Seventy Years Ago This Month at Bletchley Park

March 1940

The War.

The end of the phoney war is at last approaching, as even the most optimistic intelligence officer must have realised.  But uncertainty about the next move hangs in the air. The Allies plan to land in Norway to go to the aid of the Finns is set aside, as the Finns finally give way to the irresistible Russian force.  The new Allied plan envisages a coastal waters mining operation – with the option of landing at Narvik should the Germans be so foolish as to invade Norway further south as is half feared, half welcomed.  As the weather improves a German attack on the central European front seems inevitable, but where will the blow fall?  The Intelligence leads are there, but Allied commanders have not yet learnt to put them together and prefer to follow their hunches. 

The Air War warms up.

On 16th March the Germans bomb the fleet at Scapa Flow, damaging a cruiser. This finally persuades the Cabinet to abandon leaflet raids, and on 19th March fifty RAF bombers attack the German aeroplane base at Hornum on Sylt Island. Subsequent reconnaissance confirms the German assertions that no damage has been done.

The War in Finland.

After the renewed massive offensive by the Russians in February has finally breached the Finnish line, they know they have to make peace but manage to hang on until on 4th March the Russians mount an assault across the frozen Baltic close to Helsinki. The Soviet Government offers to negotiate peace, and finally terms are accepted on 13th March. For three and a half months the Finns have held out magnificently against the overwhelming mass of Russians. But for those few who chose to read the signs, the Russians, despite their casualties, have displayed something of that ruthlessness, tenacity, courage, and indeed skill, in particular in making use of the rigours of winter, that they would reveal in a couple of years in resisting the Germans. 

Plans to Invade Scandinavia.

The Allies know that Hitler wishes to acquire bases in Norway to protect his iron-ore supplies coming by coastal waters from the north of Sweden.  The British and French Prime Ministers had agreed to mount an expedition to help the Finns in their fight against the invading Russians.  This force would land on the coast of Norway on 20th March “to be sure of forestalling the Germans” according to Neville Chamberlain. In passing, it would take control of the iron-ore mines in Northern Sweden. But from their penetration of the British naval codes the Germans become aware of the Allied plans, and so Hitler orders that urgent preparations be made for a counter-move, an invasion of Norway and Denmark.  The Altmark incident reinforced Hitler’s decision to seize the Norwegian bases, before his attack on France. Allied plans to go to the help of the Finns are abandoned on 14th March, replaced by a sea mining campaign, though alternative British plans for landing on the Norwegian coast are being developed. Both sides plan to ignore the neutrality of Norway, Sweden, & Denmark.

BP adds to the signs of a German Attack. 

From many sources there are indications that the Germans are about to invade Scandinavia. For example, General Oster, the second-in-command of the Abwehr, the German military intelligence service, gave warnings to the Dutch and the Vatican that Germany would attack Norway on 9th April.  In March 1940, Bletchley Park makes its first contribution to the debate on what the Germans intend to do.  As so often was to happen in the future, whenever possible Germany is careful to use land-lines when planning operations, so the BP breaks into the Luftwaffe Enigma provide few clues.  Following information given by a double agent “Snow”, the Y service monitors the signals from a German spy ship in Norwegian waters. And in the second half of March BP reads the minor cyphers being used by her to radio back Intelligence information. This was the work of a special section under Oliver Strachey set up to tackle the Abwehr hand codes. (Oliver, a brother of Lytton Strachey, was an old hand from Room 40 in the Admiralty during the Great War.  In December 1941 Dilly Knox, another veteran from Room 40, was to break the machine codes of the Abwehr who use a special version of Enigma). The Naval staff are informed on 30th March; the Admiralty orders that the spy ship should not be disturbed because of the “cryptanalytical value” of her transmissions.

And then on 7th April, as a result of the signal ‘traffic analysis’ work of the young Harry Hinsley, Hut 4 sends warnings to the Admiralty that there is exceptional German naval wireless activity in the mouth of the Baltic. No Admiralty Intelligence section dissents from the view of the First Sea Lord, Winston Churchill, that a landing in Scandinavia is beyond German powers, and that the activity portends another break-out by heavy ships into the Atlantic or against the British convoys to Norway. The Norway desk officer in the Foreign Office minutes “A German descent on Narvik is surely out of the question”.  The British Air Intelligence branch is still advising that Hitler’s spring offensive would be limited to seizing Holland – to provide a base for an all-out attack on Britain. “They were conclusions” says Harry Hinsley, the Official Historian writing long after “which reflected the outlook of Whitehall at the time, in the absence of incontestable intelligence from Sigint” but also in the absence of “adequate machinery for rigorous and authoritative assessments of the massive but miscellaneous information about the enemy that was nevertheless available”.

The Early Breaks into Enigma. 

Using the sheets prepared at Bletchley Park by John Jeffreys, based on the Polish Zygalski method, the Poles and British have made the first wartime breaks of some Enigma keys in January 1940.  By the end of March some 50 key settings are solved, though generally with considerable delay, some being the Red, the Luftwaffe operational key, and others the Blue, the Luftwaffe training key.  According to the Poles, BP and the Poles broke some 126 key settings in total from January to mid-June, 83 % of these were being solved at BP.  The Poles say that over 5,000 messages were read in this period. The two teams are exchanging material over a line between Bletchley and the Château de Vignolles at Gretz-Armainvillers 30 kms to the east of Paris, until the Poles have to evacuate on 9th June. (The Polish accounts say that Enigma machines were used to preserve security!)   BP is to break the Yellow, the German Army-Air Force Norwegian campaign key, shortly after it is introduced on 8th April and it was read almost every day, and with little delay, from when it was first broken on 10th April. After Germany changed the setting up procedures for the other keys of their Army and Air Force on 1st May, the Yellow retained the old procedures, presumably because it was too difficult to change in the middle of a campaign. So the sheet method could continue to be used for this key, and the large increase of radio traffic incumbent on a military campaign proves a great help to the cryptographers.

The Value of the Early Breaks. 

Of course establishing a key setting enabled all the messages intercepted in that setting to be decrypted just as a German cypher clerk would do, typically for one day until the key setting changed at midnight. So BP is now reading hundreds of German messages. Because the war was static at the time of the first breaks, the intelligence information obtained was not of great value, but the potential for when the war went mobile is – or should have been - now clear! And indeed, though the Germans achieved total surprise in their invasion of Norway on 8th April, two days later BP broke the German campaign key, the Yellow, and thereafter read it continuously and almost currently until it lapsed after the end of the Norwegian campaign. Unfortunately only the Navy had made special arrangements for the feeding of such information to their forces.  But both the Red and Yellow had little information of direct value for the Fleet and the first breaks into German naval Enigma keys were not achieved until March 1941.  (By contrast, the Germans were well informed about our naval operations, since they were reading our navy and merchant navy codes at this time). That the success of BP with Enigma had little, if any, direct impact on the conduct of the fighting in Norway nor indeed for many months to come until ways were devised for handling this valuable information, is one of the tragedies of the intelligence war.

The Intelligence Process in Hut 3 and Hut 4. 

The decrypted Enigma messages from Hut 6 went directly into Hut 3 for translation and the preparation of outgoing messages to the Ministries and, in due course, the Military Commands. Shortly after the first break, Enigma Army &  Air Force Intelligence work had been established in Hut 3. F. L. (Peter) Lucas records: ‘On a snowy January morning of 1940, in a small bleak wooden room with nothing but a table and three chairs, the first bundle of Enigma decodes appeared. The four of us who then constituted ‘Hut 3’ had no idea what they were about to disclose’. This hut was on what is now the Mansion car-park, close to the tennis courts. [When they moved to what is now called Hut 3 in the autumn of 1940 as the team grew, the original Hut 3 was renamed Hut 9, and became the Administration hut]. Their tasks were far from trivial, requiring considerable Intelligence work. But GC&CS had not been given a formal role to extract ‘Intelligence’ from the decrypted messages; that was the responsibility of the Intelligence branches in the Ministries who could also bring to bear raw material coming from other Intelligence sources such as agents, photo-recce, etc. The responsibility of GC&CS was to report, or send out, the complete, but translated, outcome of the decryption process; they were permitted to add explanatory ‘comments’, but had to make it quite clear what was the raw material and what was the comment added from GC&CS work, using their card indexes and cumulated experience.

Attack on Enigma by the “Bombe” method.

Alan Turing has been working on the design of his “bombe” machine for finding the Enigma key settings.  Alan rejects the approaches that the Poles had used, and which John Jeffreys was using, believing that the Germans would soon abandon the repeat sending of the code wheel settings. (In this he was proved right for they abandoned it on 1st May 1940, but strangely not for some navy keys).  Instead he relies on the “crib” approach where the code-breaker “guesses” what part of the message may say. The manufacture of the prototype “bombe”, a complex electro-mechanical machine, has been in progress under “Doc” Harold Keen, the research director of the British Tabulating Machine Co at Letchworth.  The first prototype bombe was installed in Hut 1 at BP on 18th March. However, this bombe, Victory, did not achieve many key settings results because of the very high number of false alarms, each of which had to be tested to find the real stop.  When this was discussed by the Enigma production team it was Gordon Welchman who pointed out that Alan was not making use of the reciprocal nature of the Enigma machine; that is if a key, say X is pressed at one setting of the machine and produced the output Y, then at the same setting pressing Y must produce the output X. For otherwise the receiver of the message could not use his machine set up in the identical way to decrypt the incoming encrypted message.  (It is said Gordon Welchman subsequently claimed that ‘For all of one day, I knew something that Alan Turing did not know’!).  Alan then devised the diagonal relay board which took account of this property and proved a great feature of the bombe design.  Maybe the bombe should best be described as the ‘Turing/Welchman bombe’! It was not until 8th August when a second bombe, Agnus, was installed in the “bombe room”, Hut 11, with a “diagonal board” relay plate that the bombes became effective.

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