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108 lines
4.5 KiB
TeX
108 lines
4.5 KiB
TeX
\section{\module{rotor} ---
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Enigma-like encryption and decryption}
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\declaremodule{builtin}{rotor}
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\modulesynopsis{Enigma-like encryption and decryption.}
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This module implements a rotor-based encryption algorithm, contributed by
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Lance Ellinghouse\index{Ellinghouse, Lance}. The design is derived
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from the Enigma device\indexii{Enigma}{device}, a machine
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used during World War II to encipher messages. A rotor is simply a
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permutation. For example, if the character `A' is the origin of the rotor,
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then a given rotor might map `A' to `L', `B' to `Z', `C' to `G', and so on.
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To encrypt, we choose several different rotors, and set the origins of the
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rotors to known positions; their initial position is the ciphering key. To
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encipher a character, we permute the original character by the first rotor,
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and then apply the second rotor's permutation to the result. We continue
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until we've applied all the rotors; the resulting character is our
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ciphertext. We then change the origin of the final rotor by one position,
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from `A' to `B'; if the final rotor has made a complete revolution, then we
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rotate the next-to-last rotor by one position, and apply the same procedure
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recursively. In other words, after enciphering one character, we advance
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the rotors in the same fashion as a car's odometer. Decoding works in the
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same way, except we reverse the permutations and apply them in the opposite
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order.
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\indexii{Enigma}{cipher}
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The available functions in this module are:
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\begin{funcdesc}{newrotor}{key\optional{, numrotors}}
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Return a rotor object. \var{key} is a string containing the encryption key
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for the object; it can contain arbitrary binary data but not null bytes.
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The key will be used
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to randomly generate the rotor permutations and their initial positions.
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\var{numrotors} is the number of rotor permutations in the returned object;
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if it is omitted, a default value of 6 will be used.
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\end{funcdesc}
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Rotor objects have the following methods:
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\begin{methoddesc}[rotor]{setkey}{key}
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Sets the rotor's key to \var{key}. The key should not contain null bytes.
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\end{methoddesc}
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\begin{methoddesc}[rotor]{encrypt}{plaintext}
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Reset the rotor object to its initial state and encrypt \var{plaintext},
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returning a string containing the ciphertext. The ciphertext is always the
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same length as the original plaintext.
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\end{methoddesc}
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\begin{methoddesc}[rotor]{encryptmore}{plaintext}
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Encrypt \var{plaintext} without resetting the rotor object, and return a
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string containing the ciphertext.
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\end{methoddesc}
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\begin{methoddesc}[rotor]{decrypt}{ciphertext}
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Reset the rotor object to its initial state and decrypt \var{ciphertext},
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returning a string containing the plaintext. The plaintext string will
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always be the same length as the ciphertext.
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\end{methoddesc}
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\begin{methoddesc}[rotor]{decryptmore}{ciphertext}
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Decrypt \var{ciphertext} without resetting the rotor object, and return a
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string containing the plaintext.
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\end{methoddesc}
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An example usage:
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\begin{verbatim}
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>>> import rotor
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>>> rt = rotor.newrotor('key', 12)
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>>> rt.encrypt('bar')
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'\xab4\xf3'
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>>> rt.encryptmore('bar')
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'\xef\xfd$'
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>>> rt.encrypt('bar')
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'\xab4\xf3'
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>>> rt.decrypt('\xab4\xf3')
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'bar'
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>>> rt.decryptmore('\xef\xfd$')
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'bar'
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>>> rt.decrypt('\xef\xfd$')
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'l(\xcd'
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>>> del rt
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\end{verbatim}
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The module's code is not an exact simulation of the original Enigma
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device; it implements the rotor encryption scheme differently from the
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original. The most important difference is that in the original
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Enigma, there were only 5 or 6 different rotors in existence, and they
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were applied twice to each character; the cipher key was the order in
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which they were placed in the machine. The Python \module{rotor}
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module uses the supplied key to initialize a random number generator;
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the rotor permutations and their initial positions are then randomly
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generated. The original device only enciphered the letters of the
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alphabet, while this module can handle any 8-bit binary data; it also
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produces binary output. This module can also operate with an
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arbitrary number of rotors.
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The original Enigma cipher was broken in 1944. % XXX: Is this right?
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The version implemented here is probably a good deal more difficult to crack
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(especially if you use many rotors), but it won't be impossible for
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a truly skillful and determined attacker to break the cipher. So if you want
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to keep the NSA out of your files, this rotor cipher may well be unsafe, but
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for discouraging casual snooping through your files, it will probably be
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just fine, and may be somewhat safer than using the \UNIX{} \program{crypt}
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command.
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\index{NSA}
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\index{National Security Agency}
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