Footnotes

• The Enigma I was developed in 1927 by Chriffrier-maschinen (later Heimsoeth und Rinke) for the German Army and first introduced in 1931. It was based on the commercial Enigma D but has a fixed reflector and a plugboard at the front that was exclusive to the German Armed Forces. The Model I was used throughout World War II.
  
  The Enigma machine was invented by a German engineer Arthur Scherbius shortly after WW1 and outwardly resembles a typewriter with QWERTY keyboard. The operator would press the key for the letter in the message to be transmitted and the enciphered letter would be lit up on the lamp board. The mechanism consisted a series of interchangeable rotors which rotated every time a key was pressed to keep the cipher continuously changing. This, combined with the plug board at the front of the machine where pairs of letters were transposed resulted in offered 159 million million million possible settings to choose from which the Germans believed made Enigma unbreakable.
  
  This example is one of a batch of 540 Enigmas supplied to Nachrichten-Geratlager, Berlin Templehof, Ringbannstrasse 9.