Some strategic messages were enciphered with a number of devices that were even more complex. From 1933 onwards the Enigma was constantly developed for use by the army, the navy, and the diplomatic service. Scherbius had already supplemented the rotor system with a plug board in 1928, which made enciphering even more complicated. The working principle is based on a number of simple electric circuits. Each one connects a key of the typewriter-like keyboard with a glow lamp that illuminates a letter in the display panel.
Every keystroke causes the illumination of a new letter.
Each current path leads to one of the three rotors, always via one front and one rear contact, up to the reversing wheel, or reflector. From there, it leads back through all the rotors and also through the plugs of the plug board. The plug board connections are varied. Every time a key is pressed, the rotors are advanced one position according to a preordained system. Three of these rotors are always selected from a set of five and used in a sequence that is determined anew on each occasion.