By Peter Calvocoressi
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Which French ministers were expected by the 34 THE ENIGMA MACHINE AND THE POLES Germans to collaborate with them. By the winter of 1937-38 the Poles reckoned that they could decypher 75 per cent of what they intercepted. On the other hand they got nothing from this source about the German invasion of Austria in March 1938 because that operation was conducted in wireless silence. The main problem of the thirties was keeping up with the various changes introduced by the Germans for the purpose of keeping one step ahead of possible decypherers, if only for the fun of it: professional pride fuels progress.
He tapped these three letters out twice over, thus getting a six-letter group which he put at the beginning of the message. If, for example, he had thought up LUC, he would tap out LUCLUC, which might be rendered by the machine into PROSHB. He then re-set his three wheels to show LUC, tapped out PROSHB and carried on with the message. On the eve of the German attack in the west in May 1940, the double encyphering of this preambular key was cancelled. It was, as we shall see, a serious error, for it was the main weapon used first by the Poles and then by the British in their attacks on the daily settings of the Enigma machine.
Intelligence expostulated: something must have gone wrong with the D/F. But the operators stuck to their story: the signals had been put on the air in Poland, not Holland. The next day five more such companies turned up in Poland, shortly followed by administrative elements of the Luftwaffe's Fliegerkorps VIII which was supposed to be in Greece and appeared from the wireless pattern to be still active there. It became obvious that the continuing transmissions from Holland and Greece were spoof.