Cryptography Reference
In-Depth Information
1. Find the color that is farthest from
C
.Callit
d
.
2. Find the best place to insert this color in the list. That is, scan
the list and find
i
such that
δ
(
C, c
i
)+
δ
(
C, c
i+1
)
is minimized.
3. Insert
d
and set it to be the new
C
.
Finding the farthest color ensures that algorithm does not get
“trapped” in one corner of the three-dimensional color cube only to
find it must make big leaps to reach the other colors.
This sorting algorithm is not perfect and there's no guarantee that
the jumps will be small. While the overall distance is minimized,
some distances are more important than others. Adding information
in the least significant bit will only produce some changes. In a three-
bit system,
001
can only change to
000
,not
010
. Sorting the entire
palette may leave one of the longer jumps between color
000
and
001
and put one of the shorter ones between
001
and
010
.Amore
sophisticated approach will try to find the best way to pair up all of
the colors in the palette so that the total distance of all of the pairs is
minimized.
Jessica J. Fridrich offered an easy tweak to the algorithm. Instead
of sorting the palette, the algorithm looks for the closest color with
the right parity for representing the bit being hidden. The parity of
the bit is
2
.[Fri99]
EzStego tries to thwart steganalysis by shipping the palette un-
sorted. Here are the steps:
R
+
G
+
Bmod
1. Begin with an unsorted palette produced by a program like
Photoshop.
2. Sort the palette so the closest colors fall next to each other in
order.
3. Encode the message by twiddling the least significant bit. Make
sure to encrypt the message with a cryptographically secure
random number stream.
4. Unsort the palette by renumbering all of the colors with their
original values from the original palette.
5. Ship the image. Any attacker looking at the palette will only see
one produced by a non-steganographic program.
6. The receiver can re-sort the palette using the same sorting al-
gorithm. If the algorithm is deterministic, it will produce the
same results.
