From: Hal <74076.1041@CompuServe.COM>
Date: Fri, 12 Mar 93 22:56:34 PST
To: <cypherpunks@toad.com>
Subject: HIDE: embeddin msgs into snd & graphics
Message-ID: <930313065252_74076.1041_DHJ21-2@CompuServe.COM>
MIME-Version: 1.0
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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.
 
Hal
74076.1041@compuserve.com