slides/keyword-break.html

   1 <!DOCTYPE html>
   2 <html>
   3   <head>
   4     <title>Breaking keyword ciphers</title>
   5     <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"/>
   6     <style type="text/css">
   7       /* Slideshow styles */
   8       body {
   9         font-size: 20px;
  10       }
  11       h1, h2, h3 {
  12         font-weight: 400;
  13         margin-bottom: 0;
  14       }
  15       h1 { font-size: 3em; }
  16       h2 { font-size: 2em; }
  17       h3 { font-size: 1.6em; }
  18       a, a > code {
  19         text-decoration: none;
  20       }
  21       code {
  22         -moz-border-radius: 5px;
  23         -web-border-radius: 5px;
  24         background: #e7e8e2;
  25         border-radius: 5px;
  26         font-size: 16px;
  27       }
  28       .plaintext {
  29         background: #272822;
  30         color: #80ff80;
  31         text-shadow: 0 0 20px #333;
  32         padding: 2px 5px;
  33       }
  34       .ciphertext {
  35         background: #272822;
  36         color: #ff6666;
  37         text-shadow: 0 0 20px #333;
  38         padding: 2px 5px;
  39       }
  40        .float-right {
  41         float: right;
  42       }
  43     </style>
  44   </head>
  45   <body>
  46     <textarea id="source">
  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
  81 * Thread-safe shared-memory code is hard.
  82
  83 The `multiprocessing` library makes this easier.
  84
  85 But before we get there, a couple of diversions...
  86
  87 ---
  88
  89 # DRYing code
  90
  91 Three cipher breaking tasks so far.
  92
  93 All working on the same principle:
  94
  95 ```
  96 find a way to enumerate all the possible keys
  97 initialise 'best so far'
  98 for each key:
  99     decipher message with this key
 100     score it
 101     if it's better than the best so far:
 102         update best so far
 103 ```
 104
 105 Repetition of code is a bad smell.
 106
 107 Separate the 'try all keys, keep the best' logic from the 'score this one key' logic.
 108
 109 ---
 110
 111 # map()
 112
 113 A common task is to apply a function to each item in a sequence, returning a sequence of the results.
 114
 115 ```python
 116 def double(x):
 117     return x * 2
 118
 119 >>> map(double, [1,2,3])
 120 [2,4,6]
 121 ```
 122
 123 * `map()` is a higher-order function: its first argument is the function that's applied.
 124
 125 How can we use this for keyword cipher breaking?
 126
 127 ---
 128
 129 # Mapping keyword decipherings
 130
 131 Define a function that takes a possible key (keyword and cipher type) and returns the key and its fitness.
 132
 133 * (Also pass in the message and the fitness function)
 134
 135 Use `map()` and `max()` to find the best key
 136
 137 ---
 138
 139 # Arity of print()
 140
 141 How many arguments does this take?
 142
 143 How do you write a function that takes this many arguments?
 144
 145 ---
 146
 147 # Function arguments
 148
 149 ## Positional, keyword
 150
 151 * Common or garden parameters, as you're used to.
 152 * `def keyword_encipher(message, keyword, Keyword_wrap_alphabet.from_a):`
 153
 154 ## Excess positional
 155 * `def mean(x, *xs):`
 156
 157 First number goes in `x`, remaining go in the tuple `xs`
 158
 159 ## Excess keyword
 160
 161 * `def myfunc(arg1=0, **kwargs):`
 162
 163 `kwargs` will be a Dict of the remaining keywords (not `arg1`)
 164
 165 ---
 166
 167 # Back to `multiprocessing`
 168
 169 What does `Pool.starmap()` do?
 170
 171 ---
 172
 173 ```python
 174 from multiprocessing import Pool
 175
 176 def keyword_break_mp(message, wordlist=keywords, fitness=Pletters, chunksize=500):
 177     helper_args = [??? for word in wordlist] # One tuple for each possible key
 178     with Pool() as pool:
 179         breaks = pool.starmap(keyword_break_worker, helper_args, chunksize)
 180         return max(breaks, key=lambda k: k[1])
 181
 182 def keyword_break_worker(???):
 183     ???
 184     return (key, fitness)
 185 ```
 186
 187 * Gotcha: the function in `Pool.starmap()` must be defined at the top level
 188     * This is definitely a "feature"
 189
 190 ---
 191
 192 # Performance and chunksize
 193
 194 Try the multiprocessing keyword break. Is it using all the resources?
 195
 196 Setting `chunksize` is an art.
 197
 198 ## Map-reduce as a general pattern for multiprocessing
 199
 200     </textarea>
 201     <script src="http://gnab.github.io/remark/downloads/remark-0.6.0.min.js" type="text/javascript">
 202     </script>
 203
 204     <script type="text/javascript"
 205       src="http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML&delayStartupUntil=configured"></script>
 206
 207     <script type="text/javascript">
 208       var slideshow = remark.create({ ratio: "16:9" });
 209
 210       // Setup MathJax
 211       MathJax.Hub.Config({
 212         tex2jax: {
 213         skipTags: ['script', 'noscript', 'style', 'textarea', 'pre']
 214         }
 215       });
 216       MathJax.Hub.Queue(function() {
 217         $(MathJax.Hub.getAllJax()).map(function(index, elem) {
 218             return(elem.SourceElement());
 219         }).parent().addClass('has-jax');
 220       });
 221       MathJax.Hub.Configured();
 222     </script>
 223   </body>
 224 </html>