Dilly began to investigate the characteristics of the Enigma machine as soon as he had access to one, probably as early as 1925. With an actual machine in front of him, he could tell how the keyboard letters were connected to the entry plate, and by battery testing could determine the internal wiring of the wheels. By tapping messages out to himself he could then begin to experiment on ways in which the wiring could be ascertained, using his own ‘crib’ for the cipher text. A method for breaking the wheel wiring was essential as the manufacturers had indicated that to increase security the wiring of the original wheels could be changed.
The instructions given to ordinary German soldiers, the Schlüsselanleitung für die Chiffriermaschine Enigma, included an example using a real machine which would have given Dilly some of the answers if only SIS had agreed to pay for it when it was first offered to them by the French in 1931. Appendix 1 is a translation of those instructions, which explain how the machine was set up to encipher a message. The technique Dilly invented to break the machine was by ‘rodding’ and ‘buttoning-up’, which he first put into practice with operational messages in the Spanish Civil War in 1937, when Mussolini and Franco obtained from Hitler improved ‘K’ models of the commercial Enigma machine with rewired wheels.
Throughout the chapters in this book, there is frequent mention of Dilly’s system of rodding for breaking messages. Appendix 2, written by Frank Carter for the Bletchley Park Trust, explains how the rods were made specifically to match each machine. Although it refers to the rods’ original use for the un-Steckered Enigma machine, there were occasions when rodding could be applied to the Steckered German service machine, such as the ‘routine operation’ put in place in confirming the authenticity of the ‘crib message’ provided in the 1930 operator’s manual when Dilly stripped off the Stecker as a reciprocal substitution super-encipherment. His fundamental methods for breaking Scherbius’s original machine had been a vital step in cryptanalysis, allowing later complications, such as the multi-turnover nature of the Abwehr machine, to be dealt with as they arose. As stated in the appendix, the rods used are those made for the German Railways ‘K’ model machine, broken on Dilly’s methods in 1940. The term ‘rotor’, as is now normal in modern cryptographic texts, has replaced the word ‘wheel’, which was always used at Bletchley Park, being a near translation of Walze, the word used in the German description of the Enigma machine.
Appendix 3, also written by Frank Carter for the Bletchley Park Trust, explains ‘buttoning-up’. The author is very grateful to Frank for providing these detailed explanations of Dilly’s techniques. Frank in turn would like to thank Keith Batey, the author’s husband, for his assistance in their compilation. Appendix 4 is Dilly’s wonderfully idiosyncratic description for Alastair Denniston of the ‘lobster hunt’ that preceded the breaking of the Abwehr Enigma, and Appendix 5, compiled by Ralph Erskine, gives a complete list of the machines broken by Dilly’s ISK section.