Enigma Machine - Operation

Operation

In use, the Enigma required a list of daily key settings as well as a number of auxiliary documents. The procedures for German Naval Enigma were more elaborate and more secure than the procedures used in other services. The Navy codebooks were also printed in red, water-soluble ink on pink paper so that they could easily be destroyed if they were at risk of being seized by the enemy. The codebook to the right was taken from captured German submarine U-505.

In German military usage, communications were divided up into a number of different networks, all using different settings for their Enigma machines. These communication nets were termed keys at Bletchley Park, and were assigned code names, such as Red, Chaffinch, and Shark. Each unit operating on a network was assigned a settings list for its Enigma for a period of time. For a message to be correctly encrypted and decrypted, both sender and receiver had to set up their Enigma in the same way; the rotor selection and order, the starting position and the plugboard connections must be identical. All these settings (together the key in modern terms) must have been established beforehand, and were distributed in codebooks.

An Enigma machine's initial state, the cryptographic key, has several aspects:

• Wheel order (Walzenlage) — the choice of rotors and the order in which they are fitted.
• Initial position of the rotors — chosen by the operator, different for each message.
• Ring settings (Ringstellung) — the position of the alphabet ring relative to the rotor wiring.
• Plug connections (Steckerverbindungen) — the connections of the plugs in the plugboard.
• In very late versions, the wiring of the reconfigurable reflector.

Note that although the ring settings (ringstellung) were a required part of the setup, they did not actually affect the message encryption because the rotors were positioned independently of the rings. The ring settings were only necessary to determine the initial rotor position based on the message setting which was transmitted at the beginning of a message, as described in the "Indicators" section, below. Once the receiver had set his rotors to the indicated positions, the ring settings no longer played any role in the encryption.

In modern cryptographic language, the ring settings did not actually contribute entropy to the key used for encrypting the message. Rather, the ring settings were part of a separate key (along with the rest of the setup such as wheel order and plug settings) used to encrypt an initialization vector for the message. The session key consisted of the complete setup except for the ring settings, plus the initial rotor positions chosen arbitrarily by the sender (the message setting). The important part of this session key was the rotor positions, not the ring positions. However, by encoding the rotor position into the ring position using the ring settings, additional variability was added to the encryption of the initialization vector.

Enigma was designed to be secure even if the rotor wiring was known to an opponent, although in practice there was considerable effort to keep the wiring secret. If the wiring is secret, the total number of possible configurations has been calculated to be around 10114 (approximately 380 bits); with known wiring and other operational constraints, this is reduced to around 1023 (76 bits). Users of Enigma were confident of its security because of the large number of possibilities; it was not then feasible for an adversary to even begin to try every possible configuration in a brute force attack.