Free The Bletchley Park Codebreakers by Michael Smith Page B

1938 indicating system when the new rotors were introduced. It is almost certain that the British were unable to intercept the SD traffic at that stage, since it seems to have been sent at low-power, and British intercept facilities were very thin on the ground.
    The Poles disclosed the design of the Zygalski sheets (called ‘
Netz verfahren
’ - net method) at the Warsaw meeting, and sent Enigma clones to London and Paris in August. GC&CS soon began to prepare two sets of sheets, based on Zygalski’s design, although the actual punching of the sheets could not start until mid-November, since a special machine to print the positions of certain repeated indicator letters, known as ‘females’, had first to be made. The first set of Zygalski sheets was completed around late December 1939, in one third of the time predicted by Knox. Making the sheets so quickly was a major achievement, since sixty sets (one for each rotor order), each with twenty-six sheets, had to be prepared, with many of the sheets containing about 1,000 holes cut precisely to give the relevant co-ordinates. Knox apparently had to resort to subterfuge and contravene Denniston’s orders for preparing them, in order for them to be ready so soon.
    The British cryptanalysts must have been dismayed when theystarted to use the sheets, since they could not solve any Enigma traffic with them. Unknown to GC&CS, crucial data received from the Poles about rotors IV and V had been incorrect. GC&CS’s failure with the sheets may have been the reason why Denniston asked Brigadier Stewart Menzies (‘C’, who was also the director of GC&CS) to see whether Rejewski and his colleagues could visit GC&CS to help with Enigma. Menzies duly wrote to Colonel Louis Rivet, the head of the
Cinquiéme Bureau de l’État Major de l’Armée
(Fifth Section of the Army General Staff – the
Services de Renseignements et de Contre-Espionnage militaire)
, whose functions included radio intelligence. But Rivet was unwilling to send the Polish cryptanalysts, who were working with the French Army at the Château de Vignolles, in Gretz- Armainvillers, near Paris, after escaping from Poland. He felt that since his Service was paying the Poles, it was entitled to keep them.
    Part of a second set of Zygalski sheets was sent to the Poles in France on 28 December 1939, together with a set of Jeffreys sheets (which were a punched sheet catalogue of the effect of two rotors and the reflector). Alan Turing took the balance of the Zygalski sheets soon after they were completed around 7 January 1940. According to Knox’s accounts, Denniston did not want the balance to be sent, perhaps for reasons of security. Knox had to threaten to resign unless they were sent immediately. It was fortunate that he took such a strong line, since GC&CS would not otherwise have learned the correct data for rotors IV and V so quickly, which might well have had disastrous consequences for its attack on Enigma. The Poles used the sheets to make the first break into wartime Enigma on 17 January, when they solved Green (the cipher used by the
Heer
in Germany’s military districts, the
Wehrkreis)
for 28 October. At around this time, the GC&CS Enigma section moved into Hut 6, pursuant to a decision taken in early December 1939.
    Hut 6 solved Green for 25 October, the first wartime key to be broken in Britain, almost immediately after Turing’s return from France with the correct information about rotors IV and V. There was great relief when soon afterwards it broke Red (the main
Luftwaffe
cipher, which was widely employed for operational and administrative purposes) for 6 January, since there had been fears that the indicating system might have changed on 1 January. Hut 6 solved about fifty daily keys in Red, Green and Blue (a
Luftwaffe
practice cipher) between mid-January and late March 1940 using the Zygalski sheets.A new cipher, Yellow, was first intercepted on 10 April, during the Norwegian campaign, and was then