The "Enigma machine" was a family of electromechanical rotor cipher-machines whose components and operating procedures evolved over time. It was the principal German military and civil cipher device from the late 1920s through World War II. Messages enciphered on Enigma were decrypted ("broken") from December 1932—just as Adolf Hitler was about to take power in Germany—by the Polish General Staff's Cipher Bureau and during World War II also by Great Britain's Bletchley Park ("Ultra") operation.
The foundations for Enigma decryption were laid by the mathematician Marian Rejewski soon after he joined the Cipher Bureau as a civilian cryptologist in September 1932. He used a mathematical theorem that has been called "the theorem that won World War II." Permutation theory and documents supplied by French military intelligence officer Gustave Bertrand, which had been bought from Hans-Thilo Schmidt ("Asché")—an employee of the German Armed Forces' Cipher Office—enabled Rejewski to reconstruct the wirings of the military Enigma's rotors and reflector. He guessed correctly (to the later chagrin of British cryptologist Alfred Dillwyn Knox) that the letters of the alphabet were wired into the machine's entry ring simply in alphabetical order.
With the Enigma reconstructed, Rejewski and fellow mathematician-cryptologists Henryk Zygalski and Jerzy Różycki devised techniques to regularly break messages. They used the "grill" and "clock," the "cyclometer" and its derivative "card catalog," and from late 1938 the "cryptological bomb" and "perforated sheets." They also exploited the machine's design quirks and German cipher clerks' security lapses. In January 1938 the Poles were reading a remarkable 75 percent of Enigma intercepts.
On 25 July 1939, in the Kabaty Woods south of Warsaw, with World War II a month off, the Poles revealed their achievements to British and French intelligence representatives. The British mathematician Gordon Welchman has written: "Ultra would never have gotten off the ground if we had not learned from the Poles, in the nick of time, the details both of the German military … Enigma machine, and of the operating procedures that were in use." Welchman subsequently contributed an important innovation to a British-produced "bomb": a "diagonal board" that increased the device's efficiency.
In September 1939 key Cipher Bureau personnel were evacuated from Poland via Romania to France. At "PC [Command Post] Bruno" outside Paris, during the "Phony War," they resumed breaking Enigma ciphers. They collaborated with Bletchley Park, eventually by teletype (for maximal security, they corresponded in Enigma, closing dispatches with a "Heil Hitler!"). The mathematicians were visited by Alan Turing, conceptual founder of the first programmable electronic computer, Colossus; the latter would come into use in late 1943 to break still more complex German ciphers that the British christened "Fish."
Enigma decryption created sustained opportunities (sometimes inadequately exploited) to look over enemy leaders' shoulders. It took time for Britain to master large-scale interception, decryption, translation, assessment, and secure distribution of Enigma traffic. Eventually Ultra provided the Allies near-unprecedented tactical and strategic advantages in the North African, Mediterranean, European, and Atlantic theaters, and—through official as well as clandestine sharing of Ultra intelligence with the Soviets—on the eastern front. Ultra enabled the Allies not only to counter German moves but also to deceive Germany about Allied intentions, most notably on the eve of the Normandy landings (June 1944).
Ultra's influence on the war has been assessed variously. A common opinion holds that Ultra shortened it by two years. Such estimates of duration, however, beg the question of outcome.
British prime minister Winston Churchill's greatest fear was that German submarine wolf packs might strangle his sea-locked country. The major factor that staved off Britain's defeat in the Battle of the Atlantic was its regained mastery of Naval-Enigma decryption, thanks to captures of Enigma machines and key tables from German submarines and weather trawlers. Had Britain capitulated, a United States deprived of the British Isles as a forward base could not have entered the European or North African theaters. The war in Europe would have been played out essentially between Germany and the Soviets. Germany, given two years' grace time, might have developed a crushing advantage in jet aircraft, missiles, perhaps atom bombs.
At war's end, Supreme Allied Commander Dwight D. Eisenhower described Ultra as having been "decisive" to Allied victory. Churchill told King George VI: "It was thanks to Ultra that we won the war."
Ultra remained secret, at Churchill's behest, until F. W. Winterbotham published The Ultra Secret in 1974.
Bertrand, Gustave. Enigma ou la plus grande énigme de la guerre 1939–1945. Paris, 1973.
Hinsley, F. H., and Alan Stripp, eds. Codebreakers: The Inside Story of Bletchley Park. Oxford, U.K., and New York, 1993.
Kahn, David. Seizing the Enigma: The Race to Break the German U-Boat Codes, 1939–1943. Boston, 1991.
Kozaczuk, Władysław. Enigma: How the German Machine Cipher Was Broken, and How It Was Read by the Allies in World War Two. Edited and translated by Christopher Kasparek. Frederick, Md., 1984.
Rejewski, Marian. "Remarks on Appendix 1 to [volume 1, 1979, of] British Intelligence in the Second World War [edited] by F. H. Hinsley." Translated from the Polish by Christopher Kasparek. Cryptologia 6, no. 1 (January 1982): 75–83.
Welchman, Gordon. The Hut Six Story: Breaking the Enigma Codes. New York, 1982.
Winterbotham, F. W. The Ultra Secret. New York, 1974.