On the outbreak of the First World War he was asked to join the Admiralty's cryptanalytical bureau. (4) "He had succeeded in breaking much of the German admirals' flag code, detecting, with his ear for metre, lines of poetry in the repeated bigrams of a message, which provided a crib." (5) Knox was also involved in the Zimmermann Telegram episode. In January 1917, the German Foreign Secretary, Arthur Zimmermann, sent a coded telegram to the German minister in Mexico City. This instructed the minister to propose an alliance with Mexico if war broke out between Germany and the United States. In return, the telegram proposed that Germany and Japan would help Mexico regain the territories that it lost to the United States in 1848 (Texas, New Mexico and Arizona). The telegram was intercepted by the the Admiralty's cryptanalytical bureau and shown to President Woodrow Wilson on 24th February and helped to bring the United States into the war. (6)

After the war he continued work in the field of codebreaking in the renamed the Government Code and Cypher School. On 21st July 1920 he married his former secretary, Olive Margaret Rickman. Over the next few years they had two sons. Following German intervention in Spain he solved the Spanish military code and collaborated with the French on Italian naval codes used in Abyssinia. (7) Frederick Winterbotham described Knox as "tall, with a rather gangling figure, unruly black hair, his eyes, behind glasses, some miles away in thought." (8)

In June 1938, Sir Stewart Menzies, the chief of MI6, received a message that the Polish Intelligence Service had encountered a man who had worked as a mathematician and engineer at the factory in Berlin where the Germans were producing the Enigma machine. The man, Richard Lewinski (not his real name), was a Polish Jew who had been expelled from Nazi Germany because of his religion. He offered to sell his knowledge of the machine in exchange for £10,000, a British passport and a French resident permit. Lewinski claimed that he knew enough about Enigma to build a replica, and to draw diagrams of the heart of the machine - the complicated wiring system in each of its rotors.

Menzies suspected that Lewinski was a German agent who wanted to "lure the small British cryptographic bureau down a blind alley while the Germans conducted their business free from surveillance". Menzies suggested that Alfred Dilwyn Knox, should go to interview Lewinski. He asked Alan Turing from Cambridge University to go with him. He had spent time working with Alonzo Church at Princeton University, developing the notion of a "universal computing machine". Knox and Turing were soon convinced that Lewinski had a deep knowledge of the machine and he was taken to France to work on producing a model of the machine.

According to Anthony Cave Brown, the author of Bodyguard of Lies (1976): "Lewinski worked in an apartment on the Left Bank, and the machine he created was a joy of imitative engineering. It was about 24 inches square and 18 inches high, and was enclosed in a wooden box. It was connected to two electric typewriters, and to transform a plain-language signal into a cipher text, all the operator had to do was consult the book of keys, select the key for the time of the day, the day of the month, and the month of the quarter, plug in accordingly, and type the signal out on the left-hand typewriter. Electrical impulses entered the complex wiring of each of the rotors of the machine, the message was enciphered and then transmitted to the right-hand typewriter. When the enciphered text reached its destination, an operator set the keys of a similar apparatus according to an advisory contained in the message, typed the enciphered signal out on the left-hand machine, and the right hand machine duly delivered the plain text. Until the arrival of the machine cipher system, enciphering was done slowly and carefully by human hand. Now Enigma, as Knox and Turing discovered, could produce an almost infinite number of different cipher alphabets merely by changing the keying procedure. It was, or so it seemed, the ultimate secret writing machine." (9) Mavis Batey, who worked at the Government Code and Cypher School, pointed out the "Polish replica moved the breaking of Enigma on from a theoretical exercise to a practical one and Knox always gave the Poles credit for the part they played." (10)

On the outbreak of the Second World War, Alfred Dilwyn Knox Mavis worked full time at the Government Code and Cypher School at Bletchley Park. Bletchley was selected simply as being more or less equidistant from Oxford University and Cambridge University since the Foreign Office believed that university staff made the best cryptographers. The house itself was a large Victorian Tudor-Gothic mansion, whose ample grounds sloped down to the railway station. According to Penelope Fitzgerald, Knox always slept in the office. (11)

As the most senior cryptographer, was allocated working space in "a row of chunky converted interlinked houses - just across the courtyard from the main house, near the stables". It became known as the "Cottage". (12) At first his department consisted of ten people, including "two very brilliant" young women, Mavis Batey and Margaret Rock. (13) Mavis later recalled. "We were all thrown in the deep end. No one knew how the blessed thing worked. When I first arrived, I was told, 'We are breaking machines, have you got a pencil? And that was it. You got no explanation. I never saw an Enigma machine. Dilly Knox was able to reduce it - I won't say to a game, but a sort of linguistic puzzle. It was rather like driving a car while having no idea what goes on under the bonnet." (14) "We were looking at new traffic all the time or where the wheels or the wiring had been changed, or at other new techniques. So you had to work it all out yourself from scratch.” (15)

Knox admitted that he liked employing women. According to Sinclair McKay, the author of The Secret Life of Bletchley Park (2010): "Dilwyn Knox... found that women had a greater aptitude for the work required - as well as nimbleness of mind and capacity for lateral thought, they possessed a care and attention to detail that many men might not have had." (16) Knox definitely had a very enlightened approach to the employment of women and was accused of being guilty of "positive discrimination".

Knox was so impressed with the work of Mavis Batey and Margaret Rock that in August 1940, that he contacted head office: "Miss Lever (Batey) is the most capable and the most useful and if there is any scheme of selection for a small advancement in wages, her name should be considered.... Miss Rock is 4th or 5th best in the whole Enigma staff and quite as useful as some of the professors." (17)

Some members of staff at Bletchley Park suggested that Knox selected so many women to join his team because he liked to be surrounded by attractive women who were given the name "Dilly's Fillies". One of these women later pointed out: "A myth has grown up that Dilly went around in 1939 looking at the girls arriving at Bletchley and picking the most attractive for the Cottage... That is completely untrue. Dilly took us on our qualifications." (18)

Another of the cryptanalyst working on the project, R. V. Jones, later explained the scale of their problem. "This was a very ingenious arrangement of three wheels, each one of which had a sequence of studs on each side, with each stud on one side being connected by a wire to a pin on the other side - the exact arrangement of the connections being one of the secrets of the machine and the pin making contact with one of the studs on the next wheel. The machine had a typewriter keyboard, and it was worked rather like a cyclometer: every time the machine was operated to encode a letter, one wheel would be turned by one space; after this wheel had moved by enough spaces to turn it through one revolution, it would click its neighbouring wheel by one space. The wheels were thus never in the same position twice. The basic encoding was effected by the passage of an electric current through the studs so that when a letter was to be encoded, the appropriate key would be pressed on the keyboard, and the resultant coded letter would be determined by the appropriate conducting path through the studs, the studs on one wheel making suitable contact with the pins on the neighbouring wheel. A further touch of ingenuity was to add a reversing arrangement at the edge of the third wheel, again with studs cross-connected so as to send the current backwards through the wheels by yet another path. The returning current lit a small electric bulb which illuminated a particular letter on a second keyboard, and thus indicated the enciphered equivalent of the letter whose key had originally been pressed." (11)

Alan Turing was put in charge of the project. By studying old decrypted messages, he discovered that many of them conformed to a rigid structure. He found that he could sometimes predict part of the contents of an undeciphered message, based on when it was sent and its source. As Simon Singh has explained: "For example, experience showed that the Germans sent a regular enciphered weather report shortly after 6 a.m. each day. So, an encrypted message intercepted at 6.05 a.m. would be almost certain to contain wetter, the German word for 'weather'. The rigorous protocol used by any military organisation meant that such messages were highly regimented in style, so Turing could even be confident about the location of wetter within the encrypted message. For example, experience might tell him that the first six letters of a particular cipher text corresponded to the plaintext letters wetter. When a piece of plaintext can be associated with a piece of cipher text, this combination is known as a crib. Turing was sure that he could exploit the cribs to crack Enigma." (12)

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The problem for Turing and the rest of the team was that at the time they had to use a "trial and error" approach. This was a very difficult task as there were 159,000,000,000,000,000,000 possible settings to check. R. V. Jones was one of those who worked with Alan Turing on this project: "Together we worked out ways in which the process might be done mechanically, with a machine that would recognize when genuine German was coming out by the frequencies with which various letters and diphthongs appeared." (13)

Alan Turing set about developing an engine that would increase the speed of the checking process. Turing finalised the design at the beginning of 1940, and the job of construction was given to the British Tabulating Machinery factory at Letchworth. The engine (called the "Bomb") was a copper-coloured cabinet that was two metres tall, two metres long and a metre wide. Its chief engineer, Harold Keen, and a team of twelve men, built it in complete secrecy. Keen later recalled: "There was no other machine like it. It was unique, built especially for this purpose. Neither was it a complex tabulating machine, which was sometimes used in crypt-analysis. What it did was to match the electrical circuits of Enigma. Its secret was in the internal wiring of (Enigma's) rotors, which 'The Bomb' sought to imitate." (14)

The first machine, named Victory, was installed at Bletchley Park on 18th March 1940. "Its initial performance was uncertain, and its sound was strange; it made a noise like a battery of knitting needles as it worked to produce the German keys." (15) A more improved version, called Agnes, was delivered on 8th August. Over the next few months 178 messages were broken on the two machines. They were described by operators as being "like great big metal bookcases". (16)

Frederick Winterbotham was the chief of Air Intelligence at MI6. He later described the moment when Major General Sir Stewart Menzies, the chief of MI6, first gave him copies of German secret messages: "It was just as the bitter cold days of that frozen winter were giving way to the first days of April sunshine that the oracle of Bletchley spoke and Menzies handled me four little slips of paper, each with a short Luftwaffe message on them... From the Intelligence point of view they were of little value, except as a small bit of administrative inventory, but to the back-room boys at Bletchley Park and to Menzies... they were like the magic in the pot of gold at the end of the rainbow. The miracle had arrived." (17)

Alfred Dilwyn Knox developed cancer in 1941. Winston Churchill was fully aware of Knox's importance in the war, intervened in an attempt to restore him to health. Churchill offered him the services of a destroyer to take him to the warmth of the Caribbean, but he was too ill to be moved. "Churchill then obtained special medical treatment to him through his own physician, Lord Moran, the president of the Royal College of Physicians, and also arranged with the United States Embassy in London to obtain supplies of fresh tropical fruit, a rarity in wartime England for which Knox had cravings." (18)

Alfred Dilwyn Knox died of cancer on 27th February 1943 at his home, Courns Wood House, Hughenden, near High Wycombe, Buckinghamshire.

(1) Mavis Batey, Oxford Dictionary of National Biography (2004-2014)

(2) Anthony Cave Brown, Bodyguard of Lies (1976) pages 18

(3) Mavis Batey, Oxford Dictionary of National Biography (2004-2014)

(4) Anthony Cave Brown, Bodyguard of Lies (1976) pages 18

(5) Mavis Batey, Oxford Dictionary of National Biography (2004-2014)

(6) Simon Singh, The Code Book: The Secret History of Codes & Code-Breaking (2000) pages 110-115

(7) Mavis Batey, Oxford Dictionary of National Biography (2004-2014)

(8) Frederick Winterbotham, The ULTRA Secret (1974) page 14

(9) Anthony Cave Brown, Bodyguard of Lies (1976) page 20

(10) Mavis Batey, Oxford Dictionary of National Biography (2004-2014)

(11) Penelope Fitzgerald, The Knox Brothers (2002) page 228

(12) Sinclair McKay, The Secret Life of Bletchley Park (2010) page 13

(13) Penelope Fitzgerald, The Knox Brothers (2002) page 229

(14) Mavis Batey, interviewed by Sinclair McKay, for his book, The Secret Life of Bletchley Park (2010) page 51

(15) The Daily Telegraph (13th November, 2013)

(16) Sinclair McKay, The Secret Life of Bletchley Park (2010) page 57

(17) Alfred Dilwyn Knox, letter to Headquarters (August 1940)

(18) Sinclair McKay, The Secret Life of Bletchley Park (2010) page 14

(11) R. V. Jones, Most Secret War: British Scientific Intelligence 1939-1945 (1978) pages 98-99

(12) Simon Singh, The Code Book: The Secret History of Codes & Code-Breaking (2000) page 170

(13) R. V. Jones, Most Secret War: British Scientific Intelligence 1939-1945 (1978) pages 100

(14) Harold Keen, interviewed by Anthony Cave Brown (c. 1970)

(15) Anthony Cave Brown, Bodyguard of Lies (1976) page 23

(16) Mary Stewart, interviewed in The Men Who Cracked Enigma (2003)

(17) Frederick Winterbotham, The ULTRA Secret (1974) page 15

(18) Anthony Cave Brown, Bodyguard of Lies (1976) page 252