Keyword ciphers

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+
+# Keyword ciphers
+
+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
+--|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|--
+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
+
+* Taking a more Pythonic approach
+
+---
+
+# The cipher
+
+* Still character-by-character substitution, still monosubstitution.
+
+Ciphertext alphabet: start with a keyword, write out the rest of the alphabet, removing duplicates.
+
+## Three variants
+
+Write out the rest of the alphabet...
+
+1. ...starting from 'a' (keywordabcf...)
+2. ...starting from the last letter of the keyword (keywordfgh...)
+3. ...starting from the largest letter of the keyword (keywordzabc...)
+
+---
+
+# A more Pythonic way
+
+_string_`.translate()` and _string_`.maketrans()`
+
+* Make the 'ciphertext' alphabet, relate to the 'plaintext' alphabet (`string.ascii_lowercase`)
+* Use those to make the translation table
+* Enciphering is simply applying `plaintext.translate(enciphering_table)`
+* Deciphering just uses a different table
+
+---
+
+# Making the cipher alphabet from a keyword
+
+Three challenges:
+
+1. How to say which type of cipher alphabet to use
+2. Where to start the rest of the alphabet
+3. Removing duplicate letters
+
+Solutions:
+
+1. Keyword arguments for procedures
+2. sort and slices
+3. Use something like an ordered set
+
+Both enciphering and deciphering need the same keyword-based alphabet, so pull this out into another procedure.
+
+---
+
+# Keyword arguments
+
+Used to:
+
+1. give a default value for a parameter
+2. allow parameters to be named (not just positional)
+
+Give our `keyword_encipher` and `keyword_decipher` procedures a keyword parameter of `wrap_alphabet`.
+
+Pass this parameter to the `keyword_alphabet` procedure.
+
+## wrap_alphabet has no inherent meaning
+Use Python 3.4's `Enum`
+```python
+from enum import Enum
+
+class Keyword_wrap_alphabet(Enum):
+    from_a = 1
+    from_last = 2
+    from_largest = 3
+```
+
+(Use integers in earlier Pythons)
+---
+
+# Deduplicating a sequence
+
+We need
+
+* Something set-like
+* Something ordered
+
+No ordered set in Python, but do have an ordered dict.
+
+* Keys of a dict are a set. 
+* Keys in an ordered dict retain their order (subsequent instances are ignored)
+
+`deduplicated_list = list(collections.OrderedDict.fromkeys(list))`
+
+---
+
+# Sorts and slices
+
+## Recap 
+Write out the rest of the alphabet...
+
+1. ...starting from 'a' (keywordabcf...)
+2. ...starting from the last letter of the keyword (keywordfgh...)
+3. ...starting from the largest letter of the keyword (keywordzabc...)
+
+* Santitise the keyword before we use it
+
+---
+# Making the keyword alphabet
+
+## Cases
+1. As we're deduplicating anyway, just add the entire alphabet to the end of the keyword, then deduplicate. 
+`deduplicate(keyword + string.ascii_lowercase)`
+
+2. and 3. How to find the appropriate letter of the keyword.
+
+`deduplicate(keyword + string_ascii_lowercase[from:] + string.ascii_lowercase)`
+
+Question: why not take a slice of the second alphabet copy?
+
+Question: what do we use as the last letter of 'character'? 'r' or 'e'?
+
+`sorted()` will put a string in lexicographical order.
+`.find()` will find an item in a sequence
+
+---
+
+# Keyword enciphering
+
+Now we've got the keyword-based cipher alphabet, simply use `.translate()` to do the enciphering/deciphering.
+
+Use `''.maketrans()` to make the translation table.
+
+Sorted!
+
+Does it pass the tests?
+
+

+ + + + + + +