slides/keyword-break.html

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  47
  48 # Breaking keyword ciphers
  49
  50 a | b | c | d | e | f | g | h | i | j | k | l | m | n | o | p | q | r | s | t | u | v | w | x | y | z
  51 --|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|--
  52 k | e | y | w | o | r | d | a | b | c | f | g | h | i | j | l | m | n | p | q | s | t | u | v | x | z
  53
  54 ----
  55
  56 # Duplicate and extend your `affine_break()` function
  57
  58 * How to cycle through all the keys? What _are_ all the keys?
  59
  60 * Look at `words.txt`
  61
  62 ---
  63
  64 # Test it.
  65
  66 * `2013/4a.ciphertext`
  67 * `2013/4b.ciphertext`
  68
  69 This will take a while. Fire up a system monitor. What's wrong?
  70
  71 ---
  72
  73 # Python, threads, and the GIL
  74
  75 Thread-safe shared-memory code is hard.
  76
  77 Python's Global Interpreter Lock prevents shooting yourself in the foot.
  78
  79 Where you want true parallelism, need different threads (Python processes).
  80 * Thread-safe shared-memory code is hard.
  81
  82 The `multiprocessing` library makes this easier.
  83
  84 But before we get there, a couple of diversions...
  85
  86 ---
  87
  88 # `map()`
  89
  90 A common task is to apply a function to each item in a sequence, returning a sequence of the results.
  91
  92 ```python```
  93 def double(x):
  94     return x * 2
  95
  96 >>> map(double, [1,2,3])
  97 [2,4,6]
  98 ```
  99
 100 * `map()` is a higher-order function: its first argument is the function that's applied.
 101
 102 How can we use this for keyword cipher breaking?
 103
 104 ---
 105
 106 # Mapping keyword decipherings
 107
 108 Define a function that takes a possible key (keyword and cipher type) and returns the key and its fitness.
 109
 110 Use `map()` and `max()` to find the best key
 111
 112 ---
 113
 114 # `print()`
 115
 116 How many arguments does this take?
 117
 118 How do you write a function that takes this many arguments?
 119
 120 ---
 121
 122 # Function arguments
 123
 124 ## Positional, keyword
 125
 126 * Common or garden parameters, as you're used to.
 127 * `def keyword_encipher(message, keyword, wrap_alphabet=0):`
 128
 129 ## Excess positional
 130 * `def mean(x, *xs):`
 131
 132 First number goes in `x`, remaining go in the tuple `xs`
 133
 134 ## Excess keyword
 135
 136 * `def myfunc(arg1=0, **kwargs):`
 137
 138 `kwargs` will be a Dict of the remaining keywords (not `arg1`)
 139
 140 ---
 141
 142 # Back to `multiprocessing`
 143
 144 What does `Pool.starmap()` do?
 145
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