Maybe no stego on eBay afterall

[Hadmut Danisch]

On Fri, Jul 19, 2002 at 06:21:53PM -0400, John S. Denker wrote:
> 
> a) Consider the following:  Suppose I take a picture with
> my CCD camera of a scene containing a more-or-less white
> area.  I choose the f/stop and exposure-time so that N
> photons are expected to hit each pixel of the CCD array
> in this area.  Elementary physics and statistics tell us 
> that there will be an uncertainty of sqrt(N) in the actual 
> photon count.
> 
> b) I repeat the operation using a brighter light, bigger
> aperture, and/or longer exposure, so that the expectation
> value is now 10N.  Then I divide by 10 to get an image
> rather comparable to the first, but with sqrt(10) less
> noise.


I disagree for several technical, physical, photographical
reasons, but we can ignore this for this discussion, since
it is a standard method of physics to measure for a 
longer time or many samples at once and take the average to
reduce noise.




>  I then add noise back in, using either
>   b1) an industrial-strength random symbol generator, or
>   b2) a well-encrypted message
> 
> I claim it is impossible for any empirical test to 
> distinguish case (a) from (b1) or (b2).  


What do you want to show? If you hide an information this
way, how should the receiver decode/separate it? The receiver would 
need to know how your picture looked like before you added 
information or how to separate information belonging to the
white picture and information belonging to your message.

If you substitute just the LSB, you presume that the LSB is
an industrial-strength random bit sequence. But if this were
the case, why the hell should digital cameras bother to include
it in the stored image? If you have an image with a depth of
8 bit, where the LSB is just random nonsense, why not simply
cut it away?

And secondly, your transmissions are eye-catching. Why 
should someone transmit images of a white wall? 

As long as you need a picture of a white wall, I wouldn't even
call it steganography, just some strange way of encoding. 
It's easier to simply concatenate a plain jpeg-header and your
encrypted message, hoping that nobody tries to view the picture.



regards
Hadmut






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