruit in strategic terms from the knowledge gained about the organisation and equipment of the Luftwaffe. And Photo Recce, including before long from the ultra long-range Spitfire (Type D), is at last performing well as the virtue of using fast, high flying aircraft becomes accepted practise.  After 5th July the Intelligence Staff issued no further invasion alert until 7th September when all home defences were brought to a state of “immediate action”.  The reduction in the number of Enigma messages increases the difficulty of the cryptographers work very considerably, making it all the more remarkable that they are able to continue to break the Luftwaffe Red key every day, though often with some delay.  Since 1st May the Germans had dropped the double sending of the encyphered message setting at the beginning of messages, so the methods that BP had used until then, based on those pioneered by the Poles, can no longer be used.  Hut 6 has to rely on the stupid mistakes that German operators make in setting up the Enigma machines, called the “Cillis” at BP, as the Turing bombe is not yet fit for use.

Progress on the Bombe. 

Alan Turing, 28 years old at this time, had started work on his bombe soon after he had joined BP from Cambridge at the outbreak of the war.  Initially he had been working in the Research team in Cottage 3 in the Stable Yard, under the brilliant, very experienced but old school code-breaker, Dilly Knox.  By now in July 1940 Alan is working as leader of the small team in Hut 8 along with Peter Twinn, Tony Kendrick, Hugh Foss and Joan Clarke (Joan Murray), concentrating primarily on breaking the Naval Enigma traffic. But at this stage there is considerable interaction with other cryptographers in Hut 6 where the team under Gordon Welchman is regularly breaking the Luftwaffe Enigma Red key and trying to break other air force and army keys. The first bombe, “Victory” had been delivered on 18th March 1940.  It is both serviced and operated by a small tri-service team of technicians under Sergeant E. Jones (who continued to be in charge of the bombes throughout the war, rising to Squadron Leader. The Wrens took over operating the bombes from the men in March 1941, but the machines continued to be serviced by RAF technicians). The first bombe was installed in Hut 1, a small hut that had many other uses including a sick bay.  German naval Enigma keys were much more difficult to break than the other services keys (e.g. the Luftwaffe Red), because the initial daily ground state indicator setting and the encrypted message settings depended upon looking up the monthly key sheet and code book sheet-for-the-day.  Alan Turing and Joan Clarke use this first bombe to test “menus”, endlessly plodding through the possible 336 wheel orders for the three code wheels in use in the naval Enigma which are known to be drawn from eight possible wheels. (BP had seven of these wheels at this time, five being the standard Enigma wheels, two special naval ones having been captured from the submarine U-33 in February, but the eighth was not captured until August 1940).  During June they read ten days of traffic for April 1940 of the widely used Deep-Waters “Dolphin” key, with the help of crib material captured from the patrol boat Schiff 26.  The code sheets for converting the message indicators into the actual machine settings, some of which Hut 8 had reconstructed from the April breaks, had been changed by the Germans on 1st July.  So Hut 8 is now using the bombe and a special machine, the “Baby”, to create a catalogue on Hollerith cards of the encyphered version of the German word “eins” (“one”) for a given day’s wheel-order and plugging at all possible machine settings. Because of the difficulty they have in working out the initial settings without the code-sheets, once they have found the wheel order and plug-board connections for the day this catalogue is used to find the message settings for naval intercepts.  Hut 8 is also using another technique invented by Turing, called Banburismus, for reducing the number of possible wheel orders.  It relies on the weakness of the message setting procedure for certain types of naval Enigma keys that on a given day all the message settings start from the encryption of the same “ground-state” position of the code-wheels.  But it was not until November 1940 when Hugh Foss in Hut 8 succeeded in breaking a naval Enigma message purely cryptographically with the help of an improved bombe. This man-power intensive Banburismus procedure was used until the number of bombes available grew to the point, in about June 1943, when it became more efficient to use bombe time to save on cryptographer’s time. 
 
Unfortunately the first Turing bombe failed to reject a significant proportion of the incorrect rotor settings, resulting in numerous “stops” which had to be worked through by hand in order to find the correct setting.  Writing at the end of the war, Sq. Ld. Jones says that after weeks of fruitless runs “...it was realised that some radical change was still required. …So the theorists, the cryptographers, the Engineers, Professors, Doctors, got together and decided what was needed”. It was probably at about this time (though writing years later he remembered it as being months earlier) that Gordon Welchman pointed out that one could reduce the number of false stops by taking account of the reciprocal properties of the Enigma machine. (If one pressed the key “A” and the lamp “M” lit, then if one pressed the key “M” at the same settings of the code-wheels it must be the lamp “A” that lights, etc; otherwise decrypting with the same machine would not work).  Early in July Welchman and Turing realise that their “diagonal relay board” implementation designed to take advantage of this property would also enable “simultaneous scanning” of all the possible 26 plug-board settings for an individual wheel setting.   This would strengthen the bombes enormously, reducing the number of false stops very considerably and enabling strong menus to be found for shorter cribs.  Later in July the young Richard Pendered, who works for Gordon Welchman in Hut 6, is busy testing out the new bombe at the Letchworth factory of BTM.  This second and much improved bombe, “Agnes”, is installed at BP in Hut 11, the “bombe hut”, on 8th August 1940.

The Bletchley Park Trust welcomes the preparation of these notes, but the authors are responsible for the statements and the views expressed.