From: Eli Brandt <ebrandt@jarthur.Claremont.EDU>
Date: Thu, 11 Mar 93 23:38:53 PST
To: cypherpunks@toad.com
Subject: Re:  HIDE: embedding msgs into snd & graphics
In-Reply-To: <9303120259.AA03718@sting.Berkeley.EDU>
Message-ID: <9303120738.AA15498@toad.com>
MIME-Version: 1.0
Content-Type: text/plain


> I don't know if you actually know GIF format (I don't) but I know that you'd
> have to do some reasonably intelligent churning of the data.  For one, it's
> just not going to be as easy as dropping a noise bit from each n-byte set;
> GIF format is fairly compressed as I understand.

What you'd want to do is uncompress the byte stream, twiddle the low
bits, and LZW it back up.  Well, that's easy enough.  The problem is
that GIFs are colormapped, and the map need have no coherence
between entries 8 and 9.  Even optimally arranged (a non-trivial
task; looks like the salesman travels colorspace...), that low bit
is probably going to be significant enough to munge things visibly.
What you'd have to do is remap the file to 128 colors, duplicate
them in the colormap, and encode your message in the choice of
identical entries.  Unfortunately, most images look bad enough
mapped to 256 colors, and will degrade further in 128.  Though I
suppose nobody really wants to look at the image anyway...

Your compression is also going to die.  This may provide a quick way
to scan for this technique, and finding a redundant colormap is a
dead giveaway of either secrecy or stupidity.  You could fix that by
tweaking twin colors slightly, adding a bit of visible noise.

I think color images, as opposed to mapped, would be the way to go
for steganography.  More room, and nobody expects them to compress.

>     -J

	 PGP 2 key by finger or e-mail
   Eli   ebrandt@jarthur.claremont.edu