The index of coincidence (IC) is a measure of the relative frequency of letters in a cipher text sample, which facilitates cryptanalysis of poly-alphabetic ciphers by allowing determination of the period t, length of the key, as an alternative to Kasiski’s method.

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Index of coincidence for completely random text over k alphabets

I'm confused about /Friedman's method 2 (using a table with column of k size of key, and n/k rows, n is the total size for cipher text) \begin{equation} m = ...