03-10-2012, 03:55 PM
Image Steganography and Steganalysis
Image Steganography.pdf (Size: 3.17 MB / Downloads: 141)
Abstract
Steganography is the art of hiding information within other information in
such a way that it is hard or even impossible to tell that it is there. There
are many different carriers for steganography but the most popular is digital
images. Recently, there it has been rumoured that terrorist cells are using
steganography to hide their secret plans, and subsequently it is becoming
increasingly important to detect the images that contain steganography such
that we can reduce foul-play. This counter-technique is known as steganalysis.
In contrast to the magnitude of papers that analyse either steganography
or steganalysis techniques, this paper provides in-depth descriptions of both.
It also offers a chance to put the theory into practice by way of a piece of
software designed to maximise learning in the fields. The paper can therefore
be split into two parts: Research, and Software Development.
Introduction to Steganography
What is Steganography?
The term Steganography refers to the art of covert communications. By implementing
steganography, it is possible for Alice to send a secret message to Bob in such a way that
no-one else will know that the message exists. Typically, the message is embedded within
another object known as a cover Work, by tweaking its properties. The resulting output,
known as a stegogramme is engineered such that it is a near identical perceptual model
of the cover Work, but it will also contain the hidden message. It is this stegogramme
that is sent between Alice and Bob. If anybody intercepts the communication, they will
obtain the stegogramme, but as it is so similar to the cover, it is a difficult task for
them to tell that the stegogramme is anything but innocent. It is therefore the duty
of steganography to ensure that the adversary regards the stegogramme - and thus, the
communication - as innocuous.
One of the oldest examples of steganography dates back to around 440 BC in Greek
History. Herodotus, a Greek historian from the 5th Century BC, revealed some examples
of its use in his work entitled "The Histories of Herodotus". One elaborate example
suggests that Histaeus, ruler of Miletus, tattooed a secret message on the shaven head
of one of his most trusted slaves. After the hair had grown back, the slave was sent to
Aristagorus where his hair was shaved and the message that commanded a revolt against
the Persians was revealed [19]. In this example, the slave was used as the carrier for the
secret message, and anyone who saw the slave as they were sent to Aristagorus would
have been completely unaware that they were carrying a message. As a result of this,
the message reached the recipient with no suspicion of covert communication ever being
raised.
How is Steganography Used?
When a steganographic system is developed, it is important to consider what the most
appropriate coverWork should be, and also how the stegogramme is to reach its recipient.
With the Internet offering so much functionality, there are many different ways to send
messages to people without anyone knowing they exist. For example, it is possible that
an image stegogramme could be sent to a recipient via email. Alternatively it might
be posted on a web forum for all to see, and the recipient could log onto the forum
and download the image to read the message. Of course, although everyone can see the
stegogramme, they will have no reason to expect that it is anything more than just an
image.
In terms of development, Steganography is comprised of two algorithms, one for embedding
and one for extracting. The embedding process is concerned with hiding a secret
message within a cover Work, and is the most carefully constructed process of the two.
A great deal of attention is paid to ensuring that the secret message goes unnoticed if
a third party were to intercept the cover Work. The extracting process is traditionally
a much simpler process as it is simply an inverse of the embedding process,
How is Steganalysis Used?
Steganalysis is very important to international security, as growing interest emerges as to
whether terrorist organisations use steganographic techniques to communicate with each
other. In fact steganalysis is taken so seriously that it is believed that US Government
agencies, including the NSA and the Pentagon are funding research for its development.
If a file is considered to contain a secret message then it is possible that the entire Work
will be modified by the steganalyst such that the integrity of the message is removed.
This means that that Bob will not be able to make sense of the message that Alice sent
to him when he attempts to extract it. A much safer solution however, is that the Work
is deleted so that it never reaches the recipient.
Steganalysis is an extremely difficult science, as it relies on insecure steganography.
As discussed in section 1.1.1, if steganography is to be successful, it should leave no
indication that a secret message exists. Thus, if the model has been created successfully,
it should be a difficult task for any third party to spot that tampering has occurred.
Jessica Fridrich [5] suggests that "the ability to detect secret messages in images is
related to the message length". This statement is based on the logic that a small message
embedded within a large carrier will result in a small percentage of manipulations, and
therefore it will be much harder to spot any artefacts within the stegogramme.
Principles of Steganography and
Steganalysis
Literature Review
In order to understand the methods for steganography and steganalysis that will be
discussed later on in the project, it is first necessary to build up a clear picture of both
fields.
This chapter will discuss the main principles of steganography and steganalysis by
firstly discussing where we currently stand in both fields, and then introducing the necessary
background knowledge that is required to properly understand the methods introduced
in this project. The JPEG compression process is also discussed as it is essential
that we understand the significance of embedding the message data within this domain
at a later stage. Finally, we introduce some evaluative metrics so that we can easily
relate to the success and failure of the steganalysis techniques in future chapters.
Past Work
Given time, it has been possible to break every steganographic system that has ever been
published. As a result of this, new techniques are developed to improve upon the flaws
of the predecessor. It is very much the same case in steganalysis where the algorithms
are often tweaked or combined in order to attack the latest steganography algorithm.
The two fields therefore operate in a ’cat and mouse’ style strategy with steganography
aiming to be ahead of the field such that covert communications may exist. There is
then a call for steganalysis to catch up such that covert communications are minimised
as much as possible.
One of the most basic steganographic techniques is achieved by manipulating the pixel
values of the cover image in sequence such that they then turn into code that can be used
to reconstruct the message when extracting. The most popular method for developing
this concept left a pattern in the beginning of the images that steganalysts bought to
light. This led to the development of a series of techniques that could detect the existence
of steganography within any stegogramme that followed this embedding strategy, and so
steganographers set about developing a new system that would render the attacks useless.
This led to the development of randomised embedding in pixel values