pocket_enigma.py

   1 from szyfrow.support.utilities import *
   2 from szyfrow.support.language_models import *
   3
   4
   5 class PocketEnigma(object):
   6     """A pocket enigma machine
   7     The wheel is internally represented as a 26-element list self.wheel_map,
   8     where wheel_map[i] == j shows that the position i places on from the arrow
   9     maps to the position j places on.
  10     """
  11     def __init__(self, wheel=1, position='a'):
  12         """initialise the pocket enigma, including which wheel to use and the
  13         starting position of the wheel.
  14
  15         The wheel is either 1 or 2 (the predefined wheels) or a list of letter
  16         pairs.
  17
  18         The position is the letter pointed to by the arrow on the wheel.
  19
  20         >>> pe.wheel_map
  21         [25, 4, 23, 10, 1, 7, 9, 5, 12, 6, 3, 17, 8, 14, 13, 21, 19, 11, 20, 16, 18, 15, 24, 2, 22, 0]
  22         >>> pe.position
  23         0
  24         """
  25         self.wheel1 = [('a', 'z'), ('b', 'e'), ('c', 'x'), ('d', 'k'),
  26             ('f', 'h'), ('g', 'j'), ('i', 'm'), ('l', 'r'), ('n', 'o'),
  27             ('p', 'v'), ('q', 't'), ('s', 'u'), ('w', 'y')]
  28         self.wheel2 = [('a', 'c'), ('b', 'd'), ('e', 'w'), ('f', 'i'),
  29             ('g', 'p'), ('h', 'm'), ('j', 'k'), ('l', 'n'), ('o', 'q'),
  30             ('r', 'z'), ('s', 'u'), ('t', 'v'), ('x', 'y')]
  31         if wheel == 1:
  32             self.make_wheel_map(self.wheel1)
  33         elif wheel == 2:
  34             self.make_wheel_map(self.wheel2)
  35         else:
  36             self.validate_wheel_spec(wheel)
  37             self.make_wheel_map(wheel)
  38         if position in string.ascii_lowercase:
  39             self.position = pos(position)
  40         else:
  41             self.position = position
  42
  43     def make_wheel_map(self, wheel_spec):
  44         """Expands a wheel specification from a list of letter-letter pairs
  45         into a full wheel_map.
  46
  47         >>> pe.make_wheel_map(pe.wheel2)
  48         [2, 3, 0, 1, 22, 8, 15, 12, 5, 10, 9, 13, 7, 11, 16, 6, 14, 25, 20, 21, 18, 19, 4, 24, 23, 17]
  49         """
  50         self.validate_wheel_spec(wheel_spec)
  51         self.wheel_map = [0] * 26
  52         for p in wheel_spec:
  53             self.wheel_map[pos(p[0])] = pos(p[1])
  54             self.wheel_map[pos(p[1])] = pos(p[0])
  55         return self.wheel_map
  56
  57     def validate_wheel_spec(self, wheel_spec):
  58         """Validates that a wheel specificaiton will turn into a valid wheel
  59         map.
  60
  61         >>> pe.validate_wheel_spec([])
  62         Traceback (most recent call last):
  63             ...
  64         ValueError: Wheel specification has 0 pairs, requires 13
  65         >>> pe.validate_wheel_spec([('a', 'b', 'c')]*13)
  66         Traceback (most recent call last):
  67             ...
  68         ValueError: Not all mappings in wheel specificationhave two elements
  69         >>> pe.validate_wheel_spec([('a', 'b')]*13)
  70         Traceback (most recent call last):
  71             ...
  72         ValueError: Wheel specification does not contain 26 letters
  73         """
  74         if len(wheel_spec) != 13:
  75             raise ValueError("Wheel specification has {} pairs, requires 13".
  76                 format(len(wheel_spec)))
  77         for p in wheel_spec:
  78             if len(p) != 2:
  79                 raise ValueError("Not all mappings in wheel specification"
  80                     "have two elements")
  81         if len(set([p[0] for p in wheel_spec] +
  82                     [p[1] for p in wheel_spec])) != 26:
  83             raise ValueError("Wheel specification does not contain 26 letters")
  84
  85     def encipher_letter(self, letter):
  86         """Enciphers a single letter, by advancing the wheel before looking up
  87         the letter on the wheel.
  88
  89         >>> pe.set_position('f')
  90         5
  91         >>> pe.encipher_letter('k')
  92         'h'
  93         """
  94         self.advance()
  95         return self.lookup(letter)
  96     decipher_letter = encipher_letter
  97
  98     def lookup(self, letter):
  99         """Look up what a letter enciphers to, without turning the wheel.
 100
 101         >>> pe.set_position('f')
 102         5
 103         >>> cat([pe.lookup(l) for l in string.ascii_lowercase])
 104         'udhbfejcpgmokrliwntsayqzvx'
 105         >>> pe.lookup('A')
 106         ''
 107         """
 108         if letter in string.ascii_lowercase:
 109             return unpos(
 110                 (self.wheel_map[(pos(letter) - self.position) % 26] +
 111                     self.position))
 112         else:
 113             return ''
 114
 115     def advance(self):
 116         """Advances the wheel one position.
 117
 118         >>> pe.set_position('f')
 119         5
 120         >>> pe.advance()
 121         6
 122         """
 123         self.position = (self.position + 1) % 26
 124         return self.position
 125
 126     def encipher(self, message, starting_position=None):
 127         """Enciphers a whole message.
 128
 129         >>> pe.set_position('f')
 130         5
 131         >>> pe.encipher('helloworld')
 132         'kjsglcjoqc'
 133         >>> pe.set_position('f')
 134         5
 135         >>> pe.encipher('kjsglcjoqc')
 136         'helloworld'
 137         >>> pe.encipher('helloworld', starting_position = 'x')
 138         'egrekthnnf'
 139         """
 140         if starting_position:
 141             self.set_position(starting_position)
 142         transformed = ''
 143         for l in message:
 144             transformed += self.encipher_letter(l)
 145         return transformed
 146     decipher = encipher
 147
 148     def set_position(self, position):
 149         """Sets the position of the wheel, by specifying the letter the arrow
 150         points to.
 151
 152         >>> pe.set_position('a')
 153         0
 154         >>> pe.set_position('m')
 155         12
 156         >>> pe.set_position('z')
 157         25
 158         """
 159         self.position = pos(position)
 160         return self.position
 161
 162
 163 def pocket_enigma_break_by_crib(message, wheel_spec, crib, crib_position):
 164     """Break a pocket enigma using a crib (some plaintext that's expected to
 165     be in a certain position). Returns a list of possible starting wheel
 166     positions that could produce the crib.
 167
 168     >>> pocket_enigma_break_by_crib('kzpjlzmoga', 1, 'h', 0)
 169     ['a', 'f', 'q']
 170     >>> pocket_enigma_break_by_crib('kzpjlzmoga', 1, 'he', 0)
 171     ['a']
 172     >>> pocket_enigma_break_by_crib('kzpjlzmoga', 1, 'll', 2)
 173     ['a']
 174     >>> pocket_enigma_break_by_crib('kzpjlzmoga', 1, 'l', 2)
 175     ['a']
 176     >>> pocket_enigma_break_by_crib('kzpjlzmoga', 1, 'l', 3)
 177     ['a', 'j', 'n']
 178     >>> pocket_enigma_break_by_crib('aaaaa', 1, 'l', 3)
 179     []
 180     """
 181     pe = PocketEnigma(wheel=wheel_spec)
 182     possible_positions = []
 183     for p in string.ascii_lowercase:
 184         pe.set_position(p)
 185         plaintext = pe.decipher(message)
 186         if plaintext[crib_position:crib_position+len(crib)] == crib:
 187             possible_positions += [p]
 188     return possible_positions
 189
 190 if __name__ == "__main__":
 191     import doctest
 192     doctest.testmod(extraglobs={'pe': PocketEnigma(1, 'a')})