1. Introduction
Steganography is the art, science, or practice in which messages, images, or files are hidden inside other messages, images, or files. The concept of steganography is not a new one; it dates back many millennia when messages used to be hidden on things of everyday use such as watermarks on letters, carvings on bottom sides of tables, and other objects. The more recent use of this concept emerged with the dawn of the digital world. Experiments have shown that data can be hidden in many ways inside different types of digital files. The main benefit of steganography is that the payload is not expected by the investigators who get to examine the computer data. The person sending the hidden data and the person meant to receive the data are the only ones who know about it; but to everyone else, the object containing the hidden data just seems like an everyday normal object. A classification of information hiding techniques is described in [1].
Generally, steganographic methods proposed in the past few years can be categorized into two types. The methods of the first type employ the spatial domain of a host image to hide secret data. In other words, secret data are directly embedded into the pixels of the host image [2-5].
Steganographic methods of the second type employ the transformed domain of a host image to hide secret data [6-10]. Transformation functions like the discrete cosine transform (DCT) or discrete wavelet transform (DWT) are first exploited to transform the pixel values in the spatial domain to coefficients in the frequency domain. Then the secret data are embedded in the coefficients.
Recently, the scheme proposed in [11] hid large amounts of data in the pixels of a true color image, which used 8 bits to represent each color component of a color pixel. The secret image can be both a grayscale and a true color image. However, the quality of the extracted color secret image is not good in terms of the peak signal-to-noise ratio (PSNR) value and visual observation. In this paper, we propose a steganographic method which hides a color or a grayscale image in a true color image.
The detection of hidden data presents a big challenge to investigators and individuals looking for hidden data. For images only, there are hundreds of billions of images on the web and looking through all of them would be a very time consuming and computationally challenging task; let alone the other types of files that data can possibly be hidden in. Even if someone manages to go through all the current images on the web, what if some new algorithm for hiding data in images emerges? Is the application used to scan the images for hidden data suitable for and capable of uncovering the hidden data? And is it feasible to go back and rescan all the images all over again with the same or other software updated to detect the hidden data by the new algorithm? The answer to the above questions is that it’s close to impossible to be able to accurately scan or attempt to detect hidden data on such a wide scope of suspect images. It is somewhat easier for investigators to scan for hidden data on a smaller scale such as an image of a hard drive, but they are still faced with the same software inaccuracy and the possibility of encountering unknown data-hiding algorithms.
In our proposed method, the local color information of an image can be preserved well because the technique of color quantization in the proposed method is image-dependent and adaptive color quantization [12]. The extracted color secret image is perceptibly almost identical to the original image, and its PSNR value is very high Moreover, the hiding capacity of the host image and the quality of the stego-image in the proposed method are also superior to that of the scheme in [11].
The remainder of this paper is organized as follows. In Section 2, a brief description of the scheme related to this paper will be given. In Section 3, we shall present our proposed method. The overall algorithm for the proposed method will be provided in Section 4. Finally, the experiments and conclusions shall be given in Sections 5 and 6.
2. Color Quantization
Color quantization is the process that drastically reduces the number of colors used in a digital color image by approximating the original pixels with their nearest representative colors. The true color image is usually quantized to reduce the size of the image to be stored or transmitted. This means the 224 colors of a true color image have to be greatly reduced to a limited number of representative colors, which is called a color table (palette).
Typically, a color table consists of 256 entries where each entry represents a color containing red, green, and blue components. In general, a palette-based image mainly consists of a color table and some image data, which contain indices corresponding to the color table entries.
Bitmap images consist of matrices of numbered points with two dimensions for grayscale and three dimensions for RGB color images. The grayscale images, also called intensity images, contain numbered values at these points, called pixels, between 0 for black and 255 for white, which can be represented as 8-bit binary strings (28 = 256). The numbers between represent gradient gray values between black and white. The RGB, abbreviated for Red, Green and Blue, images are actually three two-dimension image layers, a red, a green and a blue layer, that are combined to produce the full color image. Each layer of a color image also contains values from 0 for black to 255 for the lightest shade of the color. The RGB color scheme is referred to as an additive scheme because adding the effective value of the three layers usually produces a lighter color at that pixel.
For instance if all three values that comprise a pixel are 0, i.e. (0, 0, 0) for (red, green, blue), they produce the color black. If the three are 255, i.e. (255, 255, 255) they add to produce a much lighter shade, white. The product of these three layers can produce over 16 million different colors and is called 24-bit color because 2563 = (28)3 = 224 = 16,777,216, in which three 8-bit binary strings represent pixel colors. They are intensity, color, red layer, green layer and blue layer, from left to right. Notice that the vertical white line that appears in the center of the intensity and color images is lighter than that in the RGB layers and that the blackish colors appear black in all images. This demonstrates the additive nature of RGB color images with respect to intensity.
Messages are hidden in the least significant bits of the 8-bit binary strings representing the color numbers; hence the abbreviated name for this method is “lsb” steganography. Each character in a message has a binary representation under the ASCII (American Standard Code for Information Interchange) character system, which assigns characters with integer values between 0 and 255. This system represents a way to express all necessary single character letters, numbers, punctuations, symbols, etc. for general communication purposes. The pixel is capable of representing 224 or 16,777,216 color values. If we use the lower 2 bits of each color channel to hide data as shown in Figure 1.
3. The Proposed Scheme
In this section, we shall present the proposed the VDX method for extracting hiding a color or a grayscale secret image in a true color host image.
The proposed method is described below.
3.1. Requirements of Hiding Information Digitally
There are many different protocols and embedding techniques that enable us to hide data in a given object. However, all of the protocols and techniques must satisfy a number of requirements so that steganography can be applied correctly. The following is a list of main requirements that steganography techniques must satisfy:
Figure 1. Illustrated representing the pixel values used the lower 2 bits of each color channel to hide data.
● The integrity of the hidden information after it has been embedded inside the stego object must be correct. The secret message must not change in any way, such as additional information being added, loss of information or changes to the secret information after it has been hidden. If secret information is changed during steganography, it would defeat the whole point of the process.
● The stego object must remain unchanged or almost unchanged to the naked eye. If the stego object changes significantly and can be noticed, a third party may see that information is being hidden and therefore could attempt to extract or to destroy it.
● In watermarking, changes in the stego object must have no effect on the watermark. Imagine if you had an illegal copy of an image that you would like to manipulate in various ways. These manipulations can be simple processes such as resizing, trimming or rotating the image. The watermark inside the image must survive these manipulations, otherwise the attackers can very easily remove the watermark and the point of steganography will be broken.
● Finally, we always assume that the attacker knows that there is hidden information inside the stego object.
3.2. Embedding and Detecting a Mark
Figure 2 shows a simple representation of the generic embedding and decoding process in steganography. In this example, a secret image is being embedded inside a cover image to produce the stego image. The first step in embedding and hiding information is to pass both the secret message and the cover message into the encoder. Inside the encoder, one or several protocols will be implemented to embed the secret information into the cover message. The type of protocol will depend on what information you are trying to embed and what you are embedding it in. For example, you will use an image protocol to embed information inside images.