Free The Bletchley Park Codebreakers by Michael Smith Page A

about 28×35×15 cm (11×14×6 in) andweighed about 12 kg (26 lb). Its main components were a keyboard, lampboard, rotors, reflector (
Umkehrwalze
) and plugboard. A battery provided the electric current for the internal circuitry (see Figure 4.1). The standard machine used three rotors, which during the war were chosen from a set of five for the army, or eight for the navy. Rotor turnover notches were in a different place on rotors I to V – a weakness that was later to be exploited by Huts 6 and 8. Double notches in the special naval rotors, VI to VII, were in the same positions in all three rotors, which made it impossible for Hut 8 to differentiate between them in a procedure known as Banburismus. Unlike the rotors, the reflector in three-rotor
Wehrmacht
Enigma could not move. A later version of naval Enigma, M4, used a rotor as part of a settable reflector, but it did not rotate during use.
    The plugboard contained twenty-six dual sockets, into which cables with jacks were inserted to connect pairs of letters – A to P, B to K, and so on. It was reciprocal: in the previous examples, P swapped for A, and K for B. Ten pairs of letters were generally connected during the war, although eleven pairs would have produced the maximum number of combinations. Without a plugboard, Enigma was relatively easy to solve, but the plugboard version was a very impressive machine.
    Dilly Knox, GC&CS’s chief cryptanalyst, and other members of GC&CS tried to reconstruct Wehrmacht Enigma’s wiring in the 1930s, but were thwarted by one factor - the wiring to Enigma’s entry disc or rotor (the Eintrittwalze). Knox called the wiring the ‘QWERTZU’, on account of its link to Enigma’s keyboard. It had also stalled Rejewski, although only for a relatively short time. In January 1939, three representatives from GC&CS, including Knox, met two senior members of the Polish Cipher Bureau and Bertrand in Paris, but the Poles were under orders to disclose nothing substantive about their successes. They did well: one of the GC&CS party wrote about the Poles: ‘Practical knowledge of [Wehrmacht] enigma nil.’ However, Knox and Alastair Denniston, the operational head of GC&CS, together with Bertrand and a French cryptanalyst, were invited by the Poles to Warsaw at the end of July. To their great surprise, a Polish clone of Enigma was revealed to them on 26 July at the Cipher Bureau’s centre at Pyry, outside Warsaw, and they were told that the wiring to the entry rotor was the identity permutation (A wired to A, B to B, and so on). Knox was far from pleased that the Poles had beaten him to it, maintaining ‘a stony silence’ during the conference. The Polish methods to solve Enigma signals were explained at some length; the visitors were shown the bombas during the second part of the conference. Fortunately, when they met the Poles the next day, ‘Knox was his own bright self & won the hearts & admiration of the young men with whom he was in touch.’
    Figure 4.1 Path of current through Enigma
    Surprisingly, one cryptanalyst at GC&CS, a ‘Mrs B. B.’ (it has not been possible to identify her) ‘had seriously contemplated’ that the wiring was indeed an identity. But she had not been given a crib (known plain-text) supplied to GC&CS by Bertrand, either because organization was not Knox’s forte, or because he thought that she would be wasting her time in following it up: she had therefore been unable to make any progress. Even more surprisingly, although Knox understood her hypothesis about its wiring, he had not pursued it himself, probably because he could not believe that the Germans had been so stupid. Knox wrote that ‘had she worked on the crib we should be reaching them [the Poles]’. However, in this he was somewhat over-optimistic, since it is most unlikely that GC&CS could have recovered the wiring of rotors IV and V. Rejewski had been able to do so only because the
Sicherheitsdienst
(SD) continued to use a pre-15 September