28-12-2012, 03:59 PM
AN OVERVIEW OF IMAGE STEGANOGRAPHY
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Abstract:
Steganography is the art of hiding the fact that communication is taking place, by hiding information in
other information. Many different carrier file formats can be used, but digital images are the most
popular because of their frequency on the Internet. For hiding secret information in images, there exists a
large variety of steganographic techniques some are more complex than others and all of them have
respective strong and weak points. Different applications have different requirements of the
steganography technique used. For example, some applications may require absolute invisibility of the
secret information, while others require a larger secret message to be hidden. This paper intends to give
an overview of image steganography, its uses and techniques. It also attempts to identify the requirements
of a good steganographic algorithm and briefly reflects on which steganographic techniques are more
suitable for which applications.
Introduction
Since the rise of the Internet one of the most important factors of information technology and communication
has been the security of information. Cryptography was created as a technique for securing the secrecy of
communication and many different methods have been developed to encrypt and decrypt data in order to keep
the message secret. Unfortunately it is sometimes not enough to keep the contents of a message secret, it may
also be necessary to keep the existence of the message secret. The technique used to implement this, is called
steganography.
Steganography is the art and science of invisible communication. This is accomplished through hiding
information in other information, thus hiding the existence of the communicated information. The word
steganography is derived from the Greek words “stegos” meaning “cover” and “grafia” meaning “writing” [1]
defining it as “covered writing”. In image steganography the information is hidden exclusively in images.
The idea and practice of hiding information has a long history. In Histories the Greek historian Herodotus writes
of a nobleman, Histaeus, who needed to communicate with his son-in-law in Greece. He shaved the head of one
of his most trusted slaves and tattooed the message onto the slave’s scalp. When the slave’s hair grew back the
slave was dispatched with the hidden message [2]. In the Second World War the Microdot technique was
developed by the Germans. Information, especially photographs, was reduced in size until it was the size of a
typed period. Extremely difficult to detect, a normal cover message was sent over an insecure channel with one
of the periods on the paper containing hidden information [3]. Today steganography is mostly used on
computers with digital data being the carriers and networks being the high speed delivery channels.
Overview of Steganography
To provide an overview of steganography, terms and concepts should first be explained. An overview of the
different kinds of steganography is given at a later stage.
Steganography concepts
Although steganography is an ancient subject, the modern formulation of it is often given in terms of the
prisoner’s problem proposed by Simmons [9], where two inmates wish to communicate in secret to hatch an
escape plan. All of their communication passes through a warden who will throw them in solitary confinement
should she suspect any covert communication [10].
The warden, who is free to examine all communication exchanged between the inmates, can either be passive or
active. A passive warden simply examines the communication to try and determine if it potentially contains
secret information. If she suspects a communication to contain hidden information, a passive warden takes note
of the detected covert communication, reports this to some outside party and lets the message through without
blocking it. An active warden, on the other hand, will try to alter the communication with the suspected hidden
information deliberately, in order to remove the information [5].
Different kinds of steganography
Almost all digital file formats can be used for steganography, but the formats that are more suitable are those
with a high degree of redundancy. Redundancy can be defined as the bits of an object that provide accuracy far
greater than necessary for the object’s use and display [11]. The redundant bits of an object are those bits that
can be altered without the alteration being detected easily [5]. Image and audio files especially comply with this
requirement, while research has also uncovered other file formats that can be used for information hiding.
Figure 1 shows the four main categories of file formats that can be used for steganography.
Image steganography
As stated earlier, images are the most popular cover objects used for steganography. In the domain of digital
images many different image file formats exist, most of them for specific applications. For these different image
file formats, different steganographic algorithms exist.
Image definition
To a computer, an image is a collection of numbers that constitute different light intensities in different areas of
the image [14]. This numeric representation forms a grid and the individual points are referred to as pixels.
Most images on the Internet consists of a rectangular map of the image’s pixels (represented as bits) where each
pixel is located and its colour [15]. These pixels are displayed horizontally row by row.
The number of bits in a colour scheme, called the bit depth, refers to the number of bits used for each pixel [16].
The smallest bit depth in current colour schemes is 8, meaning that there are 8 bits used to describe the colour of
each pixel [16]. Monochrome and greyscale images use 8 bits for each pixel and are able to display 256
different colours or shades of grey. Digital colour images are typically stored in 24-bit files and use the RGB
colour model, also known as true colour [16]. All colour variations for the pixels of a 24-bit image are derived
from three primary colours: red, green and blue, and each primary colour is represented by 8 bits [14]. Thus in
one given pixel, there can be 256 different quantities of red, green and blue, adding up to more than 16-million
combinations, resulting in more than 16-million colours [16]. Not surprisingly the larger amount of colours that
can be displayed, the larger the file size [15].
Image Compression
When working with larger images of greater bit depth, the images tend to become too large to transmit over a
standard Internet connection. In order to display an image in a reasonable amount of time, techniques must be
incorporated to reduce the image’s file size. These techniques make use of mathematical formulas to analyse
and condense image data, resulting in smaller file sizes. This process is called compression [15].
In images there are two types of compression: lossy and lossless [1]. Both methods save storage space, but the
procedures that they implement differ. Lossy compression creates smaller files by discarding excess image data
from the original image. It removes details that are too small for the human eye to differentiate [15], resulting in
close approximations of the original image, although not an exact duplicate. An example of an image format
that uses this compression technique is JPEG (Joint Photographic Experts Group) [14].
Image and Transform Domain
Image steganography techniques can be divided into two groups: those in the Image Domain and those in the
Transform Domain [2]. Image – also known as spatial – domain techniques embed messages in the intensity of
the pixels directly, while for transform – also known as frequency – domain, images are first transformed and
then the message is embedded in the image [20].
Image domain techniques encompass bit-wise methods that apply bit insertion and noise manipulation and are
sometimes characterised as “simple systems” [17]. The image formats that are most suitable for image domain
steganography are lossless and the techniques are typically dependent on the image format [18].
Steganography in the transform domain involves the manipulation of algorithms and image transforms [17].
These methods hide messages in more significant areas of the cover image, making it more robust [4]. Many
transform domain methods are independent of the image format and the embedded message may survive
conversion between lossy and lossless compression [18].