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Cryptography/Index of coincidence

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The index of coincidence for a ciphertext is the probability that two letters selected from it are identical. Usually denoted by I, it is a statistical measure of the redundancy of text. The index of coincidence of totally random collection (uniform distribution) of letters is around 0.0385.[1]


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Wikipedia has related information at index of coincidence

  1. ↑ "Index of Coincidence".
 

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