General Heinz Guderian with an Enigma machine in a half-track being used as a mobile command center during the Battle of France, 1940

The main German cipher machine, derived from a Dutch invention that failed in several commercial models in the late 1920s. Various models of increasing complexity were used by the Heer, Luftwaffe, Kriegsmarine, and in diplomatic traffic. It was also used by the Reichsbahn (German railways). The Italian Navy used a derivative machine, the C38M. Polish intelligence partially broke Enigma ciphers in 1932. By 1939 the Poles had a foothold understanding of the original Dutch machine and therefore were able to rig replicas of its German descendants. The French also made headway from 1938. Polish intelligence Enigma replicas, and dearly acquired knowledge of German ciphers, were supplied by the Poles to the Western Allies in July 1939. The French and Poles passed additional information to the British in 1940. The British broke the naval code for the Italian C38M in September 1940, a year before that cipher was withdrawn. That greatly aided the Royal Navy in the Mediterranean naval campaign in 1940-1941. Naval Enigma rotors were recovered from a sunken minelayer U-boat off Scotland in February 1940. That told British intelligence that all German ships and U-boats carried them. Thereafter high priority was assigned to capture of U-boats and other enemy craft. German trawlers off Norway proved especially vulnerable: capture of Enigma code books or rotors from two trawlers led to breaking of the Kriegsmarine code. In May 1941, U-110’s Enigma machine was captured intact along with all code books. That and such capture or recovery successes were kept at the highest level of secrecy, including by deceit of captured U-boat crews or separate incarceration from other German prisoners.

The British built “bombes”-machines that mimicked and thus helped work out Enigma’s rotor sequences. There were never enough bombes to meet the demand of the code breakers at Bletchley Park, plus all the armed services and Britain’s clamoring allies. If the British had been more willing to provide technical information to the Americans-which they did not for mostly valid security reasons-it is conceivable that many more bombes would have been made much earlier. That was certainly Admiral Ernest King’s firm view, but in fairness King was not the most cooperative ally either. U. S. intelligence decided to make their own bombes in September 1942, with the first poor quality models available in May 1943. By the end of the year, 75 better quality bombes had been manufactured in the United States, greatly increasing code breaking capacity. It was still an infernal problem to decode: the two inner settings of the German naval cipher were set by officers only every two days, while naval cipher clerks changed the two outer settings every 24 hours. Enigma operators then chose three of the machine’s eight rotors, each of which had 26 point positions. All that provided 160 trillion potential combinations. On the receiving end, each U-boat had two nets of six frequencies each (“Diana” and “Hubertus”). And yet, Bletchley Park broke into the cipher.

The Kriegsmarine added a fourth rotor to its ciphers in January 1942, creating a prolonged “information blackout” that reduced enemy ability to detect wolf packs and divert convoys around them. The British made it a top priority to capture another machine from a U-boat or weather ship. U-559 was forced to the surface on October 30, 1942, by a sustained depth charge attack by five destroyers and destroyer escorts. Its documents were recovered, but the machine went down with the scuttled submarine. Still, it became clear that German operators were not fully utilizing the fourth rotor. An American ASW Support Group captured U-505 off Cape Verde in June 1944. The haul of Enigma material was enormous. It was also current and forward looking to new naval codes. Deciphering signals was greatly aided by COLOSSUS I, the first electronic computer put together by the brilliance of Alan Turing and engineers at Bletchley Park and elsewhere. It made processing and reading German ciphers faster than ever, often close to “real time.” COLOSSUS II came online in June 1944. A measure of how Enigma proved vulnerable to stiff-minded German overconfidence is the remarkable fact that the source of most intercepted signals, Admiral Karl Dönitz, went to his deathbed in 1980 convinced that no enemy ever read his Enigma ciphers.


“Secret writing machine.” Siemens & Halske T52 A German cipher machine that turned patterned holes in paper ribbons into transmittable radio pulses, or back into readable messages. Its 10-rotor system made the code-breaking task of British intelligence at Bletchley Park extremely difficult. The British did not break the Geheimschreiber until they developed the COLOSSUS I and II mechanical computers by mid-1944. When the Western Allies did break the code, they gleaned much information of high value, for the Wehrmacht used Geheimschreiber machines for its top-level headquarters’ communications.


“Station X.” The site of, and usual shorthand reference for, the British Code and Cypher School founded in 1919 and located about 80 miles north of London. During World War II it housed the critical code-breaking operation run by MI6. It employed some of the most brilliant British minds of the century-notably Alan Turing, inventor of the fi rst computer-as well as cryptanalysis specialists from Allied countries such as France, Poland, and the United States. The Americans actually took a long time to arrive and longer to be fully integrated: the first U. S. team did not reach Bletchley Park until April 25, 1943. Work at Bletchley Park was compartmentalized by “hut,” with groups in different huts listening to various of the hundreds of Luftwaffe, Kriegsmarine, or Wehrmacht codes. Signals were passed to code translators in Hut Three, which accepted its first Americans only in January 1944. There were over 10,000 people working on or otherwise supporting the extraordinarily complex and crucial work done at Bletchley Park by 1945. All their extraordinary work was kept secret for several decades after the war. Outposts of cryptanalysis tied to Bletchley Park were also maintained overseas, such as the “Combined Bureau, Middle East” in Cairo.


U.S. code for intercepts of Japanese diplomatic messages, and some military communications. This body of information is sometimes referred to as “the other ULTRA.” Cryptanalysis of the U.S. Army’s Secret Intelligence Service (SIS) broke Japanese “PURPLE” machine encryptions before the start of the Pacific War. The intercepts allowed American intelligence officers to read exchanges between Tokyo and the Japanese Embassy in Washington. While providing important insight into Japanese political and foreign policy thinking and relations, MAGIC did not provide operational or other “actionable” intelligence-mainly because Japanese diplomats were not told about Army or Navy operations in advance. MAGIC thus did not provide advance warning of the attacks on Pearl Harbor (December 7, 1941), the Philippines, or Hong Kong. MAGIC traffic from Japanese Embassy officials in Berlin and European neutral capitals provided indirect intelligence on German plans, including the build-up for BARBAROSSA in mid-1941. Useful information was gleaned from 1943 to 1944 about some secret Wehrmacht weapons research and about planned strategy and dispositions along the Atlantic Wall.


U.S. code name for the Japanese electronic cipher machine that encrypted diplomatic messages. That cipher traffic was broken and read by U. S. Army intelligence agents of the Secret Intelligence Service (SIS) by late September 1940. The intercepts that resulted were code named MAGIC. The U. S. gave a copy of their PURPLE decoder machine to the British, who then also read Japanese diplomatic ciphers. The Japanese never knew that their diplomatic traffic was read by the enemy. They revealed much of military value as a result.


“Very Special Intelligence.” Code name for the initially British system of interception and decryption of German signals intelligence from 1940. ULTRA also intercepted and decrypted Italian signals. Its intelligence was shared by the major Western Allies by formal agreement from mid-1943. Although the relationship was uneasy at first, it proved one of the major successes of the Anglo-American alliance by war’s end. The code term “ULTRA” was later applied to Allied interception of Japanese signals intelligence, though not to diplomatic or political intercepts. Vast amounts of German signals were spewed out by Enigma machines and Geheimschreiber machines used by a variety of German military, diplomatic, police, and intelligence sources. ULTRA understanding of some intercepts-the Germans used nearly 200 code ciphers during the war, many of which were never penetrated-was greatly aided by widespread and often sloppy enemy tradecraft, especially within the Luftwaffe. For instance, Luftwaffe and other German operators often repeated signals on the same topic at the same time, permitting content analysis to identify certain key terms or coded locations, which provided clues to penetrate deeper into the cipher. There was also much real heroism and risk taken by Allied agents, and sheer mental sharpness and perseverance by code breakers starting with Polish and French intelligence before the war.

Winston Churchill was a key supporter of British signals breaking. He read ULTRA reports daily. British ULTRA decrypts aided defense during the Battle of Britain in 1940, helped RAF Bomber Command carry out its extended bomber offensive, and significantly aided British 8th Army win the desert campaigns (1940-1943): intercepts revealed German logistics problems and allowed the Royal Navy and RAF to further cripple supply. Probably the single most critical contribution of ULTRA was to support Allied victory over the U-boats in the Battle of the Atlantic (1939-1945). German historian Jürgen Rohwer estimates that ULTRA intercepts reduced Allied shipping losses by 65 percent as early as the end of 1941. ULTRA intelligence was also key to understanding to what degree deception operations succeeded or failed in land campaigns, up to the level of directly influencing the operational and strategic thinking of Adolf Hitler. Notable confirmation of deception success came in the BARCLAY and related MINCEMEAT operations, and for a series of critical deceptions called COCKADE. Unknown to the Western Allies, John Cairncross was a Soviet double agent in place inside Bletchley Park and MI6. He fed Moscow ULTRA intercepts that contributed directly to the Red Army’s success at Kursk.

Such important successes made ULTRA one of the top secrets of the war. ULTRA was so crucial that some operations that might have been undertaken were not, out of fear of revealing to the Germans that Enigma codes were compromised: ULTRA was just too strategically important to risk for any one tactical or operational gain. ULTRA not only aided operations, it helped shape Allied strategy at the highest levels of leadership. The secret of ULTRA was kept by at least 20,000 people for over 30 years. It was not until the 1970s that the first quasi-official accounts were authorized, and not until 1988 that the British official history astonished the historiographical world with rich detail that illuminated and altered understanding of many key events of the war.

Suggested Reading: David Khan, Seizing the Enigma (1995). R. Lewin, The American Magic (1982). Ralph Bennett, Behind the Battle (1994); F. H. Hinsley, British Intelligence in the Second World War (1979-1990); Simon Singh, The Code Book (1999).

50 Books on World War II Recommended by John Keegan

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