Re: help

["baldman@newmail.net" <baldman@newmail.net>]

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> It'll do PICT and probably other formats - somehow I doubt that
> you'll find a steganography program that will do jpeg or mpeg,
> since those are both lossy compression formats, and steganography
> relies on very subtle shifts in the least significant bit of the pixels
> of an image. Jpeg or Mpeg compression would wipe any steganographic
> information from an image. GIF, on the other hand, would be possible

Woah!  I'm no (even remote) expert, but I can't see any reason why you
couldn't use steganography with a lossy format.  GIF itself is 'lossy',
and I'm sure there are (albeit dicey) stego progs for that.  As I see
it, the process is:

 -message-to-hide-----.
                    [stego]------> altered file
 --what-to-hide-it-in-'

The input info format/input 'hiding data' formats dont matter, as long as
the stego algo understands them.  I think you may have been confused
because it IS more easy, visually, to detect an altered file using
conventional (least significant bit) techniques on JPEG/MPEG formats
than it is to detect them on higher-quality formats such as TARGA/TIFF.
This stems from the fact that J/MPEG-style algo's use a technique called
'quantization', where they effectively 'slice' the valid range of
color-values in to 'chunks', by diving an original value (say, 255
red) by a 'quantization integer' (say, 16) to produce a smaller value
(0-15) and discard the remainder.  This is an important part of their
compression strategy.  Because the data is stored in this 'quantized'
state, changing the value (even the 'least significant' bit) will make
the resultant color jump considerably.  J/MPEGs with smaller quantization
integers may still be valid (indeed, in the case of a very-long mpeg,
_exceptional_) input files for stego... under certain situations.

If anyone is interested in low-level graphics stuff, particularly H.261/
H.263/JPEG/MPEG1/MPEG2 then I would recommend "Digital Video
Communications" by Riley + Richardson (ISBN: 0-89006-890-9).  A great
read/reference.

- baldman



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