14-01-2013, 04:12 PM
A detailed look at Steganographic Techniques and their use in an Open-Systems Environment
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Introduction
This paper's focus is on a relatively new field of study in Information Technology known
as Steganography. This paper will take an in-depth look at this technology by introducing the
reader to various concepts of Steganography, a brief history of Steganography and a look at some
of the Steganographic techniques available today. The paper will close by looking at how we can
use Steganography in an open-systems environment such as the Internet, as well as some of the
tools and resources available to help us accomplish this.
What is Steganography and why is it important?
Steganography or Stego as it is often referred to in the IT community, literally means,
"covered writing" which is derived from the Greek language. Steganography is defined by
Markus Kahn [5] as follows, "Steganography is the art and science of communicating in a way
which hides the existence of the communication. In contrast to Cryptography, where the enemy
is allowed to detect, intercept and modify messages without being able to violate certain security
premises guaranteed by a cryptosystem, the goal of Steganography is to hide messages inside
other harmless messages in a way that does not allow any enemy to even detect that there is a
second message present".
In a digital world, Steganography and Cryptography are both intended to protect
information from unwanted parties. Both Steganography and Cryptography are excellent means
by which to accomplish this but neither technology alone is perfect and both can be broken. It is
for this reason that most experts would suggest using both to add multiple layers of security.
Steganography can be used in a large amount of data formats in the digital world of
today. The most popular data formats used are .bmp, .doc, .gif, .jpeg, .mp3, .txt and .wav.
Mainly because of their popularity on the Internet and the ease of use of the steganographic tools
that use these data formats. These formats are also popular because of the relative ease by which
redundant or noisy data can be removed from them and replaced with a hidden message.
A Brief History of Steganography
The earliest recordings of Steganography were by the Greek historian Herodotus in his
chronicles known as "Histories" and date back to around 440 BC. Herodotus recorded two
stories of Steganographic techniques during this time in Greece. The first stated that King
Darius of Susa shaved the head of one of his prisoners and wrote a secret message on his scalp.
When the prisoner’s hair grew back, he was sent to the Kings son in law Aristogoras in Miletus
undetected. The second story also came from Herodotus, which claims that a soldier named
Demeratus needed to send a message to Sparta that Xerxes intended to invade Greece. Back
then, the writing medium was text written on wax-covered tablets. Demeratus removed the wax
from the tablet, wrote the secret message on the underlying wood, recovered the tablet with wax
to make it appear as a blank tablet and finally sent the document without being detected.
Romans used invisible inks, which were based on natural substances such as fruit juices
and milk. This was accomplished by heating the hidden text, thus revealing its contents.
Invisible inks have become much more advanced and are still in limited use today.
During the 15th and 16th centuries, many writers including Johannes Trithemius (author of
Steganographia) and Gaspari Schotti (author or Steganographica) wrote on Steganagraphic
techniques such as coding techniques for text, invisible inks, and incorporating hidden messages
in music.
Between 1883 and 1907, further development can be attributed to the publications of
Auguste Kerckhoff (author of Cryptographic Militaire) and Charles Briquet (author of Les
Filigranes). These books were mostly about Cryptography, but both can be attributed to the
foundation of some steganographic systems and more significantly to watermarking techniques.
During the times of WWI and WWII, significant advances in Steganography took place.
Concepts such as null ciphers (taking the 3rd letter from each word in a harmless message to
create a hidden message, etc), image substitution and microdot (taking data such as pictures and
reducing it to the size of a large period on a piece of paper) were introduced and embraced as
great steganographic techniques.
A Detailed Look at Steganography
In this section we will discuss Steganography at length. We will start by looking at the
different types of Steganography generally used in practice today along with some of the other
principles that are used in Steganography. We will then look at some of the Steganographic
techniques in use today. This is where we will look at the nuts and bolts of Steganography and
all the different ways we can use this technology. We will then close by going over
Steganalysis. Steganalysis concentrates on the art and science of finding and or destroying secret
messages that have been produced using any of the various steganographic techniques we will
cover in this paper.
Encoding Secret Messages in Images
Coding secret messages in digital images is by far the most widely used of all
methods in the digital world of today. This is because it can take advantage of the limited power
of the human visual system (HVS). Almost any plain text, cipher text, image and any other
media that can be encoded into a bit stream can be hidden in a digital image. With the continued
growth of strong graphics power in computers and the research being put into image based
Steganography, this field will continue to grow at a very rapid pace.
Before diving into coding techniques for digital images, a brief explanation of
digital image architecture and digital image compression techniques should be explained.
As Duncan Sellars [7] explains "To a computer, an image is an array of numbers
that represent light intensities at various points, or pixels. These pixels make up the images
raster data." When dealing with digital images for use with Steganography, 8-bit and 24-bit per
pixel image files are typical. Both have advantages and disadvantages, as we will explain below.
8-bit images are a great format to use because of their relatively small size. The drawback is that
only 256 possible colors can be used which can be a potential problem during encoding. Usually
a gray scale color palette is used when dealing with 8-bit images such as (.GIF) because its
gradual change in color will be harder to detect after the image has been encoded with the secret
message. 24-bit images offer much more flexibility when used for Steganography. The large
numbers of colors (over 16 million) that can be used go well beyond the human visual system
(HVS), which makes it very hard to detect once a secret message, has been encoded. The other
benefit is that a much larger amount of hidden data can be encoded into a 24-bit digital image as
opposed to an 8-bit digital image. The one major drawback to 24-bit digital images is their large
size (usually in MB) makes them more suspect than the much smaller 8-bit digital images
(usually in KB) when sent over an open system such as the Internet.