Ultra (cryptography)

• 1. Introduction

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  Ultra was the name the British gave to one of the most closely guarded secrets of the Second World War: the ability to read the enemy's most sensitive encrypted communications. Adopted in June 1941 by British military intelligence, the designation came from a simple idea. The intelligence obtained was considered more important than anything covered by the highest British security classification of the time, which was called Most Secret. So it was regarded as being Ultra Secret, and the name stuck.

  At the heart of the operation was Bletchley Park, the Government Code and Cypher School in the English countryside. There, mathematicians, linguists, engineers, and clerks worked in relentless secrecy to unravel the coded radio traffic of Nazi Germany, Fascist Italy, and Imperial Japan. The name Ultra eventually became the standard designation among the western Allies for all such intelligence derived from broken enemy communications.

  Winston Churchill reportedly told King George VI that it was thanks to the secret weapon of General Menzies, put into use on all the fronts, that the Allies won the war. Dwight D. Eisenhower, at the end of hostilities, described Ultra as having been decisive to Allied victory. The official historian of British Intelligence, Sir Harry Hinsley, offered a more measured verdict: without Ultra, the war would have been something like two to four years longer. What was the secret that Churchill and Eisenhower were guarding so fiercely? How did ordinary codebreakers crack machines the Germans believed were unbreakable? And why did the world have to wait until 1974 to find out?

• 2. The Enigma Machine

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  Marian Rejewski and the Polish Cipher Bureau first broke German military Enigma in December 1932. That single fact reshapes the story of the war entirely. The achievement required brilliant mathematics, the services of a spy inside the German office responsible for administering encrypted communications, and what the sources describe simply as good luck.

  Enigma was a family of electro-mechanical rotor cipher machines that produced a polyalphabetic substitution cipher. A variant of the commercial Model D was first used by the Reichswehr in the 1920s, when it was widely considered unbreakable. The German Army, Navy, Air Force, Nazi party, Gestapo, and German diplomats all used versions of the machine, though each variant was configured differently and required different cryptanalytic treatment.

  The Poles read Enigma through the outbreak of the Second World War and continued their work in France. Then, at the turn of 1939, Germany made its systems ten times more complex. The increase in complexity required a tenfold increase in Polish decryption equipment, which the Poles could not meet. On the 25th of July 1939, the Polish Cipher Bureau handed reconstructed Enigma machines and their decryption techniques to the French and British. Gordon Welchman, one of Bletchley Park's most important figures, later wrote that Ultra would never have got off the ground without what the Poles provided in the nick of time.

  At Bletchley Park, the key figures against Enigma included mathematicians Alan Turing and Hugh Alexander, and Harold Keen, chief engineer at the British Tabulating Machine Company. German cryptographic personnel, interrogated after the war, believed that attacking Enigma was theoretically possible but would require impracticable amounts of effort and investment. That belief proved fatally wrong.

• 3. Lorenz and the Tunny Machines

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  In June 1941, Germany began introducing a different category of cipher system entirely: on-line stream cipher teleprinter machines for strategic point-to-point radio links. The British gave the family the codename Fish. The principal system was the Lorenz SZ 40/42, which Bletchley Park named Tunny.

  Breaking Tunny was, in many respects, harder than breaking Enigma. The British codebreakers had no knowledge of the machine producing the traffic and none of the head-start the Poles had given them against Enigma. The work eventually required Colossus machines, the first digital programme-controlled electronic computers.

  US Army Lieutenant Arthur J. Levenson, who worked on both Enigma and Tunny at Bletchley Park, described in a 1980 interview what the Tunny decrypts revealed about the Normandy defences. Rommel had been appointed Inspector General of the West and had inspected all the defences along the Normandy beaches, then sent a detailed message that Levenson estimated was 70,000 characters long. Bletchley Park decrypted it as a small pamphlet before D-Day: how wide the V-shaped trenches were to stop tanks, how much barbed wire, every detail of the Western defences.

  The volume of Tunny intelligence was much smaller than that from Enigma, but its strategic weight was often far higher. It carried communications between Wehrmacht high command. The eventual bulk decryption of Lorenz-enciphered messages contributed significantly, and perhaps decisively, to the defeat of Nazi Germany. Key figures in the Tunny effort included mathematicians W. T. "Bill" Tutte and Max Newman and electrical engineer Tommy Flowers, though their contribution remained far less known to the public than the Enigma story.

• 4. Keeping the Secret

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  Knowing an enemy's communications is only useful if the enemy doesn't know you know them. The British and Americans went to elaborate lengths to protect what they called the source.

  A cover name, Boniface, was used for Ultra from the beginning. British intelligence created a fictional MI6 master spy named Boniface, who supposedly controlled a network of agents throughout Germany. Intelligence obtained through codebreaking was attributed to the human intelligence network he ran. This protected Ultra from curious Allied officers who had no need to know the real source.

  Deception around naval intelligence reached extraordinary levels during the Battle of the Atlantic. In most cases where Bletchley Park's intercepts revealed the location of a U-boat in mid-Atlantic, the U-boat was not attacked immediately. A cover story had to be arranged first. A search plane might be sent out to make a plausible sighting, so that an Allied attack would seem to follow from conventional reconnaissance rather than broken ciphers.

  For attacks on Axis supply ships bound for North Africa, spotter submarines and aircraft were sent ahead to be observed by the Axis, who concluded their ships were being found by conventional means. The Axis forces came to believe there were roughly 400 Allied submarines in the Mediterranean and a huge fleet of reconnaissance aircraft on Malta. In reality, there were only 25 submarines, and at times as few as three aircraft.

  Admiral Karl Dönitz grew suspicious. He received reports of impossible encounters between U-boats and enemy vessels and on one occasion, three U-boats that had met at a tiny island in the Caribbean Sea were promptly found by a British destroyer. Dönitz ordered reviews of Enigma security, but the analyses concluded that the problem, if there was one, was not the Enigma itself. His own codebreaking group, B-Dienst, had partially broken Royal Navy traffic, which gave Dönitz reason to believe the Allies were similarly unable to read German ciphers. By 1945, most German Enigma traffic could be decrypted within a day or two, yet the Germans remained confident of its security.

• 5. The Pacific War and Allied SIGINT

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  Ultra was the designation for European theatre signals intelligence, but the western Allies were reading enemy ciphers across the globe. The United States used the codename Magic for decrypts from Japanese sources, including the Purple cipher.

  Purple was the name American codebreakers gave to the Japanese diplomatic cipher machine. Unlike Enigma, it was not a rotor machine; it was built around electrical stepping switches. It was broken by the US Army Signal Intelligence Service. The Japanese ambassador to Germany sent detailed reports encrypted on Purple, including reviews of German assessments of the military situation, reports on direct inspections of coastal defences (including, in one case, the Normandy beaches), and accounts of long interviews with Hitler.

  The chief fleet communications code of the Imperial Japanese Navy was called JN-25 by the Americans. By early 1942, the US Navy had made considerable progress decrypting Japanese naval messages. A JN-25 decrypt allowed the US to turn back a Japanese offensive in the Battle of the Coral Sea in April 1942 and set up the American victory at the Battle of Midway in June 1942.

  A JN-25 decrypt of the 14th of April 1943 provided details of Admiral Yamamoto's forthcoming visit to Balalae Island. On the 18th of April, a year to the day after the Doolittle Raid, his aircraft was shot down. Ship position reports in the Japanese Army's "2468" water transport code, decrypted by the Signal Intelligence Service starting in July 1943, helped US submarines and aircraft sink two-thirds of the Japanese merchant marine. The Italian Navy's use of the Hagelin rotor-based cipher machine C-38 was broken from June 1941 onwards by the Italian subsection of GC&CS at Bletchley Park.

• 6. Delivering Intelligence to Commanders

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  Breaking a cipher was only the beginning. The raw decrypts were often about relative trivia. Turning intercepted radio traffic into intelligence that generals and admirals could use was a subtle and labour-intensive task.

  At Bletchley Park, extensive indices tracked everything in the decrypted messages: radio frequencies, times of intercept, callsigns, unit names, ship names, weapons, technical terms, and repeated German military phrases that might be useful as cribs for future decryption. By the second half of 1941-30,000 Enigma messages a month were being deciphered, rising to 90,000 a month of Enigma and Fish decrypts combined later in the war.

  Distributing that intelligence to field commanders involved its own risks. Ultra summaries from Bletchley Park were sent over landline to a radio transmitter at Windy Ridge, operated by Section VIII of MI6 at Whaddon Hall in Buckinghamshire. From there they were transmitted to Special Liaison Units, or SLUs, attached to major army and air force commands. The activity was organised and supervised by Group Captain F. W. Winterbotham. The standard procedure was for the liaison officer to present the intelligence summary to the commander, remain with him while he studied it, then take it back and destroy it.

  By the end of the war, around 40 SLUs were serving commands around the world. The first mobile SLUs appeared during the French campaign of 1940. The first liaison officers attached to the British Expeditionary Force were Robert Gore-Browne and Humphrey Plowden. The codeword Ultra itself was reportedly suggested by Commander Geoffrey Colpoys of the Royal Navy's Operational Intelligence Centre, where it replaced the earlier codeword Hydro used for naval decrypts.

• 7. Women at Bletchley Park

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  Some 8,000 women worked at Bletchley Park, making up about three quarters of the workforce. Their contribution has been among the least publicly recognised aspects of the Ultra story.

  After encryption systems were broken, an enormous volume of work remained: recovering daily key settings, keeping pace with changes in enemy security procedures, processing, translating, indexing, analysing and distributing tens of thousands of intercepted messages every day. The more successful the codebreakers became, the more labour that effort required.

  Before the attack on Pearl Harbor, the US Navy sent letters to leading women's colleges seeking introductions to their best graduating seniors. The Army soon followed. By the end of the war, some 7,000 workers in the Army Signal Intelligence Service, out of a total workforce of 10,500, were female.

  The contrast with the Axis powers was stark. Germany and Japan had strong ideological objections to women engaging in war work. The Nazis created the Cross of Honour of the German Mother specifically to encourage women to stay at home and have children. That ideological choice denied the Axis powers the kind of analytical workforce that kept Ultra running day after day, year after year, even as the volume of intercepted traffic grew.

• 8. Secrecy, Disclosure, and Legacy

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  Two weeks after V-E Day, on the 25th of May 1945, Churchill requested that former recipients of Ultra intelligence not divulge the source or the information they had received from it. The aim was to protect future operations of the Secret Service and to deny the Axis any basis to blame Ultra for their defeat. The request was honoured for nearly three decades.

  The thirty-year rule meant no official mention of Bletchley Park appeared in public records until the mid-1970s. One practical reason for the silence was that after the war, surplus Enigma machines and Enigma-like devices were sold to countries in the developing world, whose governments remained convinced of the security of the remarkable cipher machines. Their traffic was being read; selling them the machines helped conceal that fact. Switzerland developed its own version of Enigma, called NEMA, and used it into the late 1970s. The United States National Security Agency retired the last of its rotor-based encryption systems, the KL-7 series, in the 1980s.

  Partial disclosures came first from non-British sources. In 1967, Polish military historian Wladyslaw Kozaczuk published Bitwa o tajemnice, revealing that Polish cryptologists had broken Enigma before the war. Also in 1967, David Kahn's The Codebreakers hinted at the scale of Anglo-American Enigma operations without naming Bletchley Park. The British ban was finally lifted in 1974, when F. W. Winterbotham published The Ultra Secret, the first English-language account by a direct participant.

  In 2012, Alan Turing's last two papers on Enigma decryption were released to Britain's National Archives. The GCHQ departmental historian stated that the seven decades' delay had been due to their continuing sensitivity, adding that it would not have been safe to release them earlier. In 1953, the CIA's Project ARTICHOKE was renamed Project MKUltra; the Ultra component referenced the wartime signals intelligence programme directly. The codename born in a country house in Buckinghamshire had reached into the Cold War.