Cryptography Reference
In-Depth Information
9.2.4 File Format Grief
File formats are a serious problem for anyone who would like to rou-
tinely use bit-level steganography to hide images. Many image or au-
dio file formats were designed to squeeze out some of the extra noisy
details to save space. This might be done by a special, efficient file
format like GIF or by an aggressive compression program that does
not care if it reconstructs an image that is not exactly the same. Both
of these make it hard to just hide information in the least significant
bits of an image.
Jessica J. Fridrich and
Rui Du propose hiding
information in the
parity of the sum of the
colors of multiple pixels.
The more pixels used for
each color, the smaller
the changes that need to
be made.[FD99, AP98]
The GIF file format and its 8-bit color standard is a significant
impediment because 8-bit color is quite different from 24-bit color.
It uses a table of 256 different colors that best represent the image
as a color map. The color of each pixel is described by giving the
closest color from this 256-entry table. The bits do not correspond
to the intensity of the colors at each pixel. This means that changing
the least significant bits doesn't necessarily change the intensity at
a pixel by less than 1%. Entry 128 of the table might be a saturated
ruby red while entry 129 might be a pale, washed-out indigo. They
may only differ in the least significant bit, but that can be enough to
cause major changes in the final outcome.
There are a number of different solutions to this problem. The
first is to use a smaller number of colors in the table. Instead of
choosing the 256 colors that do the best job representing the colors
of an image, the software could choose 128 colors and then choose
128 colors that are quite similar to the original 128. They might even
be the same, but that could be too suspicious. The table could be
arranged so that the two very similar colors only differ in one bit.
Here's an abbreviated example of such a table:
Yes, the newspapers
were right: snow was
general all over Ireland.
—James Joyce in
The
Dead
Entry Num.
Binary
Red Intensity Green Intensity
Blue Intensity
0
00000000
150
20
10
1
00000001
151
20
12
2
00000010
14
150
165
3
00000011
16
152
167
4
00000100
132
100
10
5
00000101
135
67
15
.
.
.
.
.
So if you want to hide the value 1 in a pixel, you would find the
closest color in the table and then choose the version of it with the
least significant bit set to one. If the closest color had red set to 15,
green set to 151, and blue set to 167, and you wanted to hide the bit
