<!DOCTYPE html>
<html>
<head>
- <title>Affine ciphers</title>
+ <title>Keyword ciphers</title>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"/>
<style type="text/css">
/* Slideshow styles */
color: #ff6666;
text-shadow: 0 0 20px #333;
padding: 2px 5px;
+ }
+ .indexlink {
+ position: absolute;
+ bottom: 1em;
+ left: 1em;
}
.float-right {
float: right;
---
+layout: true
+
+.indexlink[[Index](index.html)]
+
+---
+
# The cipher
* Still character-by-character substitution, still monosubstitution.
Write out the rest of the alphabet...
1. ...starting from 'a' (keywordabcf...)
-2. ...starting from the last unique letter of the keyword (keywordfgh...)
+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 KeywordWrapAlphabet(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?
+
</textarea>
<script src="http://gnab.github.io/remark/downloads/remark-0.6.0.min.js" type="text/javascript">
</script>
projects / cipher-training.git / blobdiff