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Re: responce to graphic encryption replies

> ... The graphic file is going to have a highly correlated 
> structure, long runs of white space etc. 

This is not the case for digital imagery. For any given band, if you
sample a significant portion of the image it would not be surprising to
see a more or less random distribution over the range.

> The statistics for such a file 
> would be different than the random distribution you'd get from using the 
> wrong key. 

If you decrypt with the wrong key, do you get a random distribution?
Would this be the case for all wrong keys? Would the statistics change
in any discernible pattern as the keys got "closer"?

> Even if the graphics format is compressed, leading to a more 
> even distribution, ...

This may give a better result. Most compressions involve a shift to the
frequency domain with a quantization of the samples. You might be able
to write a detector based on this discontinuity.

You could also try throwing an edge detector or some other morphological
recognizer at the output. It would still be cheaper than having to a man
in the loop, but you're going to spend a lot of cpu time.

Jack P. Starrantino     (215) 674-0200 (voice)
SEMCOR, Inc.            (215) 443-0474 (fax)
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Warminster, PA 18974