From: Jim choate <ravage@wixer.bga.com>
Date: Tue, 1 Mar 94 06:46:13 PST
To: cypherpunks
Subject: Re: standard for steganography?
In-Reply-To: <199403010523.VAA00389@mail.netcom.com>
Message-ID: <9403011446.AA21798@toad.com>
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I have played w/ stego some and w/ the present resolutions of images I dont
find the images have enough complexity to really hide a message of a useable
length, unless you break it up into several images.

I use a function to measure the complexity of a image based on adjacent bit
changes. The more complex an image the more bit changes. I measure it thus:

# of adjacent bit changes in image/ # of bits in image = complexity

if the complexity is too low or too high (this is counter intuitive) then you
can't hide a message. Consider an image w/ only a few bit flippings, any
message that is inserted will cause the visual image to be distorted in a
noticable way (unless it is truely expressionistic). Now consider a image w/
every other bit flipped (maximum complexity) which is in effect a
checkerboard. Any bits that get flipped change the pattern to a less complex
one (ie the checkerboard is broken up). Also you have to consider the effects
on edges and the standard deviation inherant in using anti-aliasing. This
will cause bits on the edge to be switched incorrectly for the algorith in
use. Since it is a trivial problem to measure the sd for various graphics
packages this makes a nifty test bed for finding imbedding images. Blank or
mono-chromatic areas also show the same type of errors.

I am still working on it and hope to find an error in there somewhere but so
far no go.