Cryptography Reference
In-Depth Information
for instance, first creates an optimal 32-color palette for the im-
age. Then it creates seven near duplicate colors that differ by one
pixel in either the red, green or blue component.
If one of the
32 chosen colors has the RGB profile of (64
120) ,then7ad-
ditional colors will be added with the RGB values of (64
,
250
,
,
250
,
121) ,
(64
,
251
,
120) , (64
,
251
,
121) , (65
,
250
,
120) , (65
,
250
,
121) , (65
,
251
,
120)
and (65
Less dramatic versions of this approach are also
common. MandelSteg, for instance, reduce the size of the palette to
128 different colors and then creates only one near duplicate of each
color.
This approach can hide up to three extra bits of information at
every pixel— a significant payload but one that comes with a cost
because palettes like these are also easy to detect. When clusters of
colors appear in the palette, they are often indicators of bit-twiddling
schemes like this. Natural palette creation algorithms try to choose
colors as widely dispersed as possible in order to minimize the error
of reducing the number of colors.
In other cases, the algorithms try to order the elements of the
palette to place similar colors next to each other. Flipping the least
significant bit should not distort the image too much because a sim-
ilar color should be near by. EzStego, a program written by Romana
Machado, uses this technique with some success.
,
251
,
121)
.
Section 13.7 describes
how to hide information
in the order of the
colors, a technique used
in GifShuffle.
The attacker may be able to intercept the bits if the sorting
method is publicly known or easy to figure out. If the data is pro-
tected by an additional layer of encryption, then the message will be
indistinguishable from random noise.
An attacker may still detect the presence of a message by exam-
ining the statistical profile of the bits. An encrypted hidden message
should come with an equal probability of ones and zeros. If the num-
bers of zeros and ones are equal, then the odds point toward a hidden
message.
There are two ways to thwart these attacks. The first is to use
some keyed version of the sorting routine in order to prevent an
eavesdropper from assembling the sorted palette. The palette sort-
ing algorithm is not deterministic because it tries to arrange points
in a three-dimensional space so that the distance between adjacent
points is minimized. This is a discrete version of the Traveling Sales-
man problem, a known difficult problem. EzStego uses one reason-
able approximation that makes guesses. Instead of using a random
source to make decisions, a cryptographically secure keyed random
number generator can take the place and create a keyed sorting al-
gorithm.
The second is to use the statistical mimic functions described in
Search WWH ::




Custom Search