Up to first breaking of caesar ciphers

diff --git a/slides/caesar-break.html b/slides/caesar-break.html
index f296e44142197dec2eadd10ac5d773ffb3087cca..c2556cf234bf61fe800836b70528ad93d5c005f3 100644 (file)
--- a/slides/caesar-break.html
+++ b/slides/caesar-break.html
@@ -314,6 +314,39 @@ We also need a way of passing the different functions to the keyfinding function
 
 Try them all on random ciphertexts, see which one works best.
 
 
 Try them all on random ciphertexts, see which one works best.
 

+---
+
+# Reading letter probabilities
+
+1. Load the file `count_1l.txt` into a dict, with letters as keys.
+
+2. Normalise the counts (components of vector sum to 1): `$$ \hat{\mathbf{x}} = \frac{\mathbf{x}}{\| \mathbf{x} \|} = \frac{\mathbf{x}}{ \mathbf{x}_1 + \mathbf{x}_2 + \mathbf{x}_3 + \dots }$$`
+    * Return a new dict
+    * Remember the doctest!
+
+3. Create a dict `Pl` that gives the log probability of a letter
+
+4. Create a function `Pletters` that gives the probability of an iterable of letters
+    * What preconditions should this function have?
+    * Remember the doctest!
+
+---
+
+# Breaking caesar ciphers (at last!)
+
+## Remember the basic idea
+
+```
+for each key:
+    decipher with this key
+    how close is it to English?
+    remember the best key
+```
+
+Try it on the text in `2013/1a.ciphertext`. Does it work?
+
+---
+

 
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