Re: Playfair cracker - measure of best fit?

From: Douglas A. Gwyn (DAGwyn_at_null.net)
Date: 08/09/05 

Date: Tue, 9 Aug 2005 15:18:26 GMT

Rob Sullivan wrote:
> The problem I'm having is with step 2 - compared to other Playfair
> crackers, mine can only try a fraction of the keys tried by the other
> programs in a given time. Is there a better statistic I can use, in
> terms of appropriateness and efficiency?

Yes. Pearson's chi-square is unreliable when there are fewer
than, roughly, 5 entries per bin. Kullback's information measure
works all the way down to 0 entries, is easy to aggregate (add d.f.
and add information measures), and is easy to interpret (via
inverse chi-square lookup). You can obtain documentation and an
implementation I put into the public domain long ago at URL
http://www.phaedsys.demon.co.uk/chris/crypto/i-hat.zip
(If that link is broken, just Google for "i-hat" and "Kullback".)

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