Soon after war broke out in 1914, Knox was recruited to the Royal Navy's cryptological effort in Room 40 of the Admiralty Old Building, 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 (in Room 53). In 1917, Knox followed Room 40 with its expansion into ID25. Among other tasks, he was involved in breaking: the Zimmermann Telegram which brought the USA into the war. much of the German admiral's flag code by exploiting an operator's love of romantic poetry. Between the wars Government Code and Cypher School 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. Commercial Enigma The Enigma machine became available commercially in the 1920s. In Vienna in 1925, 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. When — a decade later — Knox picked up this work, he developed a more effective algebraic system (rodding) based on the principles described by Foss. Spanish Enigma Germany's Navy 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 Franco's Enigma but knowledge of this breakthrough was not shared with the Republicans. Soon afterwards, Knox began to attack signals between Spain and Germany encrypted using steckered Enigma machines. On the eve of the Second World War Polish-French-British meetings GC&CS began to discuss Enigma with France's 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 Polish cryptographers. Knox, Hugh Foss and Alastair Denniston represented GC&CS at the first