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HIDE: embeddin msgs into snd & graphics
The considerations that Eli mentioned make it clear that embedding data in
the low bits of an image is not as trivial as it sounds, with commonly-used
image formats. In the case of GIF, Eli points out that you may have a 256
entry color table, with each pixel indexing into that table. Flipping the
low bit of a pixel may lead to a completely different color.
What you could do is to renumber the color table so that, to the extent
possible, every even-numbered color has some odd-numbered color that is
similar (close in color space), and vice versa. Then rather than just
altering the low-order bit of each pixel, you'd change the color of that
pixel to be the nearest color of opposite even-odd-ness. For the decode
step, though, you could still just check the low-order bit of the
(uncompressed) image. That renumbering step sounds like the tricky part.
I think Eli is right, too, that lossy compression is pretty much out of the
question for this application. It would be too easy to lose the message
that is encoded in the low-order bits.
Images that would be good candidates for this would be natural, scanned-in
pictures. Hand-drawn artworks and most computer-generated images would not
have enough natural randomness to allow the message to be slipped in
unnoticeably. Fortunately, nudes would fall into the useful category, and
they make up a large fraction of the images people exchange.