The plan was never implemented.Ī 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. 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. 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. Even in 1940 Bletchley had had some success in breaking Enigma keys used by the German navy. So began one of the most exciting periods of Enigma code-breaking. 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. As a result, Bletchley's resources were concentrated on breaking Enigma codes used by German U-boats in this sphere of war. 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.īy then the greatest threat to the Allied war effort came from attacks on their ship convoys in the North Atlantic. 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. 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. Only in 1941 did Enigma decrypts pay dividends. Similarly, Ultra's role in the Battle of Britain was limited: better grade intelligence came from prisoners, captured documents and improved air reconnaissance. Within a wider context, two Luftwaffe ciphers were broken, but the information gained was of little effective use. 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. 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.ĭespite providing some otherwise inaccessible information, it was some time before Ultra made any significant contribution to the war effort. The British described any intelligence gained from Enigma as 'Ultra', and considered it top secret. 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. 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.Ī 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. Helped by its closer links to the German engineering industry, the Poles managed to reconstruct an Enigma machine, complete with internal wiring, and to read the Wehrmacht's messages between 19.įew realised the significance of the work going on at Bletchley Park It was only after they had handed over details to the Polish Cipher Bureau that progress was made. Over the years the basic machine became more complicated, as German code experts added plugs with electronic circuits.īritain and her allies first understood the problems posed by this machine in 1931, when a German spy, Hans Thilo Schmidt, allowed his French spymasters to photograph stolen Enigma operating manuals, although neither French nor British cryptanalysts could at first make headway in breaking the Enigma cipher. The receiver needed to know the exact settings of these rotors in order to reconstitute the coded text. Enigma allowed an operator to type in a message, then scramble it by means of three to five notched wheels, or rotors, which displayed different letters of the alphabet.
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