Dilly Knox

The sailor in Room 53
has never, it's true, been to sea
but though not in a boat
he has served afloat —
in a bath in the Admiralty.

— Alice in ID25 by Frank Birch[17]

Soon after war broke out in 1914,[2] Knox was recruited to the Royal Navy's cryptological effort in Room 40 of the Admiralty Old Building,[18] where some of his work was done in the bath. He persuaded his superiors to have a bathtub installed in his office in the cryptanalysis section of the British Admiralty[19] (in Room 53).[17] In 1917, Knox followed Room 40 with its expansion into ID25.[2]

Among other tasks, he was involved in breaking:

• the Zimmermann Telegram which brought the USA into the war.[1]
• much of the German admiral's flag code by exploiting an operator's love of romantic poetry.[2]

Oh, if a time should ever come when we're demobilized How we shall miss the interests which once life comprised!

— Dilly the Dodo[2], Alice in ID25 by Frank Birch

During the First World War he had been elected Librarian at King's College, but never took up the appointment. After the war Knox intended to resume his research at King's, but was persuaded by his wife to remain at his secret work; indeed, so secret was this work that his own children had no idea, until many years after his death, what he did for a living, and his contribution to the war effort.[13]

The Enigma machine became available commercially in the 1920s. In Vienna in 1925,[20] Knox bought the Enigma 'C' machine evaluated by Hugh Foss in 1927 on behalf of GC&CS. Foss found "a high degree of security" but wrote a secret paper describing how to attack the machine if cribs — short sections of plain text — could be guessed.[3] When — a decade later — Knox picked up this work, he developed a more effective algebraic system (rodding) based on the principles described by Foss.[20]

The Germany Navy (Kriegsmarine) adopted Enigma in 1926, adding a plug-board (stecker) to improve security. Nazi Germany supplied non-steckered machines to Franco's Nationalists in the Spanish Civil War. On 24 April 1937, Knox broke the Spanish Enigma but knowledge of this breakthrough was not shared with the Republicans.[3][lower-alpha 2] Soon afterwards, Knox began to attack signals between Spain and Germany encrypted using steckered Enigma machines.[3]

GC&CS began to discuss Enigma with the French Deuxième Bureau in 1938, obtaining from the Bureau details of Wehrmacht Enigma supplied by Asché and signal intercepts, some of which must have been made in Eastern Europe. This led the French to disclose their links with the Biuro Szyfrów (Polish cryptographers).[3] Knox, Hugh Foss and Alastair Denniston represented the GC&CS at the first Polish–French–British meeting at Paris in January 1939.[2] The Poles were under order to disclose nothing of importance, leaving the British codebreakers disappointed. Knox's description of his system of rodding impressed the Polish codebreakers and they requested his presence at a second meeting.[3]

Knox grasped everything very quickly, almost quick as lightning. It was evident that the British had been really working on Enigma ... So they didn't require explanations. They were specialists of a different kind, of a different class.

— Marian Rejewski[2]

Knox attended the second Polish–French–British conference, held on 25–26 July 1939 at the Polish Cipher Bureau (at Pyry, south of Warsaw, Poland). The Poles began to disclose to their French and British allies their achievements in solving Enigma decryption.[21]

Although Marian Rejewski, the Polish cryptographer and mathematician who solved the plugboard-equipped Enigma used by Nazi Germany, approached the problem through permutation theory (whereas Knox applied linguistics), a good personal relationship was quickly established at the conference. The good impression made by Rejewski on Knox played an important role in increasing recruitment of mathematicians to Bletchley Park.[2] Knox was chagrined — but grateful — to learn how simple was the solution of the Enigma's entry ring (standard alphabetical order).[21]

It was such an obvious thing to do, really a silly thing to do, that nobody, not Dilly Knox or Tony Kendrick or Alan Turing, ever thought it worthwhile trying it.

— Peter Twinn[3]

After the meeting, he sent the Polish cryptologists a very gracious note in Polish, on official British government stationery, thanking them for their assistance and sending "sincere thanks for your cooperation and patience".[2] Enclosed were a beautiful scarf featuring a picture of a Derby winner and a set of paper 'batons'.[21]

I don't know how Knox's method was supposed to work, most likely he had hoped to vanquish Enigma with the batons. Unfortunately we beat him to it.

— Marian Rejewski[2]

These 'batons' were known as rods to the British and had been used to solve the Spanish Enigma. Knox's rodding method was later used to break the Italian Naval Enigma.[2]

Alan Turing worked on Enigma during the months leading to the outbreak of the Second World War in September 1939, and occasionally visited GC&CS's London HQ to discuss this problem with Knox. In the 1939 register Turing was recorded in Naphill, staying with Knox and his wife. By November 1939 Turing had completed the design of the bombe — a radical improvement of the Polish bomba.[22]

To break non-steckered Enigma machines (those without a plugboard), Knox (building on earlier research by Hugh Foss) developed a system known as 'rodding', a linguistic as opposed to mathematical way of breaking codes.[3] This technique worked on the Enigma used by the Regia Marina (Italian Navy) and the German Abwehr.[23] Knox worked in 'the Cottage', next door to the Bletchley Park mansion, as head of a research section, which contributed significantly to cryptanalysis of the Enigma.[6]

Knox's team at The Cottage used rodding to decrypt intercepted Italian naval signals describing the sailing of an Italian battle fleet, leading to the Battle of Cape Matapan in March 1941. Admiral John Godfrey, Director of Naval Intelligence credited the Allied victory at Matapan to this intelligence; Admiral Sir Andrew Cunningham, who had commanded the victorious fleet at Matapan, went to Bletchley to congratulate 'Dilly and his girls'.[2]

In October 1941, Knox solved the Abwehr Enigma.[4] Intelligence Services Knox (ISK) was established to decrypt Abwehr communications.[4] In early 1942, with Knox seriously ill, Peter Twinn took charge of running ISK and was appointed head after Knox's death.[5][4] By the end of the war, ISK had decrypted and disseminated 140,800 messages.[4] Intelligence gained from these Abwehr decrypts played an important part in ensuring the success of the Double-Cross System of MI5 and MI6, and in Operation Fortitude, the Allied campaign to deceive the Germans about D-Day.[5]

Knox's work was cut short when he fell ill with lymphoma.[24] When he became unable to travel to Bletchley Park, he continued his cryptographic work from his home in Hughenden, Buckinghamshire, where he received the CMG.[25] He died on 27 February 1943.[25] A biography of Knox, written by Mavis Batey, one of 'Dilly's girls', the female codebreakers who worked with him, was published in September 2009.[26]

• Andrew, Christopher (2011). "Chapter 1: Bletchley Park in Pre-War Perspective". In Erskine, Ralph; Smith, Michael (eds.). The Bletchley Park Codebreakers. Biteback Publishing. pp. 1–12. ISBN 978-1849540780. (Updated and extended version of Action This Day: From Breaking of the Enigma Code to the Birth of the Modern Computer Bantam Press 2001)
• Batey, Mavis (2004). "Knox, (Alfred) Dillwyn (1884–1943)". Oxford Dictionary of National Biography (online ed.). Oxford University Press. doi:10.1093/ref:odnb/37641. (Subscription or UK public library membership required.)
• Batey, Mavis (2005), "Marian and Dilly", in Rejewski, Marian (ed.), Marian Rejewski, 1905–1980: Living with the Enigma Secret, pp. 67–74
• Batey, Mavis (2009). Dilly: The Man Who Broke Enigmas. Dialogue. ISBN 978-1-906447-01-4.
• Batey, Mavis (2011). "Chapter 6: Breaking Italian Naval Enigma". In Erskine, Ralph; Smith, Michael (eds.). The Bletchley Park Codebreakers. Biteback Publishing. pp. 79–92. ISBN 978-1849540780. (Updated and extended version of Action This Day: From Breaking of the Enigma Code to the Birth of the Modern Computer Bantam Press 2001)
• Budiansky, Stephen (2000), Battle of wits: The Complete Story of Codebreaking in World War II, Free Press, ISBN 978-0-684-85932-3
• Carter, Frank, Bletchley Park Technical Articles: Rodding (PDF), archived from the original (PDF) on 11 April 2007, retrieved 14 February 2015
• Copeland, Jack (2011). "Chapter 19: Colossus and the Dawning of the Computer Age". In Erskine, Ralph; Smith, Michael (eds.). The Bletchley Park Codebreakers. Biteback Publishing. pp. 305–327. ISBN 978-1849540780. (Updated and extended version of Action This Day: From Breaking of the Enigma Code to the Birth of the Modern Computer Bantam Press 2001)
• Foss, Hugh (2011). "Chapter 3: Reminiscences on the Enigma". In Erskine, Ralph; Smith, Michael (eds.). The Bletchley Park Codebreakers. Biteback Publishing. pp. 35–39. ISBN 978-1849540780. (Updated and extended version of Action This Day: From Breaking of the Enigma Code to the Birth of the Modern Computer Bantam Press 2001)
• Fitzgerald, Penelope (2002). The Knox Brothers. Flamingo. ISBN 978-0-00-711830-4.
• Gannon, Paul (2011), Inside Room 40: The Codebreakers of World War I, London, UK: Ian Allan Publishing, ISBN 978-0-7110-3408-2
• Goebel, Greg. "Codes, Ciphers, & Codebreaking". [4.1] Room 40 & the Zimmermann telegram/German codebreakers. Retrieved 14 February 2015.
• Kahn, David (1991), Seizing the Enigma: The Race to Break the German U-boat Codes, 1939-1943, Houghton Mifflin Co., pp. 25–26, 84–85, ISBN 978-0-395-42739-2
• Keeley, Graham (24 October 2008), "Nazi Enigma machines helped General Franco in Spanish Civil War", The Times, p. 27, retrieved 15 May 2020
• Kozaczuk, Władysław (1984), Enigma: How the German Machine Cipher was Broken, and how it was Read by the Allies in World War Two - Edited and translated by Christopher Kasparek, Frederick, Maryland (a substantially revised and augmented translation of W kręgu enigmy, Warsaw, Książka i Wiedza, 1979, supplemented with additional appendices by Marian Rejewski.) (2 ed.), University Publications of America, ISBN 978-0-89093-547-7
• Sebag-Montefiore, Hugh (2000), Enigma: The Battle for the Code, London, UK: Weidenfeld & Nicolson, ISBN 0-7538-1130-8
• Smith, Michael (2010). "How It Began: Bletchley Park Goes to War". Colossus: The Secrets of Bletchley Park's Codebreaking Computers. Oxford University Press. pp. 18–35. ISBN 978-0-19-957814-6.
• Staff writer (17 November 2004), "Peter Twinn", The Telegraph, London, UK, archived from the original on 2 March 2007, retrieved 14 February 2015
• Thorpe, D.R. (2010). Supermac: The Life of Harold Macmillan. Chatto & Windus.