With excerpts from Breaking Germany's Enigma Code, BBC History
By 1941 the greatest threat to the Allied war effort came from attacks on their ship convoys in the North Atlantic. As a result, resources at Britain's Government Code and Cipher School at Bletchley Park, Buckinghamshire 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 breakthrough came in March 1941, 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 code breakers 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, May 7, 1941, the German weather ship München was captured by the HMS Somali. Prior to being boarded, the crew of the Munchen threw the ship's enigma machine overboard in a weighted bag. However, left onboard were documents on the operation of the enigma machine and vital codebooks, providing another breakthrough for Allied codebreakers.
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 code breakers, realizing 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."
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, U.S. 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.
The Enigma Code Map
The Enigma grid map was used on German U-boats for position messages, sent over to the German U-boat command. It showed several named grids, which defined certain areas. Big squares, named with 3 letters were split down to smaller grids and named with numbers from 1-99. A radio message with coordinate AM32 let the U-boat commander know he was to gravel to grid AM32 where a convoy was sighted. To know where AM32 was, he had to consult this map.
There were a lot of different issues of this map. The most used and the most famous, the 3401, showed the Northern Atlantic area.
Excellent links on the Enigma Machine ...
Bletchley Park Exhibition
The German Inigma Cipher Machine
The Rutherford Journal
Enigma Machine Captured