Military Enigma machine, model "Enigma I", used during the late 1930s and during the war; displayed at Museo Nazionale Scienza e Tecnologia Leonardo da Vinci, Milan, Italy
The mansion in 2017
Close-up of the rotors in a cipher machine
A memorial to Marian Rejewski, the mathematician who first broke Enigma and educated the British and French about Polish methods of cryptanalysis
Stephen Kettle's 2007 Alan Turing statue
The headquarters of GCHQ in 2017
First page of Al-Kindi's 9th century Manuscript on Deciphering Cryptographic Messages
Hut 6 at Bletchley Park in 2004
Enigma in use, 1943
Flow of information from an intercepted Enigma message
Allidina Visram school in Mombasa, pictured above in 2006, was the location of the British "Kilindini" codebreaking outpost during World War II
The decrypted Zimmermann Telegram.
Enigma wiring diagram with arrows and the numbers 1 to 9 showing how current flows from key depression to a lamp being lit. The A key is encoded to the D lamp. D yields A, but A never yields A; this property was due to a patented feature unique to the Enigmas, and could be exploited by cryptanalysts in some situations.
Average daily number of Signals to Commands Abroad
NUCPS banner on march in Cheltenham 1992
The Bombe replicated the action of several Enigma machines wired together. Each of the rapidly rotating drums, pictured above in a Bletchley Park museum mockup, simulated the action of an Enigma rotor.
The scrambling action of Enigma's rotors is shown for two consecutive letters with the right-hand rotor moving one position between them.
Hut 1
Enigma rotor assembly. In the Wehrmacht Enigma, the three installed movable rotors are sandwiched between two fixed wheels: the entry wheel, on the right, and the reflector on the left.
Hut 4, adjacent to the mansion, is now a bar and restaurant for the museum.
Three Enigma rotors and the shaft, on which they are placed when in use.
Hut 6 in 2004
Two Enigma rotors showing electrical contacts, stepping ratchet (on the left) and notch (on the right-hand rotor opposite D).
Bletchley's Polish Memorial, commemorating "the [prewar] work of Marian Rejewski, Jerzy Różycki and Henryk Zygalski, mathematicians of the Polish intelligence service, in first breaking the Enigma code. Their work greatly assisted the Bletchley Park code breakers and contributed to the Allied victory in World WarII."
The Enigma stepping motion seen from the side away from the operator. All three ratchet pawls (green) push in unison as a key is depressed. For the first rotor (1), which to the operator is the right-hand rotor, the ratchet (red) is always engaged, and steps with each keypress. Here, the middle rotor (2) is engaged, because the notch in the first rotor is aligned with the pawl; it will step (turn over) with the first rotor. The third rotor (3) is not engaged, because the notch in the second rotor is not aligned to the pawl, so it will not engage with the rachet.
The working rebuilt bombe, built by a team led by John Harper and switched on by the Duke of Kent, patron of the British Computer Society, on 17 July 2008. This is now located at The National Museum of Computing in Block H on Bletchley Park.
Internal mechanism of an Enigma machine showing the type B reflector and rotor stack.
A Mark 2 Colossus computer. The ten Colossi were the world's first (semi-) programmable electronic computers, the first having been built in 1943
The plugboard (Steckerbrett) was positioned at the front of the machine, below the keys. When in use during World War II, there were ten connections. In this photograph, just two pairs of letters have been swapped (A↔J and S↔O).
The stableyard cottages, where Alan Turing worked
The Schreibmax was a printing unit which could be attached to the Enigma, removing the need for laboriously writing down the letters indicated on the light panel.
Rear of the rebuilt Bombe
The Enigma Uhr attachment
The Story of Enigma workshop with Middlesex University students
German Kenngruppenheft (a U-boat codebook with grouped key codes).
Tony Sale supervising the breaking of an enciphered message with the completed Colossus computer rebuild in 2006 at The National Museum of Computing
Monthly key list number 649 for the German Air Force Enigma, including settings for the reconfigurable reflector (which only change once every eight days).
Commemorative medal for those working at Bletchley Park
Figure 2. With the inner lid down, the Enigma was ready for use. The finger wheels of the rotors protruded through the lid, allowing the operator to set the rotors, and their current position, here RDKP, was visible to the operator through a set of windows.
German U-boat model used in the film Enigma (2001)
Scherbius Enigma patent,, granted in 1928.
The Sunbeam-Talbot 2-litre used in the film Enigma (2001)
Typical glowlamps (with flat tops), as used for Enigma.
A rare 8-rotor printing Enigma model H (1929).
Heinz Guderian in the Battle of France, with an Enigma machine. Note one soldier is keying in text while another writes down the results.
Enigma in use on the Russian front
A three-rotor Enigma machine on display at Computer Museum of America and its two additional rotors.
Surviving three-rotor Enigma on display at Discovery Park of America in Union City, Tennessee, U.S.
A four-rotor Kriegsmarine (German Navy, 1. February 1942 to 1945) Enigma machine on display at the U.S. National Cryptologic Museum
A four-rotor Kriegsmarine Enigma machine on display at the Museum of the Second World War, Gdańsk, Poland
Enigma G, used by the Abwehr, had four rotors, no plugboard, and multiple notches on the rotors.
The German-made Enigma-K used by the Swiss Army had three rotors and a reflector, but no plugboard. It had locally re-wired rotors and an additional lamp panel.
An Enigma model T (Tirpitz), a modified commercial Enigma K manufactured for use by the Japanese.
An Enigma machine in the UK's Imperial War Museum
Enigma in use in Russia
Enigma in radio car of the 7th Panzer Div. staff, August 1941
A Japanese Enigma clone, codenamed GREEN by American cryptographers.
Tatjana van Vark's Enigma-inspired rotor machine.
Electronic implementation of an Enigma machine, sold at the Bletchley Park souvenir shop

During World War II, he worked at Britain's secret codebreaking centre, "Station X" at Bletchley Park, where he was one of the most important contributors.

- Gordon Welchman

During World War II, the estate housed the Government Code and Cypher School (GC&CS), which regularly penetrated the secret communications of the Axis Powers – most importantly the German Enigma and Lorenz ciphers.

- Bletchley Park

The GC&CS team of codebreakers included Alan Turing, Gordon Welchman, Hugh Alexander, Bill Tutte, and Stuart Milner-Barry.

- Bletchley Park

During the Second World War it was located at Bletchley Park, where it was responsible for breaking the German Enigma codes.


Even though the goal has been the same, the methods and techniques of cryptanalysis have changed drastically through the history of cryptography, adapting to increasing cryptographic complexity, ranging from the pen-and-paper methods of the past, through machines like the British Bombes and Colossus computers at Bletchley Park in World War II, to the mathematically advanced computerized schemes of the present.

- Cryptanalysis

Just before World War II, Welchman was invited by Commander Alastair Denniston to join the Government Code and Cypher School in the event of war.

- Gordon Welchman

Sigint missions: comprising maths and cryptanalysis, IT and computer systems, linguistics and translation, and the intelligence analysis unit


However, Welchman's main contributions were to the process of breaking the German Enigma machine cipher.

- Gordon Welchman

Around December 1932 Marian Rejewski, a Polish mathematician and cryptologist at the Polish Cipher Bureau, used the theory of permutations, and flaws in the German military-message encipherment procedures, to break message keys of the plugboard Enigma machine.

- Enigma machine

Senior staff included Alastair Denniston, Oliver Strachey, Dilly Knox, John Tiltman, Edward Travis, Ernst Fetterlein, Josh Cooper, Donald Michie, Alan Turing, Gordon Welchman, Joan Clarke, Max Newman, William Tutte, I. J. (Jack) Good, Peter Calvocoressi and Hugh Foss.


Gordon Welchman, who became head of Hut 6 at Bletchley Park, has written: "Hut 6 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 version of the commercial Enigma machine, and of the operating procedures that were in use."

- Enigma machine

The Polish transfer of theory and technology at Pyry formed the crucial basis for the subsequent World War II British Enigma-decryption effort at Bletchley Park, where Welchman worked.

- Enigma machine

Key GC&CS cryptanalysts who moved from London to Bletchley Park included John Tiltman, Dillwyn "Dilly" Knox, Josh Cooper, Oliver Strachey and Nigel de Grey.

- Bletchley Park

During World War I, inventors in several countries developed rotor cipher machines such as Arthur Scherbius' Enigma, in an attempt to minimise the repetition that had been exploited to break the Vigenère system.

- Cryptanalysis

Governments have long recognized the potential benefits of cryptanalysis for intelligence, both military and diplomatic, and established dedicated organizations devoted to breaking the codes and ciphers of other nations, for example, GCHQ and the NSA, organizations which are still very active today.

- Cryptanalysis

In 1927 Hugh Foss at the British Government Code and Cypher School was able to show that commercial Enigma machines could be broken, provided suitable cribs were available.

- Enigma machine

500 related topics


Alan Turing

Turing c. undefined 1928 at age 16
King's College, Cambridge, where Turing was an undergraduate in 1931 and became a Fellow in 1935. The computer room is named after him.
Two cottages in the stable yard at Bletchley Park. Turing worked here in 1939 and 1940, before moving to Hut 8.
A working replica of a bombe now at The National Museum of Computing on Bletchley Park
Statue of Turing by Stephen Kettle at Bletchley Park, commissioned by Sidney Frank, built from half a million pieces of Welsh slate.
Plaque, 78 High Street, Hampton
A blue plaque on the house at 43 Adlington Road, Wilmslow where Turing lived and died
Turing's OBE currently held in Sherborne School archives
The Alan Turing Building at the University of Manchester in 2008
Turing memorial statue plaque in Sackville Park, Manchester

Alan Mathison Turing (23 June 1912 – 7 June 1954) was an English mathematician, computer scientist, logician, cryptanalyst, philosopher, and theoretical biologist.

During the Second World War, Turing worked for the Government Code and Cypher School (GC&CS) at Bletchley Park, Britain's codebreaking centre that produced Ultra intelligence.

Here, he devised a number of techniques for speeding the breaking of German ciphers, including improvements to the pre-war Polish bombe method, an electromechanical machine that could find settings for the Enigma machine.

The bombe, with an enhancement suggested by mathematician Gordon Welchman, became one of the primary tools, and the major automated one, used to attack Enigma-enciphered messages.

Conel Hugh O'Donel Alexander

C. H. O'D. Alexander

Conel Hugh O'Donel Alexander (19 April 1909 – 15 February 1974), known as Hugh Alexander and C. H. O'D. Alexander, was an Irish-born British cryptanalyst, chess player, and chess writer.

He worked on the German Enigma machine at Bletchley Park during the Second World War, and was later the head of the cryptanalysis division at GCHQ for 25 years.

Other senior colleagues included Stuart Milner-Barry, Gordon Welchman, and Harry Golombek.

Alastair Denniston

Flag of a Lieutenant commander, Royal Navy

Commander Alexander "Alastair" Guthrie Denniston (1 December 1881 – 1 January 1961) was a Scottish codebreaker in Room 40, deputy head of the Government Code and Cypher School (GC&CS) and hockey player.

Following the practice of his superiors at Room 40, he contacted lecturers at Oxford and Cambridge (including Alan Turing and Gordon Welchman) asking if they would be willing to serve if war broke out.

He chose Bletchley as the location for the codebreaking effort because it was at a rail junction on a main line 47 mi north of London with good rail connections to Oxford and Cambridge and MI6 boss, Hugh Sinclair, acquired the Bletchley Park property.

On 26 July 1939, five weeks before the outbreak of war, Denniston was one of three Britons (along with Dilly Knox and Humphrey Sandwith) who participated in the trilateral Polish-French-British conference held in the Kabaty Woods south of Warsaw, at which the Polish Biuro Szyfrów (Cipher Bureau) initiated the French and British into the decryption of German military Enigma ciphers.

Joan Clarke

Clarke, c. 1940s

Joan Elisabeth Lowther Murray, MBE (née Clarke; 24 June 1917 – 4 September 1996) was an English cryptanalyst and numismatist best known for her work as a code-breaker at Bletchley Park during the Second World War.

Clarke's mathematical abilities were first discovered by Gordon Welchman, in an undergraduate Geometry class at Cambridge.

The Enigma machine was invented by the Germans to encrypt their messages; they strongly believed their code was unbreakable.

In June 1940, Clarke was recruited by her former academic supervisor, Gordon Welchman, to the Government Code and Cypher School (GC&CS).


A wartime picture of a Bletchley Park Bombe
A three-rotor Enigma with plugboard (Steckerbrett)
Depiction of a series of three rotors from an Enigma machine
The plugboard of an Enigma machine, showing two pairs of letters swapped: S–O and A–J. During World War II, ten plugboard connections were made.
A German Enigma key list with machine settings for each day of one month
The working rebuilt bombe now at The National Museum of Computing on Bletchley Park. Each of the rotating drums simulates the action of an Enigma rotor. There are 36 Enigma-equivalents and, on the right-hand end of the middle row, three indicator drums. John Harper led the "Phoenix" team that rebuilt this Bombe. It was officially switched on by the Duke of Kent, patron of the British Computer Society on 17 July 2008.
Drums on the rebuilt Bombe in action. The upper drums all rotate continuously and in synchrony.
The three drums of one of the 36 Enigma-equivalents, and the mounting plates for another, showing the 104 contacts for the wire brushes on the back of the drums. The top drum corresponds to the left-hand Enigma rotor, the middle drum to the middle rotor and the bottom drum to the right-hand rotor.
Wire brushes on the back of a drum from the rebuilt Bombe.
The letters of a crib and ciphertext expressed as a graph to provide a menu which specifies how to set up a bombe run. This example is somewhat unusual in that it contains as many as three loops.
Bombe menu based on Bletchley Park display board which gives credit to Peggy Erskine-Tulloch as the originator.
Rear view of the rebuilt Bombe. This shows the patch panels and 26-way cables used to wire up the 'menus'. It includes the 'diagonal boards' which, despite their name, are physically rectangular.
The US Navy Bombe contained 16 four-rotor Enigma-analogues and was much faster than the British three-rotor Bombes.

The bombe was an electro-mechanical device used by British cryptologists to help decipher German Enigma-machine-encrypted secret messages during World War II.

The initial design of the British bombe was produced in 1939 at the UK Government Code and Cypher School (GC&CS) at Bletchley Park by Alan Turing, with an important refinement devised in 1940 by Gordon Welchman.


A typical Bletchley intercept sheet, before decryption and translation.
A typical Bletchley intercept sheet, after decryption.
Average number of daily Ultra dispatches to field commanders during the second World War
Women cryptologists at work in the U.S. Army's Arlington Hall

Ultra was the designation adopted by British military intelligence in June 1941 for wartime signals intelligence obtained by breaking high-level encrypted enemy radio and teleprinter communications at the Government Code and Cypher School (GC&CS) at Bletchley Park.

Much of the German cipher traffic was encrypted on the Enigma machine.

Gordon Welchman wrote,

It has been alleged that "Lucy" was in major part a conduit for the British to feed Ultra intelligence to the Soviets in a way that made it appear to have come from highly placed espionage rather than from cryptanalysis of German radio traffic.

Cipher Bureau (Poland)

General Staff building (Saxon Palace), destroyed in World War II. In the photo, the arcade shelters the Tomb of the Unknown Soldier, before which stands Thorvaldsen's equestrian statue of Prince Józef Poniatowski. In this building, from 1932, the Cipher Bureau broke the German plugboard military Enigma.
The Enigma machine was an electro-mechanical rotor machine with a scrambler consisting of (from right to left) an entry drum, three rotors and a reflector. It was available commercially from the early 1920s and was modified for use by the German military who adopted it later in the decade.
Marian Rejewski ca. 1932, when he first broke Enigma
Cyclometer, devised in the mid-1930s by Rejewski to catalog the cycle structure of Enigma permutations. 1: Rotor lid closed, 2: Rotor lid open, 3: Rheostat, 4: Glowlamps, 5: Switches, 6: Letters.
A Zygalski perforated sheet (1938)
German horse artillery parading before the Saxon Palace, autumn 1939. Hitler also, in his Mercedes, took part in a parade before the Saxon Palace.
German sentries before the Saxon Palace (Polish General Staff building) where the German military Enigma had been broken and read by the Poles for several years before the war. Photo taken before 30 August 1940, when the Germans concealed Thorvaldsen's statue of Prince Józef Poniatowski.

The Cipher Bureau, in Polish: Biuro Szyfrów (, was the interwar Polish General Staff's Second Department's unit charged with SIGINT and both cryptography (the use of ciphers and codes) and cryptanalysis (the study of ciphers and codes, for the purpose of "breaking" them).

In December 1932, the Bureau began breaking Germany's Enigma ciphers.

The British sent Commander Alastair Denniston, head of Britain's Government Code and Cypher School, Dilly Knox, chief British cryptanalyst and Commander Humphrey Sandwith, head of the Royal Navy's intercept and direction-finding stations.

Former Bletchley Park mathematician-cryptologist 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."

Stuart Milner-Barry

The original Hut 6 building (photographed in 2004). Milner-Barry joined Hut 6 in early 1940, and worked in the section throughout World War II. He became head of Hut 6 in Autumn 1943.

Sir Philip Stuart Milner-Barry (20 September 1906 – 25 March 1995) was a British chess player, chess writer, World War II codebreaker and civil servant.

He worked at Bletchley Park during World War II, and was head of "Hut 6", a section responsible for deciphering messages which had been encrypted using the German Enigma machine.

Upon their return, all three soon joined the Government Code and Cypher School (GC&CS) at Bletchley Park.

Milner-Barry was recruited by mathematician Gordon Welchman, who had been his contemporary at Trinity College; in turn Milner-Barry recruited Hugh Alexander.

Dilly Knox

Collar and Badge of the Grand Cross

Alfred Dillwyn "Dilly" Knox, CMG (23 July 1884 – 27 February 1943) was a British classics scholar and papyrologist at King's College, Cambridge and a codebreaker.

He then joined the Government Code and Cypher School (GC&CS).

At Bletchley Park he worked on the cryptanalysis of Enigma ciphers until his death in 1943.

Hugh Foss

Close-up of the rotors in a cipher machine

Hugh Rose Foss (13 May 1902 – 23 December 1971) was a British cryptanalyst.

At Bletchley Park during World War II he made significant contributions both to the breaking of the German Enigma code and headed the section tasked with breaking Japanese Naval codes.

In December 1924 he joined the Government Code and Cipher School.

Gordon Welchman was told that Foss was highly esteemed by the Americans, and says that "before the war he was one of the most brilliant of the professional cryptographers of the Government Code and Cypher School".