Re: Index of Coincidence

["Rev. Ben" <samman-ben@CS.YALE.EDU>]

> I'm looking for detailed information on a statistical tool to aid in
> cryptanalysis called an "index of coincidence".  I would appreciate it if
> someone could please explain what this is, how one builds such an index
> against a given ciphertext, and how it is valuable in the cryptanalysis of
> said ciphertext. 

Given a ciphertext X with discrete components x[1], x[2], ... x[n],  I[c] 
is the probability that two random elements of x are identical.

Ben.

Ben Samman..............................................samman@cs.yale.edu
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