Marian Rejewski - Enigma Machine

Enigma Machine

In late October or early November 1932, while work on the Naval code was still underway, Rejewski was set to work, alone and in secret, on the output of the new standard German cipher machine, the Enigma I, which was coming into widespread use. While the Cipher Bureau had, by later report, succeeded in solving an earlier, plugboard-less Enigma,(Note 3) it had had no success with the Enigma I.

The Enigma machine was an electromechanical device, equipped with a 26-letter keyboard and a set of 26 lamps, corresponding to the letters of the alphabet. Inside was a set of wired drums ("rotors" and a "reflector") that scrambled the input. The machine also featured a plugboard to swap pairs of letters. To encipher a letter, the operator pushed the relevant key and noted down which of the lamps lit. Each key press caused one or more rotors to advance, and thus the encipherment varied from one key press to the next.

In order for two operators to communicate, both Enigma machines had to be set up in the same way. The large number of possibilities for setting the rotors and the plugboard combined to form an astronomical number of configurations, each of which would produce a different cipher. The settings were changed daily, with the consequence that the machine had to be "broken" anew each day if the messages were to be read continually.

To decrypt Enigma messages, three pieces of information were needed:

1. A general understanding of how Enigma functioned
2. The wiring of the rotors
3. The daily settings: the sequence and orientations of the rotors (of which there were three initially), and the plug connections on the plugboard

Rejewski had only the first at his disposal, based on information already acquired by the Cipher Bureau.