Cryptography Reference
In-Depth Information
9.2.6 Finding Edges
The Digital Invisible Ink Toolkit written by Kathryn Hempstalk in-
cludes several algorithms for encoding information in the most vi-
sual significant parts of the image. How does it identify them? By
using a Sobel filter or a Laplace filter, two standard averaging tools
that identify the pixels that are most different from their neighbors.
These are usually called the
edges
of the item.
The software package locates these edges and then tweaks the
least significant bits at these locations. The algorithms only feed the
most significant bits into the filters to avoid any problems caused by
side effects from changing the least significant bits. It is possible that
the change of the least significant bit would turn a pixel from an edge
into a non-edge making it invisible to the decoding algorithm.
One of the version called
BattleSteg
will choose pixels at random
until it finds one that is significantly edgy. Then it tweaks the least
significant bits of the pixels around it. The random selection process
is driven by the password so it the recovery process can repeat the
same sequence. FIgure 9.5 shows an image with a hidden informa-
tion and the locations where the data is stored.
9.3 Bit Twiddling
Perhaps the best way to begin experimenting with hiding informa-
tion in the noise is to download one of the experimental packages
floating around the Net. There are easily more than one hundred
Appendix A offers a list
of some of the more
prominent
steganography
packages.
programs that have been circulated publically, but many of them
seem to disappear as quickly as they appear.
9.3.1 Working with GIFs
GIF files store their images by creating a pallette of
2
n
different colors
and then mapping the colors from the image to the closest color.
This can make it harder to store data by flipping the least significant
bits because two adjacent entries in the table of colors may wildly
different even though they only differ by one significant bit.
One of the earlier programs, Hide and Seek worked around this
problem.
One version,
grey.exe
, converted color GIFs into grayscale GIFs
that would not show any of the artifacts associated with 8-bit color
