Index of Coincidence

[Perry E. Metzger]

Marc North writes:
> 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. 

There is a fairly good general description in "The Codebreakers";
there is also a book by Friedman available from Agean Park Press that
covers the topic. I don't know of any public tools to do the work,
but it isn't very hard...

Perry