Computer Security and Cryptography

6.1 THE ROTOR

The building block of a new class of enciphering machines was invented early in the twentieth century. Figure 6.1 shows a rotor or wire code-wheel, an electromechanical implementation of polyalphabetic substitution. The rotor is a disk of diameter ~4 in. and thickness ~0.4 in., made from rubber or bakelite (an early plastic), and is free to rotate about an axis perpendicular to its faces. Brass contacts arranged clockwise are evenly spaced around the circumference on each of the input and output faces, one for each letter of the signaling alphabet A, B,…, Z. Internal to the rotor body are electrical connections, 26 pairs of wires joining a contact on the rotor input face to a contact on the rotor output face.

Stationary input and output contact plates sandwich the rotor to provide for input and output. Each such plate contains contacts for the alphabet letters arranged so as to make electrical contact with those on the rotor's respective input and output faces. A signal applied to a contact on the input contact plate traverses a path composed of

• The opposing contact on the rotor input face,
• The wire within the body of the rotor,
• Connecting to a contact on the rotor output face, and finally
• Connecting to the opposing contact on the output contact plate.

A moveable ring containing the numbers 1, 2,…, 26 in Figure 6.1 (rather than A, B,…, Z) allows the rotor's rotational position to be aligned. In some benchmark position of the moveable ring, the rotor implements a ...