Affine ciphers

From 693f0533714de0185ceaa4e26b269453e7fb60d1 Mon Sep 17 00:00:00 2001 From: Neil Smith Date: Sat, 31 May 2014 18:09:06 +0100 Subject: [PATCH] More on keyword breaking --- slides/keyword-break.html | 170 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 170 insertions(+) create mode 100644 slides/keyword-break.html diff --git a/slides/keyword-break.html b/slides/keyword-break.html new file mode 100644 index 0000000..b3c0a2c --- /dev/null +++ b/slides/keyword-break.html @@ -0,0 +1,170 @@ + + + + Affine ciphers + + + + +

+
+# Breaking keyword ciphers
+
+a | b | c | d | e | f | g | h | i | j | k | l | m | n | o | p | q | r | s | t | u | v | w | x | y | z
+--|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|--
+k | e | y | w | o | r | d | a | b | c | f | g | h | i | j | l | m | n | p | q | s | t | u | v | x | z
+
+----
+
+# Duplicate and extend your `affine_break()` function
+
+* How to cycle through all the keys? What _are_ all the keys?
+
+* Look at `words.txt`
+
+---
+
+# Test it. 
+
+* `2013/4a.ciphertext`
+* `2013/4b.ciphertext`
+
+This will take a while. Fire up a system monitor. What's wrong?
+
+---
+
+# Python, threads, and the GIL
+
+Thread-safe shared-memory code is hard.
+
+Python's Global Interpreter Lock prevents shooting yourself in the foot.
+
+Where you want true parallelism, need different threads (Python processes).
+* Thread-safe shared-memory code is hard.
+
+The `multiprocessing` library makes this easier.
+
+But before we get there, a couple of diversions...
+
+---
+
+# `map()`
+
+A common task is to apply a function to each item in a sequence, returning a sequence of the results.
+
+```python```
+def double(x):
+    return x * 2
+
+>>> map(double, [1,2,3])
+[2,4,6]
+```
+
+* `map()` is a higher-order function: its first argument is the function that's applied.
+
+How can we use this for keyword cipher breaking?
+
+---
+
+# Mapping keyword decipherings
+
+Define a function that takes a possible key (keyword and cipher type) and returns the key and its fitness.
+
+Use `map()` and `max()` to find the best key
+
+---
+
+# `print()`
+
+How many arguments does this take?
+
+How do you write a function that takes this many arguments?
+
+---
+
+# Function arguments
+
+## Positional, keyword
+
+* Common or garden parameters, as you're used to.
+* `def keyword_encipher(message, keyword, wrap_alphabet=0):`
+
+## Excess positional
+* `def mean(x, *xs):`
+
+First number goes in `x`, remaining go in the tuple `xs`
+
+## Excess keyword
+
+* `def myfunc(arg1=0, **kwargs):`
+
+`kwargs` will be a Dict of the remaining keywords (not `arg1`)
+
+---
+
+# Back to `multiprocessing`
+
+What does `Pool.starmap()` do?
+
+

+ + + + + + + -- 2.34.1