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+ <textarea id="source">
+
+# Breaking 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
+
+Generally quite familiar...
+
+## Try all the keys, pick the one that looks most like Englilsh
+
+---
+
+# ...Pick one that looks most like English
+
+But the naïve Bayes score will always be the same!
+
+* Same letters, just a different order.
+
+Score by probability of substrings of letters
+
+* Bigrams, trigrams, _n_-grams
+
+---
+
+# Finding _n_-grams
+
+Given `count_2l.txt` and `count_3l.txt`, counts of bigrams and trigrams in English
+
+# Write a function that returns all the _n_-grams for a text, given _n_
+ * Assume the text is already sanitised
+
+# Build `P2l`, `P3l` (after `Pl`), `Pbigrams`, `Ptrigrams` (after `Pletters`)
+
+---
+
+# Breaking scytale
+
+What are the possible keys?
+
+---
+
+# Try all the keys...
+
+*All* the keys?
+
+What's the transposition of 'cat'?
+
+* 'bat'?
+* 'car'?
+* 'wry'?
+* 'babe'?
+* 'powwow'?
+
+---
+
+# Equivalence classes and canonical forms
+
+Lots of words yield the same transposition
+
+* They're all in the same equivalence class
+* Only need to test one from the class
+
+General idea: if there are different ways to represent something, pick one to make comparisons easier
+
+* Canonical form, canonical representation
+
+---
+
+# Finding the transpositions to try
+
+```
+For each word:
+ if it's a new transposition:
+ add it to the list
+```
+
+What data structure to use to store the transpositions?
+
+
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projects / cipher-training.git / blobdiff