peetm sends this quote from the BBC: "When the Spanish Civil War began in 1936, both Hitler's Germany and Mussolini's Italy sent troops to help the nationalists under Franco. But with the conflict dispersed across the country, some means of secure communication was needed for the German Condor Legion, the Italians and the Spanish forces under Franco. As a result, a set of modified commercial Enigma machines were delivered by Germany. ... A key figure in trying to understand it was Dilly Knox, a classicist who had been working on breaking ciphers since World War I. He was fascinated by the machine and began studying ways in which an intercepted message might in theory be broken, even writing his own messages, encrypting them and then trying to break them himself. But there was no opportunity to actually intercept a real message since German military signals were inaudible in Britain. However, the signals produced by the machines sent to Spain in 1936 were audible enough to be intercepted and Knox began work. ... Within six or seven months of having his first real code to crack, Knox had succeeded, producing the first decryption of an Enigma message in April 1937."