Cryptography

Computer Science & Statistics at University of Rhode Island

Description

Basic components of Enigma Machine

Keyboard - for inputting  plaintext or ciphertext characters.

Display Board - contains lamps that light up to indicate the ciphertext or plaintext letter.

Rotor - contains a ring with all 26 letters of the alphabet written on it and has internal wirings, which are the important part of the Enigma machine

Plugboard - Located between the keyboard and the first rotor, it allows the operator to swap pairs of letters.

Reflector - also swaps pairs of letters, the operator can manually change its settings.

Working Details 

Setup

  • Select the 3 rotor arrangement. The can be arranged as 123, 132, 213, 231, 312, 321.

  • Set the orientation of the three rotors. The rotors can be set to a particular orientation, such that the operator can see one of the letters on the rotor.

  • Set the plugboard. Using the plugboard the operator can swap the letters, so for example he can swap A with Q. The encrypted letter is different with different swaps. 

The electrical wiring between the keyboard, plugboard, internal wiring of the rotors, reflector, and the display board make the 26 electrical circuits in the machine, one for each letter of the alphabet. Rotating the rotors changes these circuits. The first rotor is rotated by one position after every key is pressed. The second rotor is rotated by one position after the first rotor make one complete rotation (i.e. one position for every 26 letters), and the third rotates after the second rotor makes the one complete rotation (i.e. one position for every 262 = 676 letters).

         

Once the operator sets the machine, he just needs to enter the letters of plaintext or ciphertext using the keyboard. When the operator presses a letter, an electric pulse passes through one of the circuits and illuminates a letter on the display board. This letter is the corresponding ciphertext/plaintext letter

 

Example

First the Enigma machine is set up by positioning the rotors and wiring the plugboard. 

Lets follow what happens when the O key is pressed.  The example is explained with respect to the above figure. Note in the figure straight connections are set for the plugboard. The encrypted letter depends on the initial set up and the number of letters encrypted after the initial set up and current letter.

First the electrical pulse passes from the keyboard through the plugboard to the letter O in the first rotor, the pulse makes the twists and turns in the 3 rotors and reaches the reflector. The reflector passes the pulse back to the rotors, and the pulse is again twisted and turned in the rotors and reaches the display board through the plugboard  and  illuminates the letter K, which is the encrypted letter.

If a swap is used between A and O, the encrypted letter for A will be K and a encrypted letter for O will not be K but X, if X is the encrypted letter of A without the swap.