Bombe - The Principle of The Bombe

The Principle of The Bombe

To decipher German military Enigma messages required that the following settings of the machine were discovered. Once these were known, all the messages for that network for that day (or pair of days in the case of the German navy) could be decrypted.

Internal settings (that required the lid of the Enigma machine to be opened)

• The selection of rotors in use in the Enigma's scrambler, and their positions on the spindle (Walzenlage or "wheel order"). Possible wheel orders numbered 60 (three rotors from a choice of five) for army and air force networks and 336 (three rotors from a choice of eight) for the naval networks.
• The positions of the alphabet ring's turnover notch in relation to the core of each rotor in use (Ringstellung or "ring settings"). There are 26 possible ring-settings for each rotor.

External settings (that could be changed without opening the machine)

• The plugboard connections (Steckerverbindungen or "stecker values"). The ten leads could be arranged in 158 million million million ways.
• The scrambler rotor positions at the start of enciphering the message key (the Grundstellung or "indicator-setting") — up to May 1940; or thereafter the initial positions of each rotor at the start of enciphering the message (the "message key") from which the indicator-setting could be derived. There are 17,576 possible three letter keys.

The bombe identified possible initial positions of the rotor cores, and the stecker partner of a specified letter for a set of wheel orders. Manual techniques were then used to complete the decryption process. In the words of Gordon Welchman "... the task of the bombe was simply to reduce the assumptions of wheel order and scrambler positions that required 'further analysis' to a manageable number."