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In school we only learned the formula without the normalizing coefficient:

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But Wikipedia has one that says:

enter image description here

I am using IC in a cryptography class to find the key length and am checking various key lengths to see which one results in an IC closest to 0.066. I have to automate the key length finding process but I don't know which formula I should use. What does the normalizing coefficient change about the value? Does it make it better for finding the key length? Which formula should I use for my case?

from freq_analysis import freqs
import string

def ioc(fname, normalize: bool = False):
    frq = list(freqs(string.ascii_uppercase, fname).values())
    n = sum(frq)
    for x in range(0, len(frq)):
        frq[x] = frq[x] * (frq[x] - 1)
    frq = sum(frq)
    if normalize:
        return frq26 / (n(n-1))
    else:
        return frq / (n*(n-1))

Full context for above code here: https://github.com/DarkFireGuy/cracking-vigenere (if you want to see context for freq_analysis and freqs())

DarkFireGuy
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1 Answers1

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The normalizing coefficient makes sure that for a random text the IC will be (close to) 1.0, independent of the size of the alphabet.

Jigg
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