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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Avoiding doubled letters in encrypted text

I have an encrypted string based on an "alphabet" of 29 runes and access to successfully decrypted (runes->English) plaintext. Testing for a Vigenere cipher, I tried a Friedman attack (aka Kappa test, ...
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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 = \Bigl(\varphi(l)-\varphi(o)\Bigr)/\Bigl(\...