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Forked from enigmaticape/ic.py
Created March 1, 2013 02:27
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Calculates Index of Coincidence
#!/usr/bin/env python
import sys
import collections
# Bag em
cipher_file = open( sys.argv[ 1 ], 'rb')
cipher_text = cipher_file.read()
# remove all non alpha and whitespace and force uppercase
# SOTHATCIPHERTEXTLOOKSLIKETHIS
cipher_flat = "".join(
[x.upper() for x in cipher_text.split() \
if x.isalpha() ]
)
# Tag em
N = len(cipher_flat)
freqs = collections.Counter( cipher_flat )
alphabet = map(chr, range( ord('A'), ord('Z')+1))
freqsum = 0.0
# Do the math
for letter in alphabet:
freqsum += freqs[ letter ] * ( freqs[ letter ] - 1 )
IC = freqsum / ( N*(N-1) )
print "%.3f" % IC, "({})".format( IC )
#!/usr/bin/env python
# Generate test suite number 1 from Sinkov pp 70
# to test Index of Coincidence computation.
# Should produce an IC of .041
al = "A" * 7
al += "B" * 6
al += "C" * 9
al += "D" * 3
al += "E" * 5
al += "F" * 6
al += "G" * 8
al += "H" * 3
al += "I" * 4
al += "J" * 7
al += "K" * 13
al += "L" * 10
al += "M" * 7
al += "N" * 0
al += "O" * 1
al += "P" * 5
al += "Q" * 3
al += "R" * 6
al += "S" * 8
al += "T" * 5
al += "U" * 4
al += "V" * 8
al += "W" * 4
al += "X" * 8
al += "Y" * 5
al += "Z" * 5
print al
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