Zygalski sheets

Perforated sheets devised by Henryk Zygalski that made it possible to determine the order of the Enigma rotors.

In July 1939, at Pyry, the Poles showed them to French and British cryptologists.^1,2

According to the Foreign Intelligence Agency (AW), the sheets, together with Rejewski’s cryptologic bomb, were a response to the changes the Germans kept introducing in the way Enigma was operated; their job was to establish the order of the cipher rotors. At Pyry on 25 July 1939 the Poles handed representatives of French and British intelligence one copy of Enigma each, together with the sheets and the bombs.^1,2

According to GCHQ, the Poles were at that point ahead of the British and the French both in the amount of traffic they had read and in the design of the bomb, although their success rate had recently fallen because of German security measures. What was learnt in Poland changed GC&CS’s approach to the cryptanalysis of Enigma, and Turing’s conversations with Rejewski in Paris in January 1940 became the basis of the first British Bombe.³

More in the operation card: Breaking Enigma.