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+<html>
+ <head>
+ <title>Keyword ciphers</title>
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+ /* Slideshow styles */
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+ background: #e7e8e2;
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+ .plaintext {
+ background: #272822;
+ color: #80ff80;
+ text-shadow: 0 0 20px #333;
+ padding: 2px 5px;
+ }
+ .ciphertext {
+ background: #272822;
+ color: #ff6666;
+ text-shadow: 0 0 20px #333;
+ padding: 2px 5px;
+ }
+ .float-right {
+ float: right;
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+ <textarea id="source">
+
+# Transposition ciphers
+
+ attack the fort at dawn
+
+ a t t a c
+ k t h e f
+ o r t a t
+ d a w n
+
+ akod ttra aean cft
+
+---
+
+# Transposition ciphers
+
+Rather than changing symbols (substitution ciphers),
+
+Rearrange them.
+
+Still disguises the message.
+
+(Good ciphers do both, and more.)
+
+---
+
+# Scytale
+
+Even older than Caesar cipher.
+
+* Wrap a strip round a pole
+* Write the message along it
+* Unwind the strip
+* "Unreadable" unless reader has pole of same diameter
+
+ attack the fort at dawn
+
+ a t t a c
+ k t h e f
+ o r t a t
+ d a w n
+
+ akod ttra aean cft
+
+---
+
+# Generalising: column transposition ciphers
+
+Scytale essentially fills a grid by rows, then reads it by columns
+
+* (Deciphering is the reverse)
+
+Column transposition ciphers:
+
+* Fill a grid
+* Reorder columns based on keyword
+* Read the grid (perhaps different direction)
+
+Scytale is just a special case of column transposition.
+
+---
+
+# Grids and data structures
+
+How to represent a grid?
+
+What operations do we need to do on it?
+
+---
+
+# Grids and data structures
+
+How to represent a grid?
+
+* List of strings
+* Each row is a string
+* Rows in order in the list
+
+What operations do we need to do on it?
+
+* Fill, by rows or columns
+* Empty, by rows or columns
+* Rearrange columns
+* Calculate the size of the grid
+* Pad message to fit a rectangle of the required size
+
+---
+
+# Finding sizes
+
+Know number of columns
+
+Number of rows = ceiling(message length / columns)
+
+Paddding is (rows * columns) - message length
+
+* What to use as default padding?
+* Keyword parameter!
+
+## Fit 'thequickbrownfox' (16 letters) into grid of
+
+* 4 columns
+* 5 columns
+
+---
+
+# Fill and empty grid by rows
+
+Split message into row-sized chunks
+
+* slices and ranges
+
+Append all the rows together
+
+* `<string>.join()`
+
+Keep thinking about test cases!
+
+---
+
+# Fill and empty grid by columns
+
+Idea: fill and empty by rows, with a transposition.
+
+`zip(*rows)` and `itertools.zip_longest(*rows)`
+
+---
+
+# Swapping columns
+
+How to represent a transposition (_permutation_, to mathematicians)?
+
+How to create it from a keyword?
+
+---
+
+# Idea of a transposition
+
+Says, for each element, where it should go
+
+```
+0 1 2 3 4 5 6
+t r e a s o n
+
+a e n o r s t
+3 2 6 5 1 4 0
+```
+
+The transposition `(3, 2, 6, 5, 1, 4, 0)` says that what was in position 3 moves to position 0, what was in position 2 moves to position 1, what was in position 6 moves to position 2, ...
+
+`enumerate(_iterable_)` yields an iterator that walks over the iterator, including the element indexes.
+
+```python
+>>> [i for i in enumerate('treason')]
+[(0, 't'), (1, 'r'), (2, 'e'), (3, 'a'), (4, 's'), (5, 'o'), (6, 'n')]
+>>> [i for i in enumerate((3, 2, 6, 5, 1, 4, 0))]
+[(0, 3), (1, 2), (2, 6), (3, 5), (4, 1), (5, 4), (6, 0)]
+```
+
+Write the `transpose` and `untranspose` functions.
+
+---
+
+# Transposition from a keyword
+
+Deduplicate the keyword
+
+Sort it
+
+Use `<iterable>.index()` to find the positions of the letters in the sorted keyword
+
+---
+
+# Transposition ciphers
+
+Put it all together
+
+---
+
+# Masking the fill characters
+
+Padding characters can be distinctive.
+
+Make a function that generates a random letter, based on the `normalised_english_counts`
+
+Use `callable()` to check if the `fillvalue` should be called or just inserted
+
+ </textarea>
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