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Enigma was a cipher machine (or, more accurately, family of machines) used by the Germans during World War II for the encryption and decryption of secret messages. It looked like and old typewriter with some extra features, like the lampboard or the rotors.
Enigma machines were used initially in the 1920s commercially, and it wasn't until a little earlier than the beginning of WWII that it was adopted by the German military and government for their communications.
The Enigma machine was first adopted by the German Navy in 1926. Two years later, the German Army introduced its own variation of the machine, the Enigma G, shortly after revised to the Enigma I, also known as the Wehrmacht, or "Services", Enigma, widely used during the war by both military and government organizations.
In 1934, the Navy adopted the Army Enigma I, which after some little changes was brought into service as the Enigma M3. The main differences between both types I and M3 being the fact that while the type I used only three rotors, the Army M3 allowed to choose three out of five. The Navy M3 allowed to choose three out of a possible eight rotors.
In 1942, a new variation was introduced by the Navy for U-boat traffic. It was the Enigma M4, and added a fourth rotor to the machine. This extra rotor was fitted in the machine by replacing the M3 reflector with a combination of thin rotors and thin reflectors.
Parts of a machine
Besides the obvious keyboard, an Enigma machine was a combination of several others electrical and mechanical subsystems interconnected to produce the encryption/decryption of the messages.
An Enigma Machine
Right after a key was pressed on the keyboard, the machine generated an electric signal which, before travelling to the rotors, went to the plugboard. The plugboard allowed to arbitrarily wire pairs of letters by means of cables (normally, only 10 pairs were used at any one time). The effect was to swap those letters before and after the electrical signal traveled to the rotor units, and right after it came back, before showing the letter on the lamp board.
For example, if the letter H and X where paired in this way, whenever the operator pressed H, the signal went to the rotors as if it was the letter X which was pressed. The inverse was also true, pressing X would send an H to the rotors. Right after the signal came from the rotors, if it was an H or an X, it was swapped again before highlighting the corresponding letter.
Also known as entry stator (Eintrittswalze in german), usually connects the plugboard to the rotors. If no plugboard is present, then it connects directly the keyboard with the rotors. The connetionsmade on the military enigma were transparent, that is A>A, B>B, C>C, etc.
However, on the commercial enigma the wiring was made following the keyboard configuration, Q>A, W>B, E>C, and so on, making it an obstacle to the initial polish attempts at deducing the
machine configuration (finally solved by cryptanalyst Marian Rejewsky).
The core of the machine were its rotors (or wheels), usually three of them, aligned at the top of the case of the machine. Each rotor was a disc with 26 connections, one for each letter in the alphabet.
When the rotors where mounted on the machine side by side, the pins of each one of them rested against the contacts of the neighbouring rotor, forming and electrical connection.
By itself, a rotor performs a very simple type of encryption, a substitution cipher, in which a letter is swapped with some other. The Enigma complexity came from the interconnection of the different rotors and the stepping mechanism of these.
Every time a key was pressed, before the electrical current was sent, the right rotor stepped, rotating one position of the 26 possible. At predefined positions, this stepping would produce a stepping on the rotor situated on the left of this one.
The reflector was situated after the last rotors on the chain, that is, on the left side of the machine. It connected the outputs of the las rotor in pairs, so, whenever a signal came through, it was reflected with another letter, redirecting the current back to the rotors.
Finally, the lamp board, placed above the keyboard. Every time a key was pressed a lamp was lighted, showing the corresponding encoded letter.