From: plaz@netcom.com (Plaz)
Date: Tue, 1 Mar 94 11:40:57 PST
To: cypherpunks@toad.com
Subject: Re: standard for steganography?
Message-ID: <199403011941.LAA06715@mail.netcom.com>
MIME-Version: 1.0
Content-Type: text/plain


Jim choate <ravage@wixer.bga.com> wrote:
>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.

Present resolutions? What directions do you think it needs to go? Stego
handles 24 bit TrueColor (actually 32 bit, but Romana wisely decided not to
stuff data in the almost always blank alpha channel), and as many DPI as
you have memory for. I don't think there is much call for deeper color
resolution.

It's true that GIF only accepts 8-bit color and the defacto standard
internet transmision format for higher resolutions JPEG is a lossy
compression that annhilates the message. Is this what you are refering to?

>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.

Yes, it is recommended that noisy images are used as the message envelope.
Scanned or Frame-grabbed are good for this kind of thing.

_______________________________________________________________________
Geoff Dale         -- Cypherpunk/Extropian --         Plastic Beethoven
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