An RGB colour image steganography scheme using overlapping block-based pixel-value differencing

This paper presents a steganographic scheme based on the RGB colour cover image. The secret message bits are embedded into each colour pixel sequentially by the pixel-value differencing (PVD) technique. PVD basically works on two consecutive non-overlapping components; as a result, the straightforward conventional PVD technique is not applicable to embed the secret message bits into a colour pixel, since a colour pixel consists of three colour components, i.e. red, green and blue. Hence, in the proposed scheme, initially the three colour components are represented into two overlapping blocks like the combination of red and green colour components, while another one is the combination of green and blue colour components, respectively. Later, the PVD technique is employed on each block independently to embed the secret data. The two overlapping blocks are readjusted to attain the modified three colour components. The notion of overlapping blocks has improved the embedding capacity of the cover image. The scheme has been tested on a set of colour images and satisfactory results have been achieved in terms of embedding capacity and upholding the acceptable visual quality of the stego-image.


Introduction
In the digital world, one of the major and essential issues is to protect the secrecy of confidential data during their transmission over a public channel. In general, the confidential digital data are pre-processed before their transmission over a public channel. This pre-processing operation changes the content of the information into another form, but only an authorized person is capable of appropriately executing the  [14] have suggested a combination of LSB and PVD methods where three consecutive pixels are considered in hiding the secret message. Their scheme has improved the embedding capacity and retained the acceptable visual quality of the stego-image. Several other PVD variants [3,[15][16][17][18][19][20] are found in the literature for enhancing the PVD technique. Lee et al. [3] have introduced a tri-way PVD approach to improve the hiding capacity and to survive against several steganalyses. Tseng & Leng [15] have modified the traditional PVD-based quantization range table and introduced a new technique known as perfect square number (PSN). The secret message bits are concealed using the PSN and their proposed quantization range table. Liao et al. [16] proposed four-pixel differencing and a modified LSB substitution-based steganographic scheme. The edge region pixel is able to tolerate extensively more changes without perceptual misrepresentation than the smooth region. Swain [17] proposed another combination of LSB-and PVD-based improved image steganographic schemes where the secret message bits are hidden into 2 × 2 pixel non-overlapping blocks of a cover image. Recently, another block-based PVD steganographic scheme was presented in [18], where they have considered 3 × 3 non-overlapping image blocks. A seven-directional PVD scheme [19] is found in the literature with improved payload capacity. Conventional PVD suffers from a falling-off boundary problem in some blocks. Hence after the readjustment process, the distortions of those blocks are high when compared with the other blocks. It is of concern that sometimes it provides a low quality of stego-image. Some authors have addressed this problem and their solutions are effective with intensive computational overhead. Zhao et al. [20] proposed PVD with modulus function for improving the image quality while preserving the same embedding capacity as found in conventional PVD. Another work is found in [21] where the authors overcome the falling-off boundary problem by adopting the adaptive PVD approach.
Several researchers have employed either LSB substitution or a PVD-based steganographic approach to devise some efficient colour image steganographic schemes. In [22], the authors have enhanced the security of the colour steganographic scheme where they have not concealed secret message bits in sequential order into each colour pixel. The embedding process is realized based on a secret pseudorandom value which decides adaptively the payload capacity and the sequence of embedding secret message bits into each colour plane. Their indirect approach definitely enhances the security level. Another LSB substitution-based colour image steganography is found in [23] where the secret message bits are hidden with reference to an indicator colour plane instead of directly embedding the secret message bits in order. Another secret key-based colour image steganography is suggested by Parvez & Gutub [24] where the secret message bits are spread out over each colour plane based on some predefined secret key. A modified PVD-based steganography is proposed by Nagaraj et al. [25]. In their scheme, they used modulus 3 function with PVD for realization of secret message bits into colour pixels. Later, Prema & Manimegalai [26] proposed a colour image steganography using modified PVD. In their scheme, an RGB colour image is decomposed into non-overlapping blocks of two consecutive pixels. Three different pairs, namely (R,G), (G,B) and (B,R), are formed from two consecutive colour pixels and the secret message is embedded based on differences of colour component pairs. They have improved the hiding capacity while maintaining acceptable visual quality of the stego-image. Yang & Wang [27] devised a block-based smart pixel adjustment process where a block of two colour pixels is considered during the secret message-embedding process. However, in their scheme, hiding capacity is not excessive. Adaptive PVD-based colour image steganography is suggested in [28] where the secret message is concealed in the block level of each colour plane. The vertical and horizontal edges are exploited in each block during the message-embedding process. The above colour image steganographic schemes basically work on a colour plane instead of on colour pixels. Hence in this paper, we have proposed an RGB colour image steganography, where the secret message is concealed into each colour pixel independently. The proposed scheme chooses a colour pixel at a time and embeds the secret message into each colour pixel individually by employing the modified PVD appropriately. In the proposed scheme, the colour pixel is grouped into two pairs, namely (R,G) and (G,B), to form two overlapping blocks. PVD is applied to each pair, for embedding the secret message bits. Afterwards, the proposed readjustment process is carried on each pair to obtain the final modified stego colour components, i.e. R, G and B components. The proposed readjustment process ensures that, in the decoding process, PVD is applicable to extract the secret message bits from the stego colour pixel. The proposed scheme will improve the embedding capacity due to consideration of overlapping block concepts.
The rest of the paper is organized as follows. Section 2 presents the basic idea of the PVD method. The details of the proposed scheme are described in §3. The experimental results are presented in §4. Finally, §5 concludes the paper.

Basics of pixel-value differencing
The PVD method [13] uses grey-level images as the cover image and variable-sized secret message bit sequences are embedded into the cover image. Fewer secret message bit sequences are embedded into the smooth region compared with the edge region. Initially, the cover image is partitioned into nonoverlapping blocks of size 1 × 2 in raster scan order. Two consecutive pixels in the ith block are denoted as P i and P i+1 , respectively. The difference value, d i , between two consecutive pixels is calculated by In this method, ith block pixels P i and P i+1 will be replaced by the stego pixels P i and P i+1 . After the embedding process, the receiver side will compute the difference of the ith block d i = |P i − P i+1 |. The difference d i is used to search for the number of concealed bitstreams in the ith block using the quantization range from figure 1. The secret bitstreams are obtained after converting the decimal value of (d i − lower i ) into binary form. An example of the PVD process is illustrated below. Suppose the 4 bits binary secret message is 1011 2 and its corresponding decimal value is 11 10 . The modified difference and m are calculated as follows: Finally, as per equation (2.1), the stego pixels will be computed as follows:

Proposed scheme
The proposed colour image steganographic scheme is presented in this section. Initially, each colour pixel is decomposed into its corresponding colour components, i.e. R, G and B. Later we have formed two pairs with a combination of (R,G) and (G,B). Other ordered pairs are also acceptable, but in this work, we have implemented our scheme using the pairs like (R,G) and (G,B). (R,G) and (G,B) will form two consecutive overlapping blocks as shown in figure 3. In our scheme, we have embedded the variable secret message bits based on the difference of each pair using PVD. After embedding the secret message bits into each pair, the intermediate colour components are further readjusted to attain the final stegocolour components. A natural colour image may be dominated by particular colour components as an outcome of the data hiding process of that particular pixel, and the distortion may be large enough to be perceived. In this paper, we have avoided this circumstance by adopting a suitable threshold value. The data-hiding capacity in each colour pixel is restricted by the threshold value, so that the stego-image may retain high visual quality. Figure 4 shows the overall embedding process. The decoding process is shown in figure 5. The algorithm steps of the proposed embedding and extraction procedure are presented as follows:

Experiment results
In this section, the experimental results are presented to demonstrate the performance of the proposed scheme. The proposed scheme has been tested on a set of standard colour images, but in this paper, we present the results for six colour images where the images are selected with consideration of diverse image features to estimate the performance in terms of visual quality and embedding capability of the stego-images. The original images are shown in figure 7. The randomly generated message bits are               insignificant, as shown in figures 21-26. The stego-image quality is further estimated in terms of the peak signal-to-noise ratio (PSNR) and embedding capacity/payload.  that the distortion appearing after embedding of the secret message into the cover image is reasonably less and imperceptible to human visual perception. The proposed scheme is also compared with some other steganographic schemes in terms of embedding capacity and PSNR, and their results are given in

Conclusion
Most colour image steganography works on individual colour components instead of considering all colour components together. But in this paper, the proposed method conceals the secret message bits directly into each pixel sequentially. Conventional PVD works on the idea of overlapping blocks of colour components. The proposed readjustment process of colour components confirms the feasibility of conventional PVD-based decoding procedure. The experimental results reveal that the proposed scheme has a larger hiding capacity with acceptable imperceptibility of the stego-image. In addition, the proposed scheme is simple and easy to implement on RGB colour images.
Data accessibility. Our data have been deposited at Dryad (http://dx.doi.org/10.5061/dryad.21tm5) [29]. Authors' contributions. Both authors contributed to the design and implementation of the research, and to the writing of the manuscript.