Bletchley Park and D-Day by David Kenyon

Author:David Kenyon
Language: eng
Format: epub
ISBN: 9780300243574
Publisher: Yale University Press

CHAPTER SIX

UNDERSTANDING GERMAN EXPECTATIONS

In the summer of 1941, one of Bletchley’s senior codebreakers, Dilly Knox, was applying himself to the ciphers of the Abwehr, Germany’s intelligence service. Knox had led the early British attacks on Enigma, including on both the commercial and the military variants. His team in the former stable yard at Bletchley Park had also been successful at breaking Italian naval Enigma traffic. In summer 1941, however, an increasing amount of Abwehr traffic was being intercepted and, while much of it was enciphered using simple pencil-and-paper methods, a portion of it clearly used machine ciphers.

The format of these messages, with two groups of four letters at the start of each message, suggested that they were enciphered using variants of Enigma different from the models previously investigated. Knox had developed a number of techniques for attacking these ‘message indicators’ (as the letter groups were termed), as well as the cipher texts themselves, in order to figure out the workings of the machines. Complex mathematical methods were needed to deduce the internal wiring of each rotor, and thus work out what encipherments would be produced. Knox turned to his team of female staff to perform these calculations. Of the dozen or more women in his team, only two spoke the necessary German. One was Margaret Rock.

Her father was a Royal Navy surgeon who had been killed in 1917, when she was just fourteen years old, after his ship hit a mine laid from a German U-Boat off the Irish coast. His letters to Margaret at boarding school had urged her to pursue her studies as far as possible and to take up a worthwhile career – in stark contrast to the general expectations of women in the early twentieth century. She had taken her father’s wishes to heart, graduating in 1921 from Bedford College in London, at the time one of the relatively few institutions where women could obtain degrees. She became a business statistician, and enjoyed travelling widely with her brother John during her time off, in the course of which she learned German to add to the French she had learned at school. This combination of mathematical and linguistic skills led to her being recruited by Bletchley Park, where she arrived in spring 1940. She would go on to become one of the most senior and most trusted figures in Dilly Knox’s team. She was older than the other women, and Knox considered her one of the half-dozen finest minds in the whole of GC&CS.

Long periods of tedious analysis and calculations led finally to Margaret and her younger colleague Mavis Lever being able to unravel the secrets of one of the most widely used Abwehr Enigmas. On 28 October 1941, Knox reported to Alastair Denniston, the head of Bletchley Park, that the prospects for breaking traffic on the so-called Zählwerke (‘counter’) Enigma were very good, and indeed the first decrypted message was issued on Christmas Day.

Bletchley Park would go on to issue over 140,000 Abwehr Enigma decrypts, from several different Enigma variants.

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