Bletchley Park and Enigma: Breaking Germany's Secret Codes

Bletchley Park, a Victorian country house in Buckinghamshire, England, was the centre of British codebreaking during the Second World War. From 1939 to 1945 the staff of the Government Code and Cypher School broke the German Enigma and Lorenz ciphers on a vast scale, producing intelligence known as Ultra. Their work influenced battles from the Atlantic to North Africa and Normandy, and it also helped give birth to the electronic computer.

Background: Enigma and the Polish breakthrough

Enigma was a portable electromechanical cipher machine adopted by the German armed forces from the late 1920s. Each key press sent an electrical current through a set of rotating wheels, called rotors, and a plugboard, scrambling the letter in a way that changed with every key stroke. The number of possible settings was so vast that the Germans believed the system was secure.

The first breakthrough came in Poland. In late 1932 the mathematician Marian Rejewski of the Polish Cipher Bureau reconstructed the wiring of the army Enigma, using mathematical analysis and documents obtained by French intelligence. With Jerzy Różycki and Henryk Zygalski he developed methods and a machine called the bomba to recover daily settings. In late July 1939, at Pyry near Warsaw, the Poles shared their results and replica Enigma machines with British and French codebreakers, weeks before the invasion of Poland.

Bletchley Park at war

Admiral Sir Hugh Sinclair, head of the Secret Intelligence Service, acquired Bletchley Park in 1938 as a war station safe from bombing in London. It lay conveniently close to railway lines between Oxford and Cambridge and to main telephone routes. The Government Code and Cypher School, headed by Alastair Denniston, moved there in August 1939.

Staff were recruited from universities, the armed forces and wider society, including mathematicians, linguists, chess players and, famously, people who did well in a newspaper crossword test. Work was organised in wooden huts and later brick blocks. Hut 6 broke army and air force Enigma, Hut 8 worked on naval Enigma, and Huts 3 and 4 turned the decrypts into intelligence. Intercepts came in from listening stations such as those of the “Y” service around Britain.

The bombe and naval Enigma

Alan Turing, a Cambridge mathematician, designed the British bombe, building on the Polish idea but using a different method. It searched for Enigma settings that matched a crib, a short piece of text the codebreakers expected to appear in a message. Gordon Welchman added the “diagonal board”, which made the machine much more efficient. The first bombe began work in March 1940, and over 200 were built by the end of the war, most operated by members of the Women’s Royal Naval Service.

Naval Enigma was harder to break because the German navy used stricter procedures. Turing led the work in Hut 8, aided by captured material from German ships and U-boats such as U-110 in May 1941. Reading these signals allowed the Admiralty to steer convoys away from U-boat patrol lines in the Battle of the Atlantic. When the U-boats introduced a four-rotor Enigma in February 1942, the codebreakers were blinded for most of the year until captured documents helped them back in.

In October 1941 Turing, Welchman, Hugh Alexander and Stuart Milner-Barry wrote directly to Winston Churchill complaining of shortages of staff. Churchill ordered that they be given all they needed with extreme priority, marking his minute “Action this day”.

Lorenz and Colossus

Hitler and the German high command used a different system for their most important communications: the Lorenz teleprinter cipher, which the British called Tunny. Without ever seeing the machine, the mathematician Bill Tutte worked out its logical structure in 1942 from intercepted traffic. To speed up the work, the Post Office engineer Tommy Flowers designed Colossus, which used around 1,600 to 2,400 valves (vacuum tubes). The first Colossus was working at Bletchley early in 1944, an improved version was ready just before D-Day, and ten were in use by the end of the war.

Ultra and its impact

Intelligence from Bletchley was given the codename Ultra and shared with only a few commanders, often disguised as coming from spies or reconnaissance. It contributed to the Royal Navy’s victory at Cape Matapan in 1941, to the fighting in North Africa and to the D-Day landings, where decrypts confirmed that the Germans believed Allied deception plans. Historian Harry Hinsley, who worked at Bletchley, suggested that Ultra shortened the war by around two to four years, though other historians consider such estimates impossible to prove.

Key people

• Alan Turing designed the bombe and led the attack on naval Enigma in Hut 8. He later worked on early computers in Manchester and on mathematical biology.
• Gordon Welchman, a Cambridge mathematician, organised Hut 6 and devised the diagonal board and much of the system for handling traffic on an industrial scale.
• Dilly Knox, a veteran of First World War codebreaking, broke the Italian naval Enigma, which helped at Cape Matapan, and the Enigma used by the German military intelligence service, the Abwehr, in December 1941, shortly before his death in 1943.
• Marian Rejewski, with Jerzy Różycki and Henryk Zygalski, made the original Polish breakthrough; the three continued working in France and Rejewski and Zygalski later reached Britain.
• Tommy Flowers and Bill Tutte were respectively the engineer who built Colossus and the mathematician who reconstructed the Lorenz machine.
• Joan Clarke, one of the few women codebreakers in Hut 8, worked closely with Turing and later became deputy head of the hut.

Secrecy and legacy

Thousands of staff kept their work secret for decades. The story became public only in 1974, with F. W. Winterbotham’s book The Ultra Secret. Bletchley’s successor, GCHQ, continued the work after the war. Alan Turing, a pioneer of computer science whose ideas went back to his paper on computable numbers, was prosecuted in 1952 for homosexual acts and died in 1954. He received a posthumous royal pardon in 2013. Bletchley Park was saved from demolition and opened as a museum in the 1990s.

Myths and facts

Myth: Enigma was broken by a single genius or a single machine. It was a team effort over many years, beginning in Poland and involving thousands of people and many techniques. German operators’ mistakes, such as predictable message openings and repeated settings, were just as important as clever machines.

Myth: once broken, Enigma stayed broken. Each German service used its own networks and keys, which changed daily, and new procedures could shut the codebreakers out for months. Some networks were never read regularly.

Myth: an American submarine captured the first Enigma. The 2000 film U-571 invented this. The real captures of Enigma material, such as from U-110, were made by the Royal Navy.

Timeline

• 1932, December: Marian Rejewski reconstructs the wiring of the German army Enigma.
• 1939, late July: Polish codebreakers share their Enigma work with Britain and France at Pyry.
• 1939, August: The Government Code and Cypher School moves to Bletchley Park.
• 1940, March: The first British bombe begins operation.
• 1941, May: Enigma material is captured from U-110.
• 1941, October: Turing and colleagues write to Churchill, who orders “Action this day”.
• 1944, early: The first Colossus begins breaking Lorenz messages.
• 1974: The Ultra Secret reveals Bletchley’s wartime role to the public.

Why it matters

Bletchley Park showed how intelligence, mathematics and engineering could influence the course of a world war. It accelerated the development of electronic computing, created a model for modern signals intelligence and, through the long silence of its staff, became one of the most remarkable kept secrets of the twentieth century.