4 <title>Keyword ciphers
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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
54 * Taking a more Pythonic approach
60 * Still character-by-character substitution, still monosubstitution.
62 Ciphertext alphabet: start with a keyword, write out the rest of the alphabet, removing duplicates.
66 Write out the rest of the alphabet...
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...)
76 _string_`.translate()` and _string_`.maketrans()`
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
85 # Making the cipher alphabet from a keyword
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
95 1. Keyword arguments for procedures
97 3. Use something like an ordered set
99 Both enciphering and deciphering need the same keyword-based alphabet, so pull this out into another procedure.
107 1. give a default value for a parameter
108 2. allow parameters to be named (not just positional)
110 Give our `keyword_encipher` and `keyword_decipher` procedures a keyword parameter of `wrap_alphabet`.
112 Pass this parameter to the `keyword_alphabet` procedure.
114 ## wrap_alphabet has no inherent meaning
115 Use Python
3.4's `Enum`
117 from enum import Enum
119 class Keyword_wrap_alphabet(Enum):
125 (Use integers in earlier Pythons)
128 # Deduplicating a sequence
135 No ordered set in Python, but do have an ordered dict.
137 * Keys of a dict are a set.
138 * Keys in an ordered dict retain their order (subsequent instances are ignored)
140 `deduplicated_list = list(collections.OrderedDict.fromkeys(list))`
147 Write out the rest of the alphabet...
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...)
153 * Santitise the keyword before we use it
156 # Making the keyword alphabet
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)`
162 2. and
3. How to find the appropriate letter of the keyword.
164 `deduplicate(keyword + string_ascii_lowercase[from:] + string.ascii_lowercase)`
166 Question: why not take a slice of the second alphabet copy?
168 Question: what do we use as the last letter of 'character'? 'r' or 'e'?
170 `sorted()` will put a string in lexicographical order.
171 `.find()` will find an item in a sequence
175 # Keyword enciphering
177 Now we've got the keyword-based cipher alphabet, simply use `.translate()` to do the enciphering/deciphering.
179 Use `''.maketrans()` to make the translation table.
183 Does it pass the tests?
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