Index of Coincidence

[Rev. Ben]

> 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 at cs.yale.edu
"If what Proust says is true, that happiness is the absence of fever, then 
I will never know happiness. For I am possessed by a fever for knowledge, 
experience, and creation."                                      -Anais Nin
Want to give a soon-to-be college grad a job?         Mail me for a resume