Cryptography Reference
In-Depth Information
tighter. This is not as glaring because the distribution naturally varies
between images, but it is still worth combating.
Westfeld further enhanced the process to minimize the disrup-
tion to the file in the next version of the algorithm, F5. He uses a pro-
cess he calls matrix encoding to spread the information out among
more bits. This reduces the density and decreases the amount of dis-
tortion.
Imagine you want to store
=4
bits of data. One solution is to
pick
2
n
=16
different locations in the image and only change one of
the locations. A change at position
3 = 0011
wouldmean you wanted
to store themessage
0011
. The algorithmF5 chooses the best value of
n
n
to accommodate the data being stored. The positions are chosen
with a cryptographically secure pseudo-random bit stream.
9.6 Summary
Placing information in the noise of digitized images is one of the
most popular methods of steganography. The different approaches
here guarantee that the data will be hard to find if you're careful
about how you use the tools. The biggest problem is making sure that
you handle the differences between24-bit and 8-bit images correctly.
The Disguise
The world is filled with noise. There is no reason why
some of the great pool of randomness can't be used to hide
data. This disguise is often impossible for the average human
to notice.
How Secure Is It?
These systems are not secure if someone is look-
ing for the information. But many of the systems can produce
images that are indistinguishable form the original. If the data
is compressed and encrypted before it is hidden, it is impossi-
ble to know whether the data is there or not. This can be sub-
verted if a special header is used to identify details about the
file. Chapter 17 describes how these simple approaches can be
detected.
How to Use the Software
There are many different versions of the
software available on the Net. The Cypherpunks archive is a
good location for the programs. Others circulate throughout
the Net.
