From: sdw@meaddata.com (Stephen Williams)
Date: Wed, 2 Mar 94 07:49:46 PST
To: bill.stewart@pleasantonca.ncr.com +1-510-484-6204)
Subject: Re: stego
In-Reply-To: <9403012217.AA05764@anchor.ho.att.com>
Message-ID: <9403021550.AA05379@jungle.meaddata.com>
MIME-Version: 1.0
Content-Type: text/plain


> 
> Jim Choate's comments on steganography having problems with images
> that are too complex or too simple were interesting.
> Obviously, cartoon-like GIFs aren't a good target, though scanned
> real stuff may be fine.  Weather maps cna be good - back when I worked
> with the things, I found you could really see about5-6 bits worth of
> depth, and after that it didn't usually look much different -
> we stole one or two values from the color-map to draw lines on
> the satellite images to add state boundaries, various data values, etc.,
> but could have stolen the LSB and maybe 7th bit without major loss on
> cloud-image pictures.  (Radar pictures, on the other hand, were almost

I haven't been able to keep up with all of the Stego discussion, but on
pictures with few colors and a large amount of a small number of
colors (like cartoons, etc. with a backround), you could use the
unused color table entries.  You set these to be the same colors as
the correspondingly largest used colors and use them when juxtaposed
with their color.


Original:   [0,A]
00000000000000000000000000000000000000000000000
000000000000000A000000000000000A000000000000000
000000000000000A000000000000000A000000000000000
000000000000000A000000000000000A000000000000000
000000000000000A000000000000000A000000000000000
000000000000000A000000000000000A000000000000000
000000000000000A000000000000000A000000000000000
00000000000000000000000000000000000000000000000

Stego-colormap:  (1/0 repeat) [0=1,A=B]
01010101010101010101010101010101010101010101010
010101010101010B010101010101010B010101010101010
010101010101010B010101010101010B010101010101010
010101010101010B010101010101010B010101010101010
010101010101010B010101010101010B010101010101010
010101010101010B010101010101010B010101010101010
010101010101010B010101010101010B010101010101010
01010101010101010101010101010101010101010101010


Won't compress as well of course, but the picture is identical.
Multiple color entries at the same setting could be used to encode
more bits.  (0=1=2=3 gives 2 bits info).

You then leave the least used colors alone.

More easily detectable I suppose though since multiple color settings
were the same.  One way around that is to make them just a shade off,
which wouldn't change the actual color much.

> all black, with one or two other pixel values, compressed to 2% of original
> size, and would have been useless for hiding anything in.)
> 
> 		Bill
> 


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