The Enigma Machine
The Enigma machine, invented by Arthur Scherbius in 1918, is an electromechanical rotor-based cipher machine that was initially produced for the commercial market. While its predecessor was not very successful, the later lamp-based model known today was used by Germany during the Second World War to encrypt its communications.
It has become the most well known cipher machine in the world due to the recent publicity surrounding the work of Allied codebreakers during the Second World War.
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The Enigma machine works by means of rotors (as pictured above). The Enigma I, produced for the German Army (Wehrmacht) and Airforce (Luftwaffe), used three rotors, while machines produced for the German Navy (Kriegsmarine) initially used three, and then later, four. The Enigma I was equipped with two spare rotors while the Navy Enigma were equipped with six, later seven or eight spare rotors. In the machine, the rotors are attached together, allowing the 26 metal contacts of each rotor (one for each letter of the alphabet) to touch.
When a letter is pressed on the machine by the operator, an electrical current is produced. This current travels through the rotors, which have criss-crossed wiring. This changes the path of the current, encrypting the message. As each key is pressed, the right most rotor is moved forward (towards the operator) by one unit. Then when the rotor has completed one full revolution (26 units), the middle rotor (to its left) moves by one - and so on. This is known as stepping - controlled by pawls which attach to the rotor's black bakelite ratchet wheel. The rotation of the rotors therefore increases the code's difficulty to break as the circuit (and therefore the rotor settings) changes at each key-press.
The addition of a fourth rotor on the Kriegsmarine Enigma machine (the Enigma M4) increased its encryption by 26x. Existing 3-rotor machines were modified by replacing the existing reflector with a 'thin rotor' and a 'thin reflector' - either the β (beta) or γ (gamma) reflector. What is important to note, though, is that the name 'Enigma M4' has no relation to the number of rotors, nor 'Enigma M3' for three rotors. M stands for 'Marine' - the German word for Navy. 4 indicates that it was the fourth model.
There are two types of rotors - Wehrmacht/Luftwaffe Enigma rotors and Kriegsmarine Enigma rotors. Enigma rotors for Wehrmacht/Luftwaffe Enigma machines have a serial number starting with an 'A' and have a number wheel, while naval Enigma rotors have a serial number starting with 'M' and have a letter wheel.
In terms of the construction of rotors, several models of rotors exist. Initially, rotors had a primarily metal construction, with the thumb wheel made of aluminium and even the ratchet wheel made from metal as well. This later changed as the war progressed. The rotor pictured above has a bakelite ratchet wheel while retaining the aluminium thumb wheel. Some later-war rotors were primarily constructed out of Bakelite due to shortages of metal - with the thumb wheel, and even the number/letter wheel made from bakelite.
You can read more about the wiring of Enigma rotors here:
Crypto Museum - Enigma Rotor Wiring
Animated graphic created from original image courtesy of Bob Lord. You can visit his Crypto Museum website at: