--- /dev/null
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+<html>
+ <head>
+ <title>Pocket enigma</title>
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+ <style type="text/css">
+ /* Slideshow styles */
+ body {
+ font-size: 20px;
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+ h1, h2, h3 {
+ font-weight: 400;
+ margin-bottom: 0;
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+ background: #e7e8e2;
+ border-radius: 5px;
+ font-size: 16px;
+ }
+ .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;
+ }
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+ float: right;
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+ <body>
+ <textarea id="source">
+
+# Pocket Enigma
+
+
+
+Stateful cipher
+
+---
+
+# Pocket Enigma
+
+Emulates the Enigma machine from WWII
+
+Mechanical cipher machine
+
+* Substitution cipher
+* Substitution changes with every letter
+
+Ciphering method: advance the wheel, then follow the lines to encipher the letter
+
+## Stateful enciphering
+
+The cipher depends on the position of the wheel
+
+We need to encapsulate that state
+
+Objects have state
+
+---
+
+# The PocketEnigma object
+
+What do we want it to do?
+
+What data should the object hold?
+
+---
+
+# The PocketEnigma object
+
+What do we want it to do?
+
+* Initialise with the appropriate wheel (and possible starting position)
+* Spin the wheel to a given position
+* Advance the wheel one position
+* Look up a letter given the wheel position
+* Encipher a letter (advance the wheel then look up the letter)
+* Encipher a message (optionally give the key)
+* Make aliases for deciphering (same as enciphering)
+
+
+* Accept user-defined wheels
+ * ...and validate them
+
+What data should it hold?
+
+* A description of the wheel being used
+* The current position of the wheel
+
+---
+
+# Data structures
+
+What's a convenient representation of the wheel
+
+1. for the object to use internally
+2. for a person to use to describe the wheel
+
+They may not be the same, and we'll have to translate between them
+
+---
+
+# Data structures
+
+### Internal use: list of transpositions.
+
+```python
+[2, 3, 0, 1, 22, 8, 15, 12, 5, ...
+```
+
+so position 0 ('a') swaps with position 2 ('c'), position 3 ('d') swaps with position 1 ('b'), and so on.
+
+* This will be a nightmare to enter correctly
+
+### Exernal use: list of pairs
+
+```python
+[('a', 'c'), ('b', 'd'), ...]
+```
+
+Easier to enter
+
+* Need to validate the human-entered list, to check it's valid
+
+---
+
+# Validating the wheel description
+
+What tests?
+
+---
+
+# Validating the wheel specification
+
+What tests?
+
+* 13 elements...
+* ...each a pair...
+* ...and 26 letters mentioned overall
+
+---
+
+# Making the PocketEnigma class
+
+```python
+class PocketEnigma(object):
+ def __init__(self, wheel=1, position='a'):
+ self.wheel1 = [('a', 'z'), ('b', 'e'), ('c', 'x'), ('d', 'k'),
+ ('f', 'h'), ('g', 'j'), ('i', 'm'), ('l', 'r'), ('n', 'o'),
+ ('p', 'v'), ('q', 't'), ('s', 'u'), ('w', 'y')]
+ self.wheel2 = [('a', 'c'), ('b', 'd'), ('e', 'w'), ('f', 'i'),
+ ('g', 'p'), ('h', 'm'), ('j', 'k'), ('l', 'n'), ('o', 'q'),
+ ('r', 'z'), ('s', 'u'), ('t', 'v'), ('x', 'y')]
+ # Rest of initialisation code here
+
+ def make_wheel_map(self, wheel_spec):
+ ...
+ self.wheel_map = ...
+ ...
+
+ def validate_wheel_spec(self, wheel_spec):
+ if len(wheel_spec) != 13:
+ raise ValueError("Wheel specification has {} pairs, requires 13".
+ format(len(wheel_spec)))
+ ...
+```
+
+---
+
+# Looking up the enciphered version of a letter
+
+*Not* advancing the wheel before
+
+Keep `self.position` to record where the wheel is
+
+* `__init__` can be passed a letter, but internally it's a number
+
+But the wheel map only works if the wheel arrow is pointing at 'a'
+
+Idea:
+
+1. Rotate the source letter back `position` spaces
+2. Do the lookup
+3. Rotate the destination letter forward `position` spaces
+
+(all mod 26)
+
+
+ </textarea>
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