slides/keyword-encipher.html

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  52
  53 # Keyword ciphers
  54
  55 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
  56 --|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|--
  57 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
  58
  59 * Taking a more Pythonic approach
  60
  61 ---
  62
  63 layout: true
  64
  65 .indexlink[[Index](index.html)]
  66
  67 ---
  68
  69 # The cipher
  70
  71 * Still character-by-character substitution, still monosubstitution.
  72
  73 Ciphertext alphabet: start with a keyword, write out the rest of the alphabet, removing duplicates.
  74
  75 ## Three variants
  76
  77 Write out the rest of the alphabet...
  78
  79 1. ...starting from 'a' (keywordabcf...)
  80 2. ...starting from the last letter of the keyword (keywordfgh...)
  81 3. ...starting from the largest letter of the keyword (keywordzabc...)
  82
  83 ---
  84
  85 # A more Pythonic way
  86
  87 _string_`.translate()` and _string_`.maketrans()`
  88
  89 * Make the 'ciphertext' alphabet, relate to the 'plaintext' alphabet (`string.ascii_lowercase`)
  90 * Use those to make the translation table
  91 * Enciphering is simply applying `plaintext.translate(enciphering_table)`
  92 * Deciphering just uses a different table
  93
  94 ---
  95
  96 # Making the cipher alphabet from a keyword
  97
  98 Three challenges:
  99
 100 1. How to say which type of cipher alphabet to use
 101 2. Where to start the rest of the alphabet
 102 3. Removing duplicate letters
 103
 104 Solutions:
 105
 106 1. Keyword arguments for procedures
 107 2. sort and slices
 108 3. Use something like an ordered set
 109
 110 Both enciphering and deciphering need the same keyword-based alphabet, so pull this out into another procedure.
 111
 112 ---
 113
 114 # Keyword arguments
 115
 116 Used to:
 117
 118 1. give a default value for a parameter
 119 2. allow parameters to be named (not just positional)
 120
 121 Give our `keyword_encipher` and `keyword_decipher` procedures a keyword parameter of `wrap_alphabet`.
 122
 123 Pass this parameter to the `keyword_alphabet` procedure.
 124
 125 ## wrap_alphabet has no inherent meaning
 126 Use Python 3.4's `Enum`
 127 ```python
 128 from enum import Enum
 129
 130 class KeywordWrapAlphabet(Enum):
 131     from_a = 1
 132     from_last = 2
 133     from_largest = 3
 134 ```
 135
 136 (Use integers in earlier Pythons)
 137 ---
 138
 139 # Deduplicating a sequence
 140
 141 We need
 142
 143 * Something set-like
 144 * Something ordered
 145
 146 No ordered set in Python, but do have an ordered dict.
 147
 148 * Keys of a dict are a set.
 149 * Keys in an ordered dict retain their order (subsequent instances are ignored)
 150
 151 `deduplicated_list = list(collections.OrderedDict.fromkeys(list))`
 152
 153 ---
 154
 155 # Sorts and slices
 156
 157 ## Recap
 158 Write out the rest of the alphabet...
 159
 160 1. ...starting from 'a' (keywordabcf...)
 161 2. ...starting from the last letter of the keyword (keywordfgh...)
 162 3. ...starting from the largest letter of the keyword (keywordzabc...)
 163
 164 * Santitise the keyword before we use it
 165
 166 ---
 167 # Making the keyword alphabet
 168
 169 ## Cases
 170 1. As we're deduplicating anyway, just add the entire alphabet to the end of the keyword, then deduplicate.
 171 `deduplicate(keyword + string.ascii_lowercase)`
 172
 173 2. and 3. How to find the appropriate letter of the keyword.
 174
 175 `deduplicate(keyword + string_ascii_lowercase[from:] + string.ascii_lowercase)`
 176
 177 Question: why not take a slice of the second alphabet copy?
 178
 179 Question: what do we use as the last letter of 'character'? 'r' or 'e'?
 180
 181 `sorted()` will put a string in lexicographical order.
 182 `.find()` will find an item in a sequence
 183
 184 ---
 185
 186 # Keyword enciphering
 187
 188 Now we've got the keyword-based cipher alphabet, simply use `.translate()` to do the enciphering/deciphering.
 189
 190 Use `''.maketrans()` to make the translation table.
 191
 192 Sorted!
 193
 194 Does it pass the tests?
 195
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