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- <title>Keyword ciphers</title>
+ <title>Transposition ciphers</title>
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* Unwind the strip
* "Unreadable" unless reader has pole of same diameter
- attack the fort at dawn
+```
+attack the fort at dawn
- a t t a c
- k t h e f
- o r t a t
- d a w n
+a t t a c
+k t h e f
+o r t a t
+d a w n
- akod ttra aean cft
+akod ttra aean cft
+```
---
* Reorder columns based on keyword
* Read the grid (perhaps different direction)
+(Keyword = secret → cerst)
+```
+attack the fort at dawn
+
+s e c r t c e r s t
+--------- ---------
+a t t a c t t a a c
+k t h e f h t e k f
+o r t a t t r a o t
+d a w n w a n d
+
+ttaac htekf traot wand
+thtw tra aean akod cft
+```
+
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 a grid?
-What operations do we need to do on it?
+How to represent a grid?
---
# 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?
+What operations do we need to do on a grid?
* Fill, by rows or columns
* Empty, by rows or columns
* Calculate the size of the grid
* Pad message to fit a rectangle of the required size
+How to represent a grid?
+
+* List of strings
+* Each row is a string
+* Rows in order in the list
+
---
# Finding sizes
Know number of columns
-Number of rows = ceiling(message length / columns)
+Number of rows = `\(\left \lceil \frac{\mathrm{message\ length}}{\mathrm{columns}} \right \rceil\)`
-Paddding is (rows * columns) - message length
+Paddding is (rows ⨉ columns) - message length
* What to use as default padding?
* Keyword parameter!
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.
+`enumerate(_iterable_)` yields an iterator that walks over the iterable, including the element indexes.
```python
>>> [i for i in enumerate('treason')]
projects / cipher-training.git / blobdiff