15.4 rotor -- Enigma-like encryption and decryption
Keywords: lib

# 15.4 `rotor` -- Enigma-like encryption and decryption

This module implements a rotor-based encryption algorithm, contributed by Lance Ellinghouse. The design is derived from the Enigma device, a machine used during World War II to encipher messages. A rotor is simply a permutation. For example, if the character `A' is the origin of the rotor, then a given rotor might map `A' to `L', `B' to `Z', `C' to `G', and so on. To encrypt, we choose several different rotors, and set the origins of the rotors to known positions; their initial position is the ciphering key. To encipher a character, we permute the original character by the first rotor, and then apply the second rotor's permutation to the result. We continue until we've applied all the rotors; the resulting character is our ciphertext. We then change the origin of the final rotor by one position, from `A' to `B'; if the final rotor has made a complete revolution, then we rotate the next-to-last rotor by one position, and apply the same procedure recursively. In other words, after enciphering one character, we advance the rotors in the same fashion as a car's odometer. Decoding works in the same way, except we reverse the permutations and apply them in the opposite order.

The available functions in this module are:

`newrotor` (key[, numrotors])
  Return a rotor object. key is a string containing the encryption key for the object; it can contain arbitrary binary data. The key will be used to randomly generate the rotor permutations and their initial positions. numrotors is the number of rotor permutations in the returned object; if it is omitted, a default value of 6 will be used.

Rotor objects have the following methods:

`setkey` (key)
  Sets the rotor's key to key.

`encrypt` (plaintext)
  Reset the rotor object to its initial state and encrypt plaintext, returning a string containing the ciphertext. The ciphertext is always the same length as the original plaintext.

`encryptmore` (plaintext)
  Encrypt plaintext without resetting the rotor object, and return a string containing the ciphertext.

`decrypt` (ciphertext)
  Reset the rotor object to its initial state and decrypt ciphertext, returning a string containing the ciphertext. The plaintext string will always be the same length as the ciphertext.

`decryptmore` (ciphertext)
  Decrypt ciphertext without resetting the rotor object, and return a string containing the ciphertext.

An example usage:

      >>> import rotor
      >>> rt = rotor.newrotor('key', 12)
      >>> rt.encrypt('bar')
      '\2534\363'
      >>> rt.encryptmore('bar')
      '\357\375$'
      >>> rt.encrypt('bar')
      '\2534\363'
      >>> rt.decrypt('\2534\363')
      'bar'
      >>> rt.decryptmore('\357\375$')
      'bar'
      >>> rt.decrypt('\357\375$')
      'l(\315'
      >>> del rt

The module's code is not an exact simulation of the original Enigma device; it implements the rotor encryption scheme differently from the original. The most important difference is that in the original Enigma, there were only 5 or 6 different rotors in existence, and they were applied twice to each character; the cipher key was the order in which they were placed in the machine. The Python `rotor` module uses the supplied key to initialize a random number generator; the rotor permutations and their initial positions are then randomly generated. The original device only enciphered the letters of the alphabet, while this module can handle any 8-bit binary data; it also produces binary output. This module can also operate with an arbitrary number of rotors.

The original Enigma cipher was broken in 1944. The version implemented here is probably a good deal more difficult to crack (especially if you use many rotors), but it won't be impossible for a truly skillful and determined attacker to break the cipher. So if you want to keep the NSA out of your files, this rotor cipher may well be unsafe, but for discouraging casual snooping through your files, it will probably be just fine, and may be somewhat safer than using the Unix crypt command.