From: "Rev. Ben" <samman-ben@CS.YALE.EDU>
Date: Mon, 11 Mar 1996 08:38:58 +0800
To: Marc North <mnorth@netcom.com>
Subject: Re: Index of Coincidence
In-Reply-To: <Pine.SUN.3.91.960309113033.18346B-100000@netcom12>
Message-ID: <Pine.A32.3.91.960309200124.5902A-100000@POWERED.ZOO.CS.YALE.EDU>
MIME-Version: 1.0
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> 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
"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
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