slides/pocket-enigma-break.html

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  47
  48 # Breaking the Pocket Enigma
  49
  50 ![centre-aligned Pocket Engima](pocket-enigma-small.jpg)
  51
  52 Using cribs
  53
  54 ---
  55
  56 # Breaking the Pocket Enigma
  57
  58 A _crib_ is a piece of plaintext we believe to be in the enciphered message.
  59
  60 This is the way Enigma messages were broken in WWII.
  61
  62 There are many keys, and doing the naïve Bayes analysis was too expensive.
  63
  64 The possible keys were filtered by finding the ones that could have produced the crib. These filtered keys could then be checked manually.
  65
  66 At Bletchley Park, the filtering was done by the Bombes.
  67
  68 ---
  69
  70 # Breaking by cribs
  71
  72 ```
  73 Given a message, a wheel spec, a crib, and the location of that crib:
  74     Go through each key in turn
  75     If the plaintext matches the crib:
  76         Add it to the list of possible keys
  77 Return the possible keys
  78 ```
  79
  80 ## Example
  81 ```python
  82 pocket_enigma_break_by_crib('kzpjlzmoga', 1, 'l', 3)
  83 ['a', 'j', 'n']
  84 ```
  85
  86 Using wheel 1, the `j` (`ciphertext[3]`) could become `l`  if the wheel starts the message on a, j, or n.
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