Few realised the significance of the work going on at Bletchley Park
With German invasion imminent in 1939, the Poles opted to share their secrets with the British, and Britain's Government Code and Cipher School (GC&CS) at Bletchley Park, Buckinghamshire, became the centre for Allied efforts to keep up with dramatic war-induced changes in Enigma output.
A host of top mathematicians and general problem-solvers was recruited, and a bank of early computers, known as 'bombes', was built - to work out the vast number of permutations in Enigma settings.
The Germans were convinced that Enigma output could not be broken, so they used the machine for all sorts of communications - on the battlefield, at sea, in the sky and, significantly, within its secret services. The British described any intelligence gained from Enigma as 'Ultra', and considered it top secret.
Only a select few commanders were made aware of the full significance of Ultra, and it was mostly used only sparingly, to prevent the Germans thinking their ciphers had been broken.
Despite providing some otherwise inaccessible information, it was some time before Ultra made any significant contribution to the war effort. Although, thanks to the information from the Poles, the British had learned to read parts of the Wehrmacht's signals traffic, regular decrypts only became possible in the Norwegian campaign - and then they were of marginal operational use.
Within a wider context, two Luftwaffe ciphers were broken, but the information gained was of little effective use. Similarly, Ultra's role in the Battle of Britain was limited: better grade intelligence came from prisoners, captured documents and improved air reconnaissance.
Only in 1941 did Enigma decrypts pay dividends. In the spring they provided evidence of a German military build-up prior to the invasion of Greece, although the Allies did not have a large enough military force to exploit this breakthrough.
In March, Bletchley's reading of the Italian navy's Enigma material helped Admiral Cunningham's Mediterranean fleet defeat the Italians at the Battle of Matapan. And in the autumn, the cryptanalysts broke ciphers used by Marshal Rommel's Panzer army, both within its own units and in communications with Rome and Berlin, giving the Allies an important advantage in North Africa.
Pinching the codes
By then the greatest threat to the Allied war effort came from attacks on their ship convoys in the North Atlantic. As a result, Bletchley's resources were concentrated on breaking Enigma codes used by German U-boats in this sphere of war. If the Allies could find out in advance where U-boats were hunting, they could direct their ships, carrying crucial supplies from North America, away from these danger zones.
So began one of the most exciting periods of Enigma code-breaking. Even in 1940 Bletchley had had some success in breaking Enigma keys used by the German navy.
It soon became clear that the best way of keeping up with rapid changes in ciphers and related technology was to capture Enigma machines and code-books on board German vessels.
In the Admiralty, where the Operational Intelligence Centre (OIC) was a leading user of Ultra, Commander Ian Fleming, Personal Assistant to the Director of Naval Intelligence, showed his talent for fantastical plots when he suggested a plan (known as Operation Ruthless) to crash-land a captured German plane in the English channel, and to overpower the patrol boat that came to rescue its supposed survivors, thereby gaining access to Enigma materials. The plan was never implemented.
A break-through came in March 1941, however, when the German trawler Krebs was captured off Norway, complete with two Enigma machines and the Naval Enigma settings list for the previous month. This allowed German Naval Enigma to be read, albeit with some delay, in April, by codebreakers at Bletchley.
Around this time, Harry Hinsley, a Bletchley codebreaker, suggested that German weather and supply ships, as well as war ships, probably carried Naval Enigma details. This idea was proved correct when, in May 1941, the German weather ship München was attacked and found with Enigma code-books for June on board.
The capture of the supply ship Gedania and weather ship Lauenburg in June yielded codebooks for the following month, and opened the way to the reading of Naval Enigma almost concurrently with events.
The ambush of three German U-boats off Cape Verde in September, however, coupled with a dramatic fall in the number of Allied ships sunk in the North Atlantic, led the German Admiral Karl Dönitz to question if the navy's cipher had been compromised.
Although he was dissuaded by his experts, the Germans redoubled their efforts to tighten Enigma's security, and the Bletchley Park codebreakers, realising what they were up against, wrote to British Prime Minster Winston Churchill complaining that they were not being given enough resources. Churchill replied with a famous 'Action This Day' memorandum: 'Make sure they have all they want on extreme priority and report to me that this had been done'.
Shortening the war
In February 1942 the Germans hit back by introducing a new fourth wheel (multiplying the number of settings another 26 times) into their Naval Enigma machines. The resulting 'net' was known to the Germans as 'Triton' and to the British as 'Shark'. For almost a year Bletchley could make no inroads into Shark, and Allied losses in the Atlantic again increased alarmingly.
In December 1942 Shark was broken, but German innovations meant that the Allies had to wait until August the following year before Naval Enigma was regularly read again. By then the Americans were active combatants, providing much-needed computer power to Bletchley.
By D-Day in June 1944 Ultra was no longer so important. But still no one wanted the Germans to sense that Enigma was being read. When, a few days before the Normandy landings, an American task force captured a German U-boat with its Enigma keys, Admiral Ernest King, US Commander in Chief of the Atlantic Fleet, threatened to court-martial the officer in charge for endangering 'Operation Overlord', as the plan for the D-Day landings was known.
By how much did Ultra intelligence, gained from reading Enigma ciphers, shorten the war? Harry Hinsley, based at Bletchley during the war, suggests it was a significant asset. If it did not keep Rommel out of Egypt in 1941, it certainly did so the following year, by preventing him exploiting his victory at Gazala.
As General Alexander put it, 'The knowledge not only of the enemy's precise strength and disposition, but also how, when and where he intends to carry out his operations brought a new dimension to the prosecution of the war.'
The loss of Egypt in 1942 would have set back the re-conquest of North Africa and upset the timetable for the invasion of France. According to Hinsley, Overlord would probably have been deferred until 1946.
But by then the Germans might have hit back with V-weapons and worse. Enigma successes always needed complementing with other intelligence material, but the fact that the Allies kept Enigma secret until 1974 shows how much it meant to them.