Editing Enigma machine (section)

=== Rotors ===
[[File:Enigma rotors with alphabet rings.jpg|thumb|Enigma rotor assembly. In the Enigma I, three movable rotors are sandwiched between two fixed wheels: the entry wheel, on the right, and the reflector on the left.]]
{{Main|Enigma rotor details}}
The rotors (alternatively ''wheels'' or ''drums'', ''Walzen'' in German) form the heart of an Enigma machine. Each rotor is a disc approximately {{convert|10|cm|in|abbr=on}} in diameter made from [[Ebonite]] or [[Bakelite]] with 26 [[brass]], spring-loaded, [[electrical contact]] pins arranged in a circle on one face, with the other face housing 26 corresponding electrical contacts in the form of circular plates. The pins and contacts represent the [[alphabet]]&nbsp;— typically the 26 letters A–Z, as will be assumed for the rest of this description. When the rotors are mounted side by side on the spindle, the pins of one rotor rest against the plate contacts of the neighbouring rotor, forming an electrical connection. Inside the body of the rotor, 26 wires connect each pin on one side to a contact on the other in a complex pattern. Most of the rotors are identified by Roman numerals, and each issued copy of rotor I, for instance, is wired identically to all others. The same is true for the special thin beta and gamma rotors used in the [[Enigma-M4|M4]] naval variant.

[[File:enigma-rotors.jpg|thumb|left|Three Enigma rotors and the shaft, on which they are placed when in use]]
By itself, a rotor performs only a very simple type of [[encryption]], a simple [[substitution cipher]]. For example, the pin corresponding to the letter ''E'' might be wired to the contact for letter ''T'' on the opposite face, and so on. Enigma's security comes from using several rotors in series (usually three or four) and the regular stepping movement of the rotors, thus implementing a polyalphabetic substitution cipher.

Each rotor can be set to one of 26 starting positions when placed in an Enigma machine. After insertion, a rotor can be turned to the correct position by hand, using the grooved finger-wheel which protrudes from the internal Enigma cover when closed. In order for the operator to know the rotor's position, each has an ''alphabet tyre'' (or letter ring) attached to the outside of the rotor disc, with 26 characters (typically letters); one of these is visible through the window for that slot in the cover, thus indicating the rotational position of the rotor. In early models, the alphabet ring was fixed to the rotor disc. A later improvement was the ability to adjust the alphabet ring relative to the rotor disc. The position of the ring was known as the ''Ringstellung'' ("ring setting"), and that setting was a part of the initial setup needed prior to an operating session. In modern terms it was a part of the [[initialization vector]].

[[File:Enigma rotors and spindle showing contacts rachet and notch.jpg|thumb|left|Two Enigma rotors showing electrical contacts, stepping ratchet (on the left) and notch (on the right-hand rotor opposite '''D''')]]
Each rotor contains one or more notches that control rotor stepping. In the military variants, the notches are located on the alphabet ring.

The Army and Air Force Enigmas were used with several rotors, initially three. On 15 December 1938, this changed to five, from which three were chosen for a given session. Rotors were marked with [[Roman numerals]] to distinguish them: I, II, III, IV and V, all with single turnover notches located at different points on the alphabet ring. This variation was probably intended as a security measure, but ultimately allowed the Polish [[Clock (cryptography)|Clock Method]] and British [[Banburismus]] attacks.

The Naval version of the ''[[Wehrmacht]]'' Enigma had always been issued with more rotors than the other services: At first six, then seven, and finally eight. The additional rotors were marked VI, VII and VIII, all with different wiring, and had two notches, resulting in more frequent turnover. The four-rotor Naval Enigma (M4) machine accommodated an extra rotor in the same space as the three-rotor version. This was accomplished by replacing the original reflector with a thinner one and by adding a thin fourth rotor. That fourth rotor was one of two types, ''Beta'' or ''Gamma'', and never stepped, but could be manually set to any of 26 positions. One of the 26 made the machine perform identically to the three-rotor machine.