clock method

A method devised by Jerzy Różycki that made it possible to determine the choice and setting of an Enigma rotor.

Description after Różycki's biographical note on the website of the Foreign Intelligence Agency. According to the AW, the method required advanced mathematical calculations, because the Germans periodically changed the order of the rotors and their starting positions.¹

The AW points out that from 1 September 1936 Enigma keys were reconstructed daily, but because the Cipher Bureau's archive was destroyed in September 1939, it is hard today to establish Różycki's individual contribution precisely. The clock method was one of several Polish tools, alongside Rejewski's cyclometer and card catalogue, Zygalski sheets and the cryptologic bomb.¹

The Cipher Bureau presented all this work to French and British delegations on 25–26 July 1939 at Pyry near Warsaw; according to GCHQ, this changed the British GC&CS's approach to the cryptanalysis of Enigma.^1,2

More in the operation card: Breaking Enigma.