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 AnarchyPPL - Anarch (Adjudicator) ExI-Freegate Virtual Branch Head plaz@netcom.com 66 Pyramid Plaza plaz@io.com Freegate, Metaverse@io.com 7777 "Subvert the domination paradigm!"