ssreeni1 / the-enigma

Recreated the WW2 Enigma machine that completes encryption and decryption functions with user changeable rotor and plugboard settings. Parses through large text files and outputs the encrypted messages in cyclic permutations.

The-Enigma

This project involves building a simulator for a generalized version of the Enigma (which itself had several different versions.) My Enigma takes descriptions of possible initial configurations of the machine and messages to encode or decode (the Enigma algorithms were reciprocal, meaning that encryption is its own inverse operation.)

The Enigmas effect a substitution cipher, on the letters of a message. That is, at any given time, the machine performs a permutation---a one-to-one mapping---of the alphabet onto itself. The alphabet consists solely of the 26 letters in one case (there were various conventions for spaces and punctuation). Plain substitution ciphers are easy to break. The Enigma, however, implements a progressive substitution, different for each subsequent letter of the message. This made decryption considerably more difficult.

The device consists of a simple mechanical system of (partially) interchangeable rotors (Walzen) that sit side-by-side on a shaft and make electrical contact with each other. Most of these rotors have 26 contacts on both sides, which are wired together internally so as to effect a permutation of signals coming in from one side onto the contacts on the other (and the inverse permutation when going in the reverse direction). To the left of the rotors, one could select one of a set of reflectors (Umkehrwalzen), with contacts on their right sides only, and wired to connect half of those contacts to the other half. A signal starting from the right through one of the 26 possible contacts will flow through wires in the rotors, "bounce" off the reflector, and then come back through the same rotors (in reverse) by a different route, always ending up permuted to a letter position different from where it started.

Each rotor and each reflector implements a different permutation, and the overall effect depends on their configuration: which rotors and reflector are used, what order they are placed in the machine, and which rotational position they are initially set to. This configuration is the first part of the secret key used to encrypt or decrypt a message. In this machine, the selected rotors in a machine's configuration will be referred to as 1–N, with 1 being the reflector, and N the rightmost rotor. In our simulator, N will be a configuration parameter. In actual Enigma machines, it was fixed for any given model (the Navy used four and the Wehrmacht used three.)

The overall permutation changes with each successive letter because some of the rotors rotate after encrypting a letter. Each rotor has a circular ratchet on its right side and a an "alphabet ring" (Ringstellung) on its left side that fits over the ratchet of the rotor to its left. Before a letter of a message is translated, a spring-loaded pawl (lever), one to the right of each rotating rotor, tries to engage the ratchet on the right side of its rotor and thus rotate that rotor by one position, changing the permutation performed by the rotor. The lever on the rightmost rotor (N) always succeeds, so that rotor N (the "fast" rotor) rotates one position before each character. The pawls pushing the other rotors, however, are normally blocked from engaging their rotors by the ring on the left side of the rotor to their right. This ring usually holds the pawl away from its ratchet, preventing the rotor wheel to its left from moving. However, the rings have notches in them (either one or two in the original Enigma machines), and when the pawl is positioned over a notch in the ring for the rotor to its right, it slips through to its own rotor and pushes it forward. A "feature" of the design called "double stepping" (corrected in other versions of the Enigma, since it reduced the period of the cipher) is that when a pawl is in a notch, it also moves the notch itself and the rotor the notch is connected to, so that the rotors on both sides of the pawl move.