Five rotors were supplied with every Enigma. Each rotor carried the serial number of the Enigma, but could have been used in a different machine. Rotors also carried a Roman number from I to V that indicated one of the 5 internal rotor wiring configurations. Any three of the five rotors numbered I to V could be put into the 3-rotor Enigma in an order determined by the code book instructions for the specific day of the month.

The three-rotor ENIGMA, the standard German electronic ciphering machine widely used in World War II. It derives from a 1919 patent of a Dutch inventor, H.A. Koch; an early design marketed by Dr. Arthur Scherbius was bought out by the German military in 1929 and placed in service. ENIGMA in several variants was used by the German Navy, the Wehrmacht, the Luftwaffe, the state railroad system, the Abwehr (intelligence) and the SS.

It was designed with a complex, interchangeable series of three rotors bearing the 26-character alphabet, a 'reflector' and a plugboard with movable connecting cords that connected pairs of letters. As an added precaution, the base or starting settings for the rotors was changed every 24 hours, according to pre-printed setting registers furnished in advance or supplied daily by courier. It has been calculated that the 3-rotor ENIGMA, with plugboard in use, made possible a total of 15 quintillion possible readings for each character.

ENIGMA was widely regarded by the Germans as too complex to be broken, but in the 1930s a team of Polish analysts (Marian Rejewski, Jerzy Rszycki and Henryk Zygalski) made remarkable progress in working out the machine's basic system, identified its vulnerabilities and succeeded in deciphering much of the encrypted German radio traffic. Their findings, including plans for very useful mechanical devices known as 'bombes', which aided in the decryption operation, were secretly passed on in 1939 to French and British investigators. An elite team of cryptanalysts, mathematicians and engineers including Alan Turing (1912-54) were established in a top-secret facility at Bletchley Park. For the rest of the war that legendary team's heroic and unstinting efforts gradually accomplished the seemingly insurmountable task of deciphering an enormous volume of encrypted communications. The critical intelligence deriving from their decipherment was dubbed ULTRA and was employed cautiously but to great effect during the war; some commentators credit ULTRA with shortening the war by some two years.