Ultra was the designation adopted by British military intelligence in June 1941 for wartime signals intelligence obtained by breaking high-level encrypted enemy radio and teleprinter communications at the Government Code and Cypher School (GC&CS) at Bletchley Park. Ultra eventually became the standard designation among the western Allies for all such intelligence. The name arose because the intelligence thus obtained was considered more important than that designated by the highest British security classification then used (Most Secret) and so was regarded as being Ultra secret. Several other cryptonyms had been used for such intelligence. British intelligence first designated it Boniface—presumably to imply that it was the result of human intelligence. The U.S. used the codename Magic for its decrypts from Japanese sources.
Much of the German cipher traffic was encrypted on the Enigma machine. Used properly, the German military Enigma would have been virtually unbreakable; in practice, shortcomings in operation allowed it to be broken. The term "Ultra" has often been used almost synonymously with "Enigma decrypts". However, Ultra also encompassed decrypts of the German Lorenz SZ 40/42 machines that were used by the German High Command, and the Hagelin machine and other Italian and Japanese ciphers and codes such as PURPLE and JN-25.
Many observers, at the time and later, regarded Ultra as immensely valuable to the Allies. Winston Churchill was reported to have told King George VI, when presenting to him Stewart Menzies (head of the Secret Intelligence Service and the person who controlled distribution of Ultra decrypts to the government): "It is thanks to the secret weapon of General Menzies, put into use on all the fronts, that we won the war!" F. W. Winterbotham quoted the western Supreme Allied Commander, Dwight D. Eisenhower, at war's end describing Ultra as having been "decisive" to Allied victory. Sir Harry Hinsley, Bletchley Park veteran and official historian of British Intelligence in World War II, made a similar assessment about Ultra, saying that it shortened the war "by not less than two years and probably by four years"; and that, in the absence of Ultra, it is uncertain how the war would have ended.
Since Ultra was revealed in the middle 1970s, historians have altered the historiography of World War II. For example, Andrew Roberts, writing in the 21st century, states, "Because of the invaluable advantage of being able to read Rommel's Enigma communications, Montgomery knew how short the Germans were of men, ammunition, food and above all fuel. When he put Rommel's picture up in his caravan he wanted to be seen to be almost reading his opponent's mind. In fact he was reading his mail".
^ a b Hinsley & Stripp 1993, p. xx.
^ Lewin 2001, p. 64.
^ Winterbotham 1974, pp. 154, 191.
^ Hinsley 1996.
^ Roberts 2009, p. 297.