slides/keyword-encipher.html

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  47
  48 # 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 * Taking a more Pythonic approach
  55
  56 ---
  57
  58 # The cipher
  59
  60 * Still character-by-character substitution, still monosubstitution.
  61
  62 Ciphertext alphabet: start with a keyword, write out the rest of the alphabet, removing duplicates.
  63
  64 ## Three variants
  65
  66 Write out the rest of the alphabet...
  67
  68 1. ...starting from 'a' (keywordabcf...)
  69 2. ...starting from the last letter of the keyword (keywordfgh...)
  70 3. ...starting from the largest letter of the keyword (keywordzabc...)
  71
  72 ---
  73
  74 # A more Pythonic way
  75
  76 _string_`.translate()` and _string_`.maketrans()`
  77
  78 * Make the 'ciphertext' alphabet, relate to the 'plaintext' alphabet (`string.ascii_lowercase`)
  79 * Use those to make the translation table
  80 * Enciphering is simply applying `plaintext.translate(enciphering_table)`
  81 * Deciphering just uses a different table
  82
  83 ---
  84
  85 # Making the cipher alphabet from a keyword
  86
  87 Three challenges:
  88
  89 1. How to say which type of cipher alphabet to use
  90 2. Where to start the rest of the alphabet
  91 3. Removing duplicate letters
  92
  93 Solutions:
  94
  95 1. Keyword arguments for procedures
  96 2. sort and slices
  97 3. Use something like an ordered set
  98
  99 Both enciphering and deciphering need the same keyword-based alphabet, so pull this out into another procedure.
 100
 101 ---
 102
 103 # Keyword arguments
 104
 105 Used to:
 106
 107 1. give a default value for a parameter
 108 2. allow parameters to be named (not just positional)
 109
 110 Give our `keyword_encipher` and `keyword_decipher` procedures a keyword parameter of `wrap_alphabet`.
 111
 112 Pass this parameter to the `keyword_alphabet` procedure.
 113
 114 ## wrap_alphabet has no inherent meaning
 115 Use Python 3.4's `Enum`
 116 ```python
 117 from enum import Enum
 118
 119 class Keyword_wrap_alphabet(Enum):
 120     from_a = 1
 121     from_last = 2
 122     from_largest = 3
 123 ```
 124
 125 (Use integers in earlier Pythons)
 126 ---
 127
 128 # Deduplicating a sequence
 129
 130 We need
 131
 132 * Something set-like
 133 * Something ordered
 134
 135 No ordered set in Python, but do have an ordered dict.
 136
 137 * Keys of a dict are a set.
 138 * Keys in an ordered dict retain their order (subsequent instances are ignored)
 139
 140 `deduplicated_list = list(collections.OrderedDict.fromkeys(list))`
 141
 142 ---
 143
 144 # Sorts and slices
 145
 146 ## Recap
 147 Write out the rest of the alphabet...
 148
 149 1. ...starting from 'a' (keywordabcf...)
 150 2. ...starting from the last letter of the keyword (keywordfgh...)
 151 3. ...starting from the largest letter of the keyword (keywordzabc...)
 152
 153 * Santitise the keyword before we use it
 154
 155 ---
 156 # Making the keyword alphabet
 157
 158 ## Cases
 159 1. As we're deduplicating anyway, just add the entire alphabet to the end of the keyword, then deduplicate.
 160 `deduplicate(keyword + string.ascii_lowercase)`
 161
 162 2. and 3. How to find the appropriate letter of the keyword.
 163
 164 `deduplicate(keyword + string_ascii_lowercase[from:] + string.ascii_lowercase)`
 165
 166 Question: why not take a slice of the second alphabet copy?
 167
 168 Question: what do we use as the last letter of 'character'? 'r' or 'e'?
 169
 170 `sorted()` will put a string in lexicographical order.
 171 `.find()` will find an item in a sequence
 172
 173 ---
 174
 175 # Keyword enciphering
 176
 177 Now we've got the keyword-based cipher alphabet, simply use `.translate()` to do the enciphering/deciphering.
 178
 179 Use `''.maketrans()` to make the translation table.
 180
 181 Sorted!
 182
 183 Does it pass the tests?
 184
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