Re: responce to graphic encryption replies

[Jiri Baum <jirib@sweeney.cs.monash.edu.au>]

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Hello jps@monad.semcor.com (Jack P. Starrantino)
  and cypherpunks@toad.com
 
JPS wrote:
> > ... 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.

Even for a picture (as opposed to text fax) you have correlation.
If you do not, you cannot compress it (losslessly).

In general you would want to find the key which leads to least entropy
(ie most compressible file). In other words, find key with minimum
I(key) + I(ciphertext decrypted-by key).

> > 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?

Given security thru obscurity, who knows?

> Would this be the case for all wrong keys? Would the statistics change

Given security thru obscurity, who knows?

> in any discernible pattern as the keys got "closer"?

Given security thru obscurity, who knows?

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

That cannot be so: once it is compressed, you can't encrypt it as a
picture, you have to encrypt it as a bitstream.

...
> 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.

Much easier than OCR.


Going back to compressing images now... HINT anyone?

Jiri
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If you want an answer, please mail to <jirib@cs.monash.edu.au>.
On sweeney, I may delete without reading!
PGP 463A14D5 (but it's at home so it'll take a day or two)
PGP EF0607F9 (but it's at uni so don't rely on it too much)

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