slides/pocket-enigma-break.html

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  52
  53 # Breaking the Pocket Enigma
  54
  55 ![centre-aligned Pocket Engima](pocket-enigma-small.jpg)
  56
  57 Using cribs
  58
  59 ---
  60
  61 layout: true
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  63 .indexlink[[Index](index.html)]
  64
  65 ---
  66
  67 # Breaking the Pocket Enigma
  68
  69 A _crib_ is a piece of plaintext we believe to be in the enciphered message.
  70
  71 This is the way Enigma messages were broken in WWII.
  72
  73 There are many keys, and doing the naïve Bayes analysis was too expensive.
  74
  75 The possible keys were filtered by finding the ones that could have produced the crib. These filtered keys could then be checked manually.
  76
  77 At Bletchley Park, the filtering was done by the Bombes.
  78
  79 ---
  80
  81 # Breaking by cribs
  82
  83 ```
  84 Given a message, a wheel spec, a crib, and the location of that crib:
  85     Go through each key in turn
  86     If the plaintext matches the crib:
  87         Add it to the list of possible keys
  88 Return the possible keys
  89 ```
  90
  91 ## Example
  92 ```python
  93 pocket_enigma_break_by_crib('kzpjlzmoga', 1, 'l', 3)
  94 ['a', 'j', 'n']
  95 ```
  96
  97 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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