Yesterday we heard about SNOW, the first of the great Double-Cross agents. Today, in the second part of our series from a mystery contributor, we hear about Dilly, one of the greatest wartime codebreakers...
It was at this point that a leading Bletchley Park codebreaker produced what may well have been Bletchley's greatest triumph. Dilly Knox, the subject of Mavis Batey's wonderfully affectionate biography Dilly: The Man Who Broke Enigmas, was quite possibly the most brilliant of Britain's wartime codebreakers.
Mavis was one of Dilly's two main assistants and a leading codebreaker in her own right so she knows what she's talking about!
Dilly had played the main role in Bletchley's initial breaks into the Enigma cipher and was now working with a small number of young female codebreakers like Mavis, trying to break the machine ciphers that no-one else at Bletchley could crack. So it was Dilly who was working on the German secret service Enigma cipher.
The main military Enigma machine looked like a small typewriter encased in a wooden box. There were series of lights, one for every letter of the alphabet, on top of the machine. The operator typed each letter of the plain-text message into the machine. If you pressed one of the keys, it sent an electrical current through the machine, which lit up the enciphered letter. The machine contained three wheels which rotated one at a time, changing the encipherment process for every letter typed into the machine.
But the German secret service, or Abwehr, machine had four wheels instead of three and those four wheels rotated far more often than on the standard machine, making it much more complex. It seemed that cracking the Abwehr Enigma was an impossible task. But on 8 December 1941, assisted by Mavis and his other chief assistant, Margaret Rock, Dilly finally managed to break it.
How he did it is told in human terms in Mavis's biography Dilly and in even more detailed terms by her husband Keith, another of the great codebreakers, in the book Bletchley Park Codebreakers, edited by Ralph Erskine and Michael Smith, one of our wonderful Dialogue Espionage Classics series. Dilly's break into Abwehr Enigma deserves to be told in all its glory. It was one of the most important moments of the intelligence war. At last, the MI5 and MI6 officers running the Double Cross system knew that the Germans believed everything they were being told. The stage was now set for the climax of this extraordinary operation.
Tune in tomorrow to hear the story of Garbo, the most important of the double agents.
N.B. Dilly is currently on special offer at £4.99!