Page "Rotor machine" Paragraph 9

It is relatively straightforward to create a machine for performing simple substitution.

We can consider an electrical system with 26 switches attached to 26 light bulbs ; when you turn on any one of the switches, one of the light bulbs is illuminated.

If each switch is operated by a key on a typewriter, and the bulbs are labelled with letters, then such a system can be used for encryption by choosing the wiring between the keys and the bulb: for example, typing the letter < tt > A </ tt > would make the bulb labelled < tt > Q </ tt > light up.

However, the wiring is fixed, providing little security.