Editing Cryptanalysis of the Enigma (section)

===Structure===
The mechanism of the Enigma consisted of a [[keyboard (computing)|keyboard]] connected to a [[battery (electricity)|battery]] and a [[Enigma machine#Entry wheel|current entry plate]] or wheel (German: ''Eintrittswalze''), at the right hand end of the scrambler (usually via a [[plugboard]] in the military versions).<ref>{{Harvnb|Alexander|c. 1945}} "Background" Para. 2 Alexander (c. 1945) "Background" Para. 2</ref> This contained a set of 26 contacts that made electrical connection with the set of 26 spring-loaded pins on the right hand rotor. The internal wiring of the core of each rotor provided an electrical pathway from the pins on one side to different connection points on the other. The left hand side of each rotor made electrical connection with the rotor to its left. The leftmost rotor then made contact with the [[Enigma machine#Reflector|reflector]] (German: ''Umkehrwalze''). The reflector provided a set of thirteen paired connections to return the current back through the scrambler rotors, and eventually to the lampboard where a lamp under a letter was illuminated.<ref>{{Harvnb|Ellsbury|1998a}}</ref>

Whenever a key on the keyboard was pressed, the [[Enigma machine#Stepping|stepping motion]] was actuated, advancing the rightmost rotor one position. Because it moved with each key pressed it is sometimes called the ''fast rotor''. When a notch on that rotor engaged with a [[Enigma machine#Stepping|pawl]] on the middle rotor, that too moved; and similarly with the leftmost ('slow') rotor.

There are a huge number of ways that the connections within each scrambler rotor—and between the entry plate and the keyboard or plugboard or lampboard—could be arranged. For the reflector plate there are fewer, but still a large number of options to its possible wirings.<ref>{{Harvnb|Churchhouse|2002|pp=202–204}}</ref>

Each scrambler rotor could be set to any one of its 26 starting positions (any letter of the alphabet). For the Enigma machines with only three rotors, their sequence in the scrambler—which was known as the ''wheel order (WO)'' to [[Allies of World War II|Allied]] cryptanalysts—could be selected from the six that are possible.

{| class="wikitable" | border=1 style="margin: 1em auto 1em auto"
|+ Possible rotor sequences—also known as ''Wheel Order (WO)''
|-
! width="70pt"| Left
! width="70pt"| Middle
! width="70pt"| Right
|-
| align="center" | I ||align="center" | II ||align="center" | III
|-
| align="center" | I || align="center" | III || align="center" | II
|-
| align="center" | II || align="center" | I || align="center" | III
|-
| align="center" | II || align="center" | III || align="center" | I
|-
| align="center" | III || align="center" | I || align="center" | II
|-
| align="center" | III || align="center" | II || align="center" | I
|}

[[File:Enigma-plugboard.jpg|right|thumbnail|The plugboard (''Steckerbrett'') was positioned at the front of the machine, below the keys. In the above photograph, two pairs of letters have been swapped (A↔J and S↔O). During World War II, ten leads were used, leaving only six letters 'unsteckered'.]]

Later Enigma models included an ''alphabet ring'' like a tyre around the core of each rotor. This could be set in any one of 26 positions in relation to the rotor's core. The ring contained one or more notches that engaged with a pawl that advanced the next rotor to the left.<ref>{{citation |last=Sale |first=Tony |author-link=Anthony Sale |title=The components of the Enigma machine |series=Enigma rotors (or wheels) |url=https://www.codesandciphers.org.uk/enigma/enigma2.htm |access-date=1 January 2010}}</ref>

Later still, the three rotors for the scrambler were selected from a set of five or, in the case of the German Navy, eight rotors. The alphabet rings of rotors VI, VII, and VIII contained two notches which, despite shortening the period of the substitution alphabet, made decryption more difficult.

Most military Enigmas also featured a [[plugboard]] (German: ''Steckerbrett''). This altered the electrical pathway between the keyboard and the entry wheel of the scrambler and, in the opposite direction, between the scrambler and the lampboard. It did this by exchanging letters reciprocally, so that if ''A'' was plugged to ''G'' then pressing key ''A'' would lead to current entering the scrambler at the ''G'' position, and if ''G'' was pressed the current would enter at ''A''. The same connections applied for the current on the way out to the lamp panel.

To decipher German military Enigma messages, the following information would need to be known.

'''Logical structure of the machine''' (unchanging)
*The wiring between the keyboard (and lampboard) and the entry plate.
*The wiring of each rotor.
*The number and position(s) of turnover notches on the rings of the rotors.
*The wiring of the reflectors.
'''Internal settings''' (usually changed less frequently than external settings)
*The selection of rotors in use and their ordering on the spindle (''Walzenlage'' or "wheel order").
*The positions of the alphabet ring in relation to the core of each rotor in use (''Ringstellung'' or "ring settings").
'''External settings''' (usually changed more frequently than internal settings)
*The plugboard connections (''Steckerverbindungen'' or "stecker values").
*The rotor positions at the start of enciphering the text of the message.

Discovering the logical structure of the machine may be called "breaking" it, a one-off process except when changes or additions were made to the machines. Finding the internal and external settings for one or more messages may be called "solving"{{sfn|Huttenhain|Fricke|1945|p=2}} – although breaking is often used for this process as well.