Bombes, Cribs and Colossi
- Codebreakers: The Inside Story of Bletchley Park edited by F.H. Hinsley and Alan Stripp
Oxford, 321 pp, £17.95, August 1993, ISBN 0 19 820327 6
Just before the outbreak of war the Government Code and Cypher School (GC & CS) moved from London to Bletchley Park in Buckinghamshire. In 1939, some hundred or so people were working there; by the end of the war there were about seven thousand. This book contains reminiscences by 30 of them, describing what they did and what life was like at Bletchley.
GC & CS came under ‘C’, the head of the Secret Service, and was nominally a department of the Foreign Office. In the years between the wars it had grown out of a group of code-breakers who, in World War One, had worked in the famous Room 40 at the Admiralty. Some of that group were still active in 1939; in particular Oliver Strachey, ‘Dilly’ Knox (whose work on the Abwehr version of Enigma is described in the book) and Commander Denniston, who became the first head of Bletchley. Already in 1938, Denniston had started to seek out the mathematicians, scholars and chess champions who reported to Bletchley on the outbreak of war. This slightly haphazard method of recruitment continued throughout the war. A number of the authors tell how their Oxford or Cambridge tutor (or friend of their tutor) asked them if they would like to do secret work, and how, at an interview in London, they were asked if they played chess, did crosswords or knew foreign languages. Those reading Classics were asked if they would like to learn Japanese. There were also experts in modern languages (two were known as ‘the walking dictionaries’) and, by 1943, many mathematicians (six of whom were, or were to become, fellows of the Royal Society). A Lancashire businessman, Wing-Commander Eric James, praised here for his skill in dealing with ‘tiresome intrigues’ and ‘something like chaos’, eventually got things running smoothly; after the war he became head of GCHQ at Cheltenham.
Enciphered enemy messages, broadcast in morse or teleprinter code, were sent to Bletchley by teleprinter or despatch rider from a chain of intercept (or ‘Y’) stations. Co-operation between these and Bletchley was essential, though not always straightforward. At the beginning the rule was that all decoding should be done at Bletchley; but much of the traffic was in low-grade hand cyphers, and it was gradually realised that this could best be decoded by Bletchley-trained people at the ‘Y’ station. This was particularly true of Japanese traffic. The important ‘Y’ stations for this were in East Africa, India and the Far East, and one of the book’s contributors, Hugh Denham, says that the work done at Bletchley on Japanese naval messages was almost without value.
At the RAF intercept station at Cheadle, there was a group of four ‘computors’, so called because officially their job was to decode air-to-ground traffic using daily keys provided by Bletchley: in fact, they often found the keys before Bletchley did. The computors believed the Air Ministry’s estimate of the size of the German bomber force in the west was much too high – Peter Gray Lucas refers to this as the ‘St Ives story’ (‘As I was going to St Ives, I met a man with seven wives’). But to get a better estimate it was necessary to know which aircraft belonged to which unit, which was difficult because the call-signs (three-place alpha-numeric) were continually changing; analysis at Bletchley had not found any discernible pattern. One night when traffic was light, two of the Cheadle computors sorted out the messages not by statistical means, but according to the style apparent in the choice of call-sign and found that this gave a coherent classification. Subsequently they were able to show that the size of a unit was nine aircraft, not 12 as the Air Ministry had supposed. Lucas gives this as an example of the ‘creative anarchy’ which contributed so much to Bletchley’s success.
The messages most copiously available were those encoded by the Enigma machine; in the last year of the war there were some 84,000 of these a month. About one third of Codebreakers is about the decoding, translation and evaluation of Enigma messages. Enigma was used by a large number of different German networks: maybe as many as a hundred, each with its own rules of procedure and daily keys. By 1943, decrypts were being mass-produced. Many messages were routinely broken, perhaps a few hours after the keys were changed, at midnight; others were never broken.