Enigma rotor details

This article contains technical details about the rotors of the Enigma machine.

Understanding the way the machine encrypts requires taking into account the current position of each rotor, the ring setting and its internal wiring.

The right side of a rotor, showing the pin electrical contacts. The Roman numeral V identifies the wiring of the rotor.

The left side of an Enigma rotor, showing the flat (plate) electrical contacts. A single turnover notch is visible on the left edge of the rotor.

Since the same wires are used for forwards and backwards legs, a major cryptographic weakness is that no letter can map to itself.

In order to understand the effect of rotation on the rotors we demonstrate this by some examples.

As an example, let us take rotor type I of Enigma I (see table below) without any ring setting offset. You can see that an A is encoded as an E, a B encoded as a K, and a K is encoded as an N. Notice that every letter is encoded into another.

In the case of the reflectors, we take Wide B (Reflector B in the table below) where an A is returned as a Y and the Y is returned as an A. Notice that the wirings are connected as a loop between two letters.

When a rotor has stepped, you must take into account the offset to know what the output is, and where it enters the next rotor.

If for example rotor I is in the B-position, an A enters at the letter B which is wired to the K. Because of the offset this K enters the next rotor in the J position.

With the rotors I, II and III (from left to right), wide B-reflector, all ring settings in A-position, and start position AAA, typing AAAAA will produce the encoded sequence BDZGO.

The ring settings, or Ringstellung, are used to change the position of the internal wiring relative to the rotor. They do not change the notch or the alphabet ring on the exterior. Those are fixed to the rotor. Changing the ring setting will therefore change the positions of the wiring, relative to the turnover-point and start position.

The ring setting will rotate the wiring. Where rotor I in the A-position normally encodes an A into an E, with a ring setting offset B-02 it will be encoded into K

As mentioned before these encodings only happen after the key is pressed and the rotor has turned. Tracing the signal on the rotors AAA is therefore only possible if a key is pressed while the rotors were in the position AAZ.

With the rotors I, II, III (from left to right), wide B-reflector, all ring settings in B-position, and start position AAA, typing AAAAA will produce the encoded sequence EWTYX.

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