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annotate Doc/lib/librotor.tex @ 5817:48b94de85bf3 legacy-trunk

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Extensive changes by AMK.
author Guido van Rossum <guido@python.org>
date Thu, 17 Jul 1997 16:29:42 +0000
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1 \section{Built-in Module \sectcode{rotor}}
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2 \bimodindex{rotor}
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3
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4 This module implements a rotor-based encryption algorithm, contributed by
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5 Lance Ellinghouse. The design is derived from the Enigma device, a machine
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6 used during World War II to encipher messages. A rotor is simply a
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7 permutation. For example, if the character `A' is the origin of the rotor,
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8 then a given rotor might map `A' to `L', `B' to `Z', `C' to `G', and so on.
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9 To encrypt, we choose several different rotors, and set the origins of the
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10 rotors to known positions; their initial position is the ciphering key. To
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11 encipher a character, we permute the original character by the first rotor,
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12 and then apply the second rotor's permutation to the result. We continue
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13 until we've applied all the rotors; the resulting character is our
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14 ciphertext. We then change the origin of the final rotor by one position,
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15 from `A' to `B'; if the final rotor has made a complete revolution, then we
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16 rotate the next-to-last rotor by one position, and apply the same procedure
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17 recursively. In other words, after enciphering one character, we advance
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18 the rotors in the same fashion as a car's odometer. Decoding works in the
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19 same way, except we reverse the permutations and apply them in the opposite
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20 order.
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21 \index{Ellinghouse, Lance}
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22 \indexii{Enigma}{cipher}
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23
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24 The available functions in this module are:
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25
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26 \renewcommand{\indexsubitem}{(in module rotor)}
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27 \begin{funcdesc}{newrotor}{key\optional{\, numrotors}}
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28 Return a rotor object. \var{key} is a string containing the encryption key
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29 for the object; it can contain arbitrary binary data. The key will be used
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30 to randomly generate the rotor permutations and their initial positions.
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31 \var{numrotors} is the number of rotor permutations in the returned object;
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32 if it is omitted, a default value of 6 will be used.
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33 \end{funcdesc}
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34
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35 Rotor objects have the following methods:
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36
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37 \renewcommand{\indexsubitem}{(rotor method)}
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38 \begin{funcdesc}{setkey}{key}
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39 Sets the rotor's key to \var{key}.
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40 \end{funcdesc}
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41
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42 \begin{funcdesc}{encrypt}{plaintext}
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43 Reset the rotor object to its initial state and encrypt \var{plaintext},
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44 returning a string containing the ciphertext. The ciphertext is always the
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45 same length as the original plaintext.
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46 \end{funcdesc}
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47
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48 \begin{funcdesc}{encryptmore}{plaintext}
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49 Encrypt \var{plaintext} without resetting the rotor object, and return a
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50 string containing the ciphertext.
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51 \end{funcdesc}
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52
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53 \begin{funcdesc}{decrypt}{ciphertext}
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54 Reset the rotor object to its initial state and decrypt \var{ciphertext},
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55 returning a string containing the ciphertext. The plaintext string will
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56 always be the same length as the ciphertext.
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57 \end{funcdesc}
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58
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59 \begin{funcdesc}{decryptmore}{ciphertext}
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60 Decrypt \var{ciphertext} without resetting the rotor object, and return a
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61 string containing the ciphertext.
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62 \end{funcdesc}
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63
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64 An example usage:
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65 \bcode\begin{verbatim}
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66 >>> import rotor
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67 >>> rt = rotor.newrotor('key', 12)
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68 >>> rt.encrypt('bar')
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69 '\2534\363'
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70 >>> rt.encryptmore('bar')
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71 '\357\375$'
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72 >>> rt.encrypt('bar')
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73 '\2534\363'
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74 >>> rt.decrypt('\2534\363')
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75 'bar'
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76 >>> rt.decryptmore('\357\375$')
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77 'bar'
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78 >>> rt.decrypt('\357\375$')
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79 'l(\315'
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80 >>> del rt
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81 \end{verbatim}\ecode
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82
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83 The module's code is not an exact simulation of the original Enigma device;
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84 it implements the rotor encryption scheme differently from the original. The
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85 most important difference is that in the original Enigma, there were only 5
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86 or 6 different rotors in existence, and they were applied twice to each
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87 character; the cipher key was the order in which they were placed in the
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88 machine. The Python rotor module uses the supplied key to initialize a
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89 random number generator; the rotor permutations and their initial positions
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90 are then randomly generated. The original device only enciphered the
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91 letters of the alphabet, while this module can handle any 8-bit binary data;
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92 it also produces binary output. This module can also operate with an
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93 arbitrary number of rotors.
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94
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95 The original Enigma cipher was broken in 1944. % XXX: Is this right?
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96 The version implemented here is probably a good deal more difficult to crack
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97 (especially if you use many rotors), but it won't be impossible for
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98 a truly skilful and determined attacker to break the cipher. So if you want
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99 to keep the NSA out of your files, this rotor cipher may well be unsafe, but
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100 for discouraging casual snooping through your files, it will probably be
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101 just fine, and may be somewhat safer than using the Unix \file{crypt}
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102 command.
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103 \index{National Security Agency}\index{crypt(1)}
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104 % XXX How were Unix commands represented in the docs?
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105