Re: Nitch (also re: steganography)

[jimn8@netcom.com (Jim Nitchals)]

an4914@anon.penet.fi writes:
> 
> Terribly sorry to frighten you, Jim.  Nitch is a (very) little-known
> nickname of mine from quite some time ago.  There are probably only
> five people who ever knew that it had been given to me.
> 
> Funny name collision.  I never expected it.  Hope you don't get fired.

Fired?  Hehe.. I'd sincerely expect more from my employers than to
get upset at something I post to a mailing list.

Thanks for the reply.  Now.. to increase the signal-to-noise ratio a
bit.. (hopefully a few people on the list read this one anyway) there
was a mention of using the LSB of a picture for steganography, and the
ensuing difficulty in hiding the results.

An idea I've had is to hide the data as the exclusive-or of several
LSB's within the image.  There are two problems with using the LSB
of *every* pixel: the obvious distribution of 1's and 0's, and the
resulting loss of compressability of the resulting image.

A 320x200 image can hold 8,000 bytes of data in the LSB, which is
probably more capacity than most messages need.  If instead we use
1 bit of steganographic data for 8 pixel LSB's, the capacity is
still about 1K, and it should be easier to hide the steganographic
signature because only 1 LSB out of 8 needs to be changed if the
parity is wrong.

If using GIF or other lossless encoder, we can tweak LSB's in ways
that actually *reduce* compressed file size.  As long as we're
tossing out the LSB as usable picture information, let's get some
benefit from it.

Assuming matching JPEG encoders and decoders on the sending and
receiving end, a JPEG image could theoretically store steganographic
data by tweaking the quantized DCT coefficients until they matched
the desired steganographic output.  The extra beauty here is that
not all JPEG decoders are built the same, so the output may not
be bit-for-bit the same.  Decoding the steganographic data would
require not only a knowledge of the algorithm, but a matching JPEG
decoder as well.