The Enigma machine is an electro-mechanical device. It consists of a keyboard (German QWERTZ layout), a lamp panel, representing the alphabet, and three or four rotors. These rotors advance each time a key is depressed. Pressing a key is translated, by rotors and plugboard, into a burning lamp, representing the encoded letter. There is a battery compartment in the machine and a plug to connect the machine on an exterior power supply. A small transformator enables the use of AC current. The current flows from the battery through the depressed bi-directional switch to the plugboard. The plugboard allows rewiring the connections between keyboard and fixed entry wheel. Next, the current proceeds from entry wheel through the wirings of the three (Wehrmacht Enigma) or four (Kriegmarine M4) rotors and enters the reflector. The reflector returns the current, via a different path back through the rotors and entry wheel, and proceeds through the plugs and bi-directional switches to light-up the lamp.

1937 Enigma Manual - English Translation
http://www.ilord.com/enigma-manual1937-english.html

Rotors:

(Walzen in German) are the most important element of the machine. These round disks, approximately 10 cm in diameter, are made from metal or bakelite. A disk consists of a round casing with fixed notch and visible alphabet or number ring. The center of the rotor is a rotatable small disk with side 26 flat contact on one side, scramble wired to 26 spring loaded contacts on the other side. Changing the position of the internal wiring, relative to the notch and alphabet, is called the ring setting or Ringstellung. The internal wiring is different for each rotor. This wiring represents a substitution encryption. The combination of several rotors, in ever changing positions relative to each other, is what makes the encryption so complex. Each rotor has on his left a notch, on his right a ratchet. These are used by the stepping mechanism to advance the rotors.

Reflector:

The reflector was a unique feature of the Enigma machine. On the internal wiring of all movable rotors, each letter can be wired with any other letter. An 'A' could be wired to 'F,' where the 'F' is wired to 'K.' In the reflector, the connections are made in pairs. If the 'A' is wired to the 'F,' this means the 'F' is also wired to the 'A,' resulting in a reciprocal encryption. The advantage for the operator is clear. Encryption and decryption is possible with the same machine setting and wiring. Unfortunately, a letter can never be encrypted into itself, and this feature opened the door to cryptanalysis, making the job easier to the codebreakers.

Stepping Mechanism:

Each time a key is depressed, the position of the rotors is changed. This results in a different substitution encryption each time for the same given letter. The first rotor, on the right side, steps on each depressed key. The middle rotor advances once on every 26 advances of the first rotor. The third, slowest rotor, on every 26 advances of the middle. Depressing a key will move the pawl axe with three pawl upwards. Each pawl is placed both over the notch ring of the rotor to its right, and the ratchet of the rotor to it's left. If a rotor is in his notch position, his pawl can drop down. On the next step, this pawl pushes the ratchet of the rotor, left to the pawl. Since there is no rotor on the right side of the first rotor, the pawl will engage the first rotor on every depressing of a key. If no key is pressed, the pawl axe presses the pawls against pads on the bottom of the machine, pushing the pawls away from the rotors. The rotors can now be moved freely by hand, in both directions. Following the current flow during encryption, one must take in account that depressing a key will first move the rotors, and then send the current through the rotors.

Accessorizing the Enigma Machine

• Schreibmax - a handy feature used on the Kriegsmarine Enigma. This little printer could print the alphabet on a small paper ribbon. This excluded the need for a second operator, reading the lamps and writing the letters down. The Schreibmax was placed on top of the Enigma machine and was connected to the lamp panel; to install the printer, the lamp cover and all light bulbs had to be removed.
• Remote lamp panel - if the machine was equipped with an extra panel, the wooden case of the Enigma was wider and could store the extra panel. There was a lamp panel version that could be connected afterwards, but that required, just as with the Schreibmax, the lamp panel and light bulbs to be removed. The remote panel made it possible for a person to read the decrypted text, without giving the operator access to it.
• Uhr (clock) - an extra plugboard switch for Luftwaffe, introduced in 1944. There was a little box containing a switch with 40 positions. It replaced the default plugs. After connecting the plugs, as determined in the daily key sheet, the operator could turn the switch in one of the 40 positions, each position resulting in a different combination of plug wiring. Most of these plug connections are unlike the default plugs not pair-wise.

Security Properties of the Enigma Cipher Machine:

• The various versions of Enigma provided different levels of security. The presence of a plugboard (stecker) significantly increased the complexity of the machine. In general, unsteckered Enigma could be attacked using hand methods, while breaking versions with a plugboard was more involved, and often required the use of machines.
• The Enigma machine had a number of properties that proved very useful to cryptanalysts. Firstly, a letter could never be encrypted to itself (with the exception of the early models A and B, which lacked a reflector). The simple fact that no coded letter could ever be the original letter was vital to the codebreakers in their quest to unravel the messages. As they studied the intercepts, it became clear that the Germans kept repeating certain set phrases.
• It was self-reciprocal: encryption is performed identically to decryption. This imposed constraints on the type of scrambling that Enigma could provide at each position, and the property was used in a number of codebreaking methods.
• A weakness in many versions of the Enigma was that the rightmost wheel would rotate a constant number of places before the next would rotate.
• Besides less than ideal inherent characteristics of the machine, the way in which Enigma was used — that is, as a cryptosystem — proved to be the greatest weakness in practice. Mistakes by operators were common, and many of the officially specified procedures for using Enigma provided a variety of avenues for attack. It has been suggested, by some of those working on its cryptanalysis at Bletchley Park, that the Enigma would have been unbreakable in practice had its operators not been so error prone, and had its prescribed procedures been better designed.

"They were given the manuals, they were told exactly what to do and how to use the machine, but part of the problem was this myth that the Enigma machine was completely unbreakable. And this was buried deep in the German psyche, so therefore they thought why bother, you know, nobody can break these messages if we use these keys because they're easy." (Tony Sale Interview - Decoding Nazi Secrets Documentary)

""The Germans made many mistakes, and though the breakthrough was the result of immense effort in several fields it was one particular error that led directly to the conquest of Enigma. When the operator had set up his machine according to the instructions for the day, and was about to encipher a signal, he would begin by tapping at random a small group of letters. The machine gave him an encipherment of this group, which he now incorporated at the start of the signal. A recipient of the message would then know, from these few letters, how to set the rotors of his own machine for deciphering that particular text. It was, one might say, a key built into the message itself. In their meticulous way, however, the Germans repeated the group at the beginning of each message. To Hut 6, (in Bletchley Park) once the significance of the letters was realized, this duplication offered great possibilities."

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Enigma Cipher Machine Homepage: Jamie Brownell

References