I am looking for a comprehensive list of the IOC values of any or all classical ciphers. I can't find a list, only pieces. Each cipher has an average IOC value, and I was wondering if there was a comprehensive list of these values for every classical cipher available. Can anyone share a list of these IOC values?

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    $\begingroup$ the IOC is obtained from a ciphertext corresponding to some given concrete plaintext. Under a statistical model for the natural language (English, say) the plaintext belongs to, it should be close to the collision probability $\sum_{x\in \{A,B,\ldots,Z\}} p_x^2$ of the natural language. So, what are you exactly asking? $\endgroup$
    – kodlu
    Commented Oct 11, 2020 at 0:14
  • $\begingroup$ @kodlu Added a bit more, take a look please. $\endgroup$
    – Neo1009
    Commented Oct 12, 2020 at 2:04

1 Answer 1


The index of coincidence (IOC) measures the likelihood of drawing two matching characters by randomly selecting two characters from a given ciphertext. It also applies to digraphs, etc.

So, the chance of drawing a given character or grouping from a given ciphertext is: (number of times that character or grouping appears / length of the text).

The index of coincidence applies to ciphertexts, not to ciphers. Friedman's IOC does not apply to a cipher itself--not to the Alberti cipher, the Vigenère cipher, etc.

See: William F. Friedman, The Index of Coincidence and Its Applications in Cryptanalysis, Aegean Park Press, 1996.

A declassified NSA document about the IOC that links to that august institution.


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