Editing Rotor machine (section)

==Mechanization==
It is straightforward to create a machine for performing simple substitution. In an electrical system with 26 switches attached to 26 light bulbs, any one of the switches will illuminate one of the bulbs.
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 {{mono|A}} would make the bulb labelled {{mono|Q}} light up. However, the wiring is fixed, providing little security.

Rotor machines change the interconnecting wiring with each key stroke. The wiring is placed inside a rotor, and then rotated with a gear every time a letter is pressed. 
So while pressing {{mono|A}} the first time might generate a {{mono|Q}}, the next time it might generate a {{mono|J}}. Every letter pressed on the keyboard increments the rotor position and get a new substitution, implementing a polyalphabetic substitution cipher.

Depending on the size of the rotor, this may, or may not, be more secure than hand ciphers. If the rotor has only 26 positions on it, one for each letter, then all messages will have a (repeating) key 26 letters long. Although the key itself (mostly hidden in the wiring of the rotor) might not be known, the methods for attacking these types of ciphers don't need that information. So while such a ''single rotor'' machine is certainly easy to use, it is no more secure than any other partial polyalphabetic cipher system.

But this is easy to correct. Simply stack more rotors next to each other, and gear them together. After the first rotor spins "all the way", make the rotor beside it spin one position. Now you would have to type 26 × 26 = 676 letters (for the [[Latin alphabet]]) before the key repeats, and yet it still only requires you to communicate a key of two letters/numbers to set things up. If a key of 676 length is not long enough, another rotor can be added, resulting in a period 17,576 letters long.

In order to be as easy to decipher as encipher, some rotor machines, most notably the [[Enigma machine]], embodied a [[symmetric-key algorithm]], i.e., encrypting twice with the same settings recovers the original message (see [[involution (mathematics)|involution]]).