Ultra and the breaking of Enigma

Ultra was the British codeword, adopted in June 1941, for intelligence obtained by breaking the highest-grade enemy machine ciphers. Most of it came from the German Enigma machine, but the label also covered the Lorenz teleprinter cipher used by the German high command and the Hagelin machine used by the Italian navy. The work centered on the Government Code and Cypher School, the forerunner of GC&CS and GCHQ, at Bletchley Park in Buckinghamshire⁶. Before June 1941 the material went out under other cover names: BONIFACE for army and air force decrypts, implying a well-placed agent in Berlin, and HYDRO for naval traffic⁶.

The Polish break

On 1 September 1932 the Polish General Staff's Cipher Bureau hired three young mathematicians from Poznań University: Marian Rejewski, Jerzy Różycki and Henryk Zygalski. In December 1932 Rejewski reconstructed the wiring of the German military Enigma, combining permutation theory with operating manuals and monthly keys that French intelligence had bought from a German informant, Hans-Thilo Schmidt⁷. The Poles then read German traffic for years, devising perforated sheets (Zygalski's method) and an electromechanical aid, the bomba, to recover daily settings⁷.

German changes to procedure late in 1938 cut deeply into Polish success. With war approaching and British and French guarantees to Poland in place, the Poles shared everything. On 25 and 26 July 1939, in the Pyry forest outside Warsaw, they briefed Alastair Denniston, head of GC&CS, and Dilly Knox, its leading Enigma specialist, together with French officers¹. GCHQ's own account says the visitors found the Poles far ahead of them, both in how much traffic they had read and in having built the first electromechanical device for cryptanalysis¹. Foreign Office historians judge that this saved the British at least a year².

Bletchley Park and the bombe

GC&CS moved to Bletchley Park as war began. In January 1940 Alan Turing traveled to France to meet the Polish cryptanalysts, who had escaped there after Poland fell; those conversations, GCHQ says, were fundamental to the design of the British bombe¹. Turing's machine was not a copy of the Polish bomba. It tested guessed plaintext, or "cribs," against the machine's possible settings, an approach that kept working after the Germans closed the loophole the Poles had exploited⁷. The first bombe, named Victory, arrived on 18 March 1940; there were about 210 by the end of the war⁷.

Naval Enigma was harder. Turing's Hut 8 broke into U-boat traffic in 1941, helped by material seized from the weather ships München and Lauenburg and from the submarine U-110⁷. On 1 February 1942 the U-boat network switched to a four-rotor machine, and the Atlantic went dark for most of a year. Regular reading resumed in December 1942, after code material was recovered from U-559⁷. From May 1943 the US Navy, with National Cash Register in Dayton, Ohio, produced 121 fast four-rotor bombes, and Anglo-American cooperation became routine⁷.

On 21 October 1941 Turing, Gordon Welchman, Hugh Alexander and Stuart Milner-Barry went over their superiors' heads and wrote directly to Churchill, saying that naval keys were being delayed by twelve hours a day for lack of clerks and typists. Churchill ordered at once that their needs be met as a matter of top priority, and by 18 November the head of Secret Intelligence Service (MI6) reported that staff were arriving fast; the papers are in the National Archives as HW 1/155³.

The Lorenz cipher, which the British called Tunny, was a separate problem. Bletchley broke it without ever seeing the machine, and from 1944 attacked it with Colossus, the electronic machine designed by the Post Office engineer Tommy Flowers^5,6. GCHQ says Colossus helped confirm that Hitler believed the invasion would come at the Pas-de-Calais rather than Normandy⁵.

Getting it to commanders safely

Using Ultra without betraying it was as hard as producing it. Army and air force summaries were written in Hut 3 and sent by the Secret Intelligence Service's radio network to Special Liaison Units attached to major headquarters. A liaison officer typically handed a summary to the commander, waited while he read it, and took it back to destroy⁶. Attacks were often given a cover story: before Axis supply ships bound for North Africa were struck, an aircraft would be sent to be seen "finding" them⁶. A postwar US Army study records that about 28 American officers served as Ultra advisers to commands in Europe under a letter of 15 March 1944 from General George Marshall to General Dwight Eisenhower, which required each to advise on using Ultra "in such fashion that the security of the source is not endangered"⁴.

What it achieved

Ultra fed directly into the The Double-Cross System: decrypts of Abwehr traffic showed that German intelligence trusted the double agents run by Security Service (MI5), which made deceptions such as Operation Mincemeat and Operation Fortitude possible and allowed their effect to be measured⁶. Decrypts also helped in the Mediterranean supply war, the Battle of the Atlantic, and Normandy⁶. For the parallel American effort against Japan, see Magic and the breaking of Purple. In July 1945 Eisenhower wrote to the SIS chief, Sir Stewart Menzies, that the intelligence had been "of priceless value to me"⁶.

Secrecy and disclosure

According to Winterbotham, Churchill asked recipients in May 1945 to keep the source secret, and they largely did for nearly three decades⁶. Polish and French accounts of the prewar break appeared in 1967 and 1973; the British ban was lifted in 1974, the year Winterbotham published The Ultra Secret⁶. The official history followed between 1979 and 1988⁶. A Director of GCHQ acknowledged in 2014 that the first volume, published in 1979, understated the Polish and French contribution, and that it took nine years to correct⁸. Postwar interrogation of German cryptographic staff (see TICOM) found that German experts had considered Enigma theoretically vulnerable but believed an attack too costly to be practical⁶.

Read next

20 February 1939 – September 1945 Magic and the breaking of Purple In September 1940 a small US Army team rebuilt Japan's top diplomatic cipher machine without ever seeing one. The decrypts, called Magic, shaped American policy before Pearl Harbor and fueled a long argument afterward. September 1939 – May 1945 The Double-Cross System How MI5 caught or recruited Germany's wartime spies in Britain and ran them as double agents, steered by the Twenty Committee, until their reports could carry the D-Day deception. January – September 1944 Operation Fortitude The Allied deception that invented armies in Scotland and southeast England in 1944 to make Germany expect landings in Norway and the Pas-de-Calais, and to see Normandy as a feint. 1915–1970s Cipher machines From Hebern's rotor to Enigma, Purple, Lorenz and SIGABA, the electromechanical machines that carried the secrets of the Second World War, and the codebreakers who read most of them. 1914–1990s Signals intelligence Intelligence from intercepted communications and electronic emissions, from Room 40 and the Zimmermann telegram to Ultra, Venona, the Berlin tunnel and NSA's domestic programs exposed by the Church Committee. August 1942 – 19 May 1944 The Lucy spy ring From Lucerne, the German émigré publisher Rudolf Roessler fed German high command secrets to Soviet intelligence in 1942–1943. Who his sources were, and how the material reached him, is still argued over.