International Journal of Engineering Trends and Technology (IJETT) – Volume 29 Number 1 - November 2015 A Review on current Image Steganography trends with focus on Spatial Domain Techniques Saleema.A1, Dr.T.Amarunnishad*2 # II year M.Tech, Computer Science and Engineering, TKM College of Engineering, Kollam, Kerala, India Abstract — Steganography is a communication method which involves sending secret information in appropriate carriers. Since it has an interesting fact of concealing the message as well as the existence of the message, steganography is on its evolutionary path to unearth new platforms. As the field of steganalysis is growing exponentially, the need for developing strong steganographic algorithms is also growing. Since the use of steganography is spreading across various fields, the goal of increasing the embedding capacity, security and image quality is being the major concerns. Here we analyze the current trends and recent advancements in image steganography mainly giving focus on the spatial domain techniques. This paper reviews some recent state of the art techniques from the literature. Also we mention the various classifications and some of the quality assessment measures and attacks. Keywords — Digital Image Steganography, Adaptive Steganography, Reversible and Irreversible Steganography, Embedding capacity, Imperceptibility, Stego image, Stegnanalysis. I. INTRODUCTION Steganography deals with the art of hiding information with an interesting property of hiding the mere existence of the secret information. What makes steganography more preferable than cryptography is its extra layer of security provided by the undetectability of the presence of secret information. Cryptography is the practice of scrambling a message to an obscured form to prevent others from understanding it while steganography is the study of obscuring the message so that it cannot be seen. Steganography requires two files – the cover/carrier file and the secret file. Various multimedia carriers like audio, video, text, image etc can act as a cover media to carry the secret information. Also the secret can be of any type, which in most cases is converted into a bitstream. The resultant file after embedding can also be called as a stego-file. In image steganography the cover file will be an image and the secret may be plain text, cipher text (or another image).In this review, we focus on image steganography. computational complexity of insertion and extraction processes. Payload capacity refers to the amount of information that can be embedded in the cover without affecting the quality of the cover medium. For image steganography, it is usually expressed in bpp (bits per pixel) which indicates the number of bits that can be concealed in a particular pixel. Security/Robustness refers to the capability of a stego-image to resist the statistical and visual steganalytic attacks as well as some image manipulations like rotation, scaling, cropping etc. Another important thing to consider is the imperceptibility which means the changes introduced by the steganographic algorithm should not be noticeable by the human eye. Also the time required to embed as well as extract the secret data should be considered while designing a steganographic algorithm. III. IMAGE STEGANOGRAPHY MODEL The major components of image steganographic model are cover image, secret image and stego-image. Cover image is the place where we conceal our secret. The secret image to be hidden is converted into a bit stream and using the stego-key, the insertion algorithm is performed. After insertion a stego-image is formed which looks exactly similar to that of the cover image, but having the secret data in it. This stego image should be resistant to steganalytic attacks as they are being send through the insecure communication channels. Upon receiving the stego-image, using the stegokey the receiver can extract the bit stream by performing the extraction process (Reverse of insertion process). Later the bit stream is converted to image. The stego-key is the one which should be predetermined and shared by both the sender and the receiver. Figure 1 shows an image steganography model. II. REQUIREMENTS OF STEGANOGRAPHY The important issues to be considered in steganography are the payload capacity, security/robustness, imperceptibility and ISSN: 2231-5381 http://www.ijettjournal.org Page 51 International Journal of Engineering Trends and Technology (IJETT) – Volume 29 Number 1 - November 2015 ((2 SSIM ( Ic, Is) 2 Ic ( Ic Is 2 Is (224 1) 0.01)2 )((2 Ic ((224 1) 0.01)2 )( Ic2 Is 2 Is (224 1) 0.03)2 ) ((224 1) 0.03)2 ) (3) where Ic , Is are the mean of cover and stego images respectively, Ic , Is is the covariance of 2 Ic Figure 1: Image Steganography Model IV. PERFORMANCE MEASURES Performance of a steganographic algorithm depends on the following factors. * Security * Imperceptibility * Embedding capacity * Computational complexity Like any other image processing techniques, there are statistical as well as visual measures for performance evaluation in image steganography. Since the field of steganalysis is fast growing, the effort required in designing robust steganographic algorithms is also increasing. Some basic measures of testing the image quality include PSNR, MSE, SSIM etc. PSNR is defined as the ratio between the maximum possible power of a signal and the power of corrupting noise that affects the fidelity of its representation. The signal in this case is the original image, and the noise is the error introduced by steganography. It is preferable if the stego-image generated have a PSNR of more than 40 dB. PSNR can be calculated using equation (1). PSNR 10 log10 max 2 MSE 2 , Is are the variance cover and stego images and of cover and stego images respectively. Correlation measure can be used to find out the correlation between cover and stego images. A correlation value closer to 1 indicates that there is less distortion and the stego image is more similar to that of the cover. Otherwise the stego image is distorted. It is calculated using the equation (4). M N pij p qij q i 1 j 1 r M N pij i 1 j 1 p 2 M N qij q 2 i 1 j 1 (4) Where p and q indicate the average pixel value of cover image and stego image respectively. MxN is the image size. Some other image quality measures include Normalized cross correlation, Average Difference, Maximized difference, Image fidelity [1] etc. There are some metrics like Minkowski summation [2], Watson metric [3], Universal quality index [4], Spearman’s rank correlation coefficient [5] which can be used for image quality assessment. In order to assure security/robustness of steganography algorithms, it should be proven that they are resilient to some known steganalytic attacks. 2 Some classical attacks include Chi square ( ) attack, Regular Singular (RS) attack, Pixel Value Differencing, LSB flipping etc. Where max is the maximum pixel intensity and MSE Chi square attack was proposed by Westfield and (Mean Squared Error) for a colour image can be Pfitzmann (2000). It is based on statistical analysis of calculated by equation (2) a Pair of Values (PoVs) that are exchanged during MSER MSEG MSEB sequential concealing. The idea of the statistical attack MSE 3 (2) is to compare the theoretically expected frequency distribution in steganograms with some sample Where MSER , MSEG , MSEB corresponds to the distribution observed in the possibly changed carrier MSE of Red, Green and Blue components respectively. medium [6]. We can find out the probability that some embedding has taken place by the degree similarity of SSIM is the Structural Similarity Metric the observed sample distribution and the theoretically expected frequency distribution. proposed by Wang et.al (2004) to measure the Regular Singular Steganalysis proposed by Fridrich et.al [7] is commonly used in random LSB similarity between two identical images. This metric steganography. It is based on some alternations made can be calculated by equation (3). in the Least Significant Bit plane of an image and the classification of pixels into three groups. The ISSN: 2231-5381 (1) http://www.ijettjournal.org Page 52 International Journal of Engineering Trends and Technology (IJETT) – Volume 29 Number 1 - November 2015 embedding length depends on the counts of the three groups. Figure 2: General Classification of Image Steganography V. IMAGE STEGANOGRAPHY METHODS A. General Classification and current trends Image steganography methods can be broadly classified into three namely spatial domain based, frequency domain based and compressed domain based (Figure 2). Each method has advantages as well as disadvantages. As we analyse, the spatial domain techniques is found to achieve a better trade off with the basic requirements of steganographic i.e. Spatial domain techniques can achieve high embedding capacities with better security and robustness and considerable computation time. In the recent years researchers of image steganography have introduced some efficient methods by combining the traditional LSB and PVD like approaches with new algorithms. Some researchers concentrates on making use of some features of cover images to find out better embedding regions and then concealing more secret data on those regions According to the human vision sensitivity complex regions are well suited to hide data than smooth regions. Also in some approaches the cover image is subjected to some sort of preprocessing to increase the payload capacity. To achieve low cost, optimal and adaptive data concealing some soft computing based approaches are being used with steganographic algorithms. It is found that introduction of soft computing techniques like fuzzy Logic, Adaptive Neural Network, Particle Swarm Optimization, Ant Colony optimization etc can give excellent results in image steganography. ISSN: 2231-5381 Figure 3: Classification of Spatial Domain Techniques. B. Recent Advancements in Spatial Domain Techniques The Spatial Domain Techniques for image steganography can be classified as in figure 3. The most popular spatial domain techniques used in image steganography are LSB based approaches. These include LSB replacement, LSB matching, LSB Matching Revisited etc. The major advantages of LSB based techniques include less degradation or imperceptibility and reasonable embedding capacity. The disadvantage is that such methods are less robust and sometimes the hidden data can be easily destroyed by some known steganalytic attacks. In [8], El Emam proposed a steganographic algorithm, hiding a large amount of data (up to 75% of the cover image size). The pixels for hiding are selected randomly and he designed a new concept defined by main cases and their sub cases for each byte in one pixel of a color image, to find the number of bits to be replaced with the secret data. The algorithm consists of three layers, one for adaptive segmentation of the cover image, one for encryption of secret message and the third for information hiding. Later he improved the method by adding an additional layer (forth layer) in [9]. This layer which is based on a neural network adds additional security against statistical as well as visual attacks. Further the method is modified by introducing the concept of Genetic Algorithm [10]. Also a new method for information hiding based on byte characteristics and variance distribution is proposed in [10]. Thus it turned out to be an intelligent method to conceal a large amount of secret message with hybrid adaptive neural network and modified adaptive genetic algorithm. This can be http://www.ijettjournal.org Page 53 International Journal of Engineering Trends and Technology (IJETT) – Volume 29 Number 1 - November 2015 viewed as a significant advancement in the research area of image steganography. In [11], a high capacity image steganography using multi layer embedding which results in good quality stego image and low computational complexity is proposed. The method is found to be better than the NMI [12] and INP [13] hiding methods. Neighbor mean interpolation (NMI) and Interpolating Neighboring Pixels (INP) integrate image interpolation with data hiding in spatial domain. A low distortion data embedding method is proposed in [14]. This method explores pixel value differencing and base decomposition. This method is based on a range table for characterizing the host image as in some other methods like PVD [I5] and HPVD [16]. But the advantage of this method over PVD and HPVD are the degree simplification and less pixel variation. It is found out that while embedding the same or slightly larger quantities of messages, this scheme produces the highest embedded image quality compared with the other two. Replacing the LSB of cover images with secret data and authentication code will result in distortion of stego image. So in [17], a novel method of image sharing is introduced by applying the Optimal Pixel Adjustment Process (OPAP) process to enhance the image quality under different payload capacity and various authentication bits conditions. The use of an image identification number makes this method distinct and more secure than other schemes. The authors proposed a new steganography algorithm using hybrid fuzzy neural networks. Here data is embedded using LSB approach and a post processing of the stego image is carried out using the fuzzy neural networks. This method could produce stego images of high visual quality and payload capacity. C. Adaptive Techniques Another classification of image steganography is Adaptive Steganography. This category may include spatial as well as transform domain techniques. Such techniques embed data adaptively by considering the concepts of human vision and results in high payload capacity as well as low distortion. Most of the PVD based techniques comes under this category as they are edge adaptive. In [18] and [19], they propose an edge adaptive scheme which can select the embedding regions according to the size of secret message and the difference between two consecutive pixels in the cover image. Here they tried to embed as much data as possible in the sharper edge regions than the smoother regions by adjusting a few parameters. Instead of selecting the pixel/pixel pair for embedding using a pseudorandom generator, they made use of the relationship between the image content and the size of the secret message. ISSN: 2231-5381 A method called as HBC is proposed in [20], which also have the same property of edge adaptive scheme. Since they just modifies the LSBs while keeping the most significant bits unchanged (edge adaptive case of LSB replacement), there is a problem of asymmetry in their stegos. A similar adaptive method which concentrates on edge areas for hiding with spatial LSB domain systems which outperforms the PVD and LSB replacement methods is proposed in [21] .They combine the method of LSB replacement with another skillful method for readjusting the pixel values in order to increase the quality of stego images. In [22] and [23] Adaptive pixel pair matching is used. They combined with image steganography certain characteristics of human visual system and found to be more resistant toward the visual and statistical attacks. In effect these methods hide more data on the edges identified on the host image. A new scheme for grayscale images that can efficiently assign dynamic embedding capacity and produce high stego image quality is proposed in [24] in which they exploit the Human Visual Sensitivity and local complexity of images. They have taken consideration of local texture characteristics of cover image and thereby adjust the number of secret bits that can be embedded in each pixel inside a block. This method assures strong linear dependency between histograms of cover and stego images. D. Reversible and Irreversible Steganography Image Steganographic methods can be of two types namely irreversible as well as reversible. Most of the above methods so far discussed are irreversible, which means the original cover image cannot be recovered after the hidden messages have been extracted. Reversible/Lossless Steganographic method aims at extracting the embedded information as well as recovering the original cover media. Although certain distortions are introduced during embedding, they should be able to remove such distortions. Reversible data hiding schemes can be of two types. Difference expansion based and Histogram shifting based. The DE based methods can provide better embedding capacity but as it accumulates image degradation between pixel pairs, it is not well suited for multi layer embedding. In histogram shifting based methods, although there is limited embedding capacity, image distortion is found to be less. In [25] , a reversible data hiding scheme is used which is based on histogram shifting imitated reversible data hiding to embed data by using the pixel shifting strategy. The results show that this scheme is suitable for single as well as multi layer embedding with low distortion. The superiority of this method with others resides in its high visual quality. A reversible hiding method for BTC compressed color images is proposed in [26]. They applied Genetic Algorithm for the optimization of bitmaps to http://www.ijettjournal.org Page 54 International Journal of Engineering Trends and Technology (IJETT) – Volume 29 Number 1 - November 2015 improve the compression rate and secret data is embedded in the optimal common bitmap. Reversibility is achieved by the order of quantization levels. This method can embed more than three bits per BTC encoded block and also good in image quality. In [27], the reversible data hiding scheme is classified into 4 types. Type-I are the initial data hiding algorithms based on lossless compression, which are not so satisfactory since the introduced distortion is usually large. Type-ll are those based on Difference Expansion technique, in which the use of a location map results in consuming the embedding capacity. Type-III algorithms are based on predictionerror expansion technique and Type-IV are based on Histogram shifting. Also they mention another type of algorithms named integer transform based reversible data hiding. They proposed PDE based scheme based on a PDE predictor which iteratively updates the prediction to yield more accurate one. Tians method in 2003 was a significant breakthrough in reversible hiding based on Difference Expansion with high embedding capacity and visual quality. Later this method was improved by Kieu ang Chang in [28] by proposing a method which horizontally and vertically embeds one secret bit into one cover pixel pair. This method can give a capacity of more than 2 bpp with five layer embedding, assuring higher PSNR. Lossless data hiding by preserving the file size is nowadays being a new research area since these have wide applications today. An improved VLC based lossless data hiding scheme which directly embeds data into the bit stream of JPEG images is proposed in [29] which can output a stego image with exactly the same content as the original one preserving the same file size. With enlargement of file size, this method assures significant improvement in embedding capacity. An adaptive reversible data hiding scheme based on integer transform is proposed in [30] which concentrates on achieving high capacity. Here tuning of integer transform is performed and data is embedded adaptively according to the block type determined by the pre estimated distortion. There are some lossless hiding algorithms with 'Vacating room for embedding after encryption’ (VRAE) and 'Reserving room for embedding before encryption' (RRBE). Reserving room before encryption is found to be more effective and easier. Such a technique is proposed in [31] which empty out room by embedding LSBs of some pixels into other pixels and then encrypt the image, so the positions of these LSBs in the encrypted image can be used to embed data. E. Cover image pre-processing Some of the methods for steganogaphy introduce a preprocessing stage before applying the data ISSN: 2231-5381 embedding algorithm. This is because the cover image selected has a significant impact in achieving the basic steganographic requirements like payload capacity, imperceptibility, transparency etc. Some images may not have enough contents to hide data securely. If we appropriately do some preprocessing on the cover image by making use of the lucrative effects of image processing, we can amplify the data embedding rate for the particular cover image. In some other methods particular regions of the cover image is selected in some manner so that data embedding in those selected regions seems to be more imperceptible. These methods make use of the properties like local complexity, human vision sensitivity etc. Sajedi and Jamzad [32] proposed a boosted steganography scheme with cover image processing with the goal of increasing the undetectability of stego images. Here the main idea of the preprocessing stage is to impose more variation in the pixel intensities of cover images compared to the original ones. The improvement they obtained is due to the properties of contrast enhancement methods and Successive Mean Quantization Transform (SMQT) enhancement. Their steganographic scheme consists of two stages one to preprocess and second for embedding. They reveal that Sharpening and Histogram Equalization provide more embedding capacity than SMQT transform. The SMQT enhancement results in more normal images and the enhancement is not apparent visually. But Sharpening and HE enhancement may be detectable with some simple processing while SMQT that has a local enhancement is not detectable easily. The image whose quality is enhanced by SMQT transform is usually perceptually indistinguishable from the original image. In an adaptive method proposed in [24], DerChyuan Lou classified pixels into three levels based on the variance of the local complexity of the cover image and they determined the level of complexity a pixel belonged to by taking human vision sensitivity into consideration. In their scheme the standard deviation is adopted to analyze the local complexity so as to estimate number of bits that can be concealed in a block of image. A higher standard deviation means the texture of the block is rougher and therefore the block can carry more secret data. In [22] and [33] a candidate region in the stego image is found out for data hiding by scanning the cover image using a spatial frequency and luminance masking. Thereafter an adaptive pixel pair matching is used to hide data in the candidate region. They plotted some characteristics of the human visual system which can be combined with image steganography to avoid visual and statistical detection of the embedded data. They highlighted the facts that the human eye is less sensitive to noise in the high resolution bands having orientation of 45. Also the eye is less sensitive to noise in those areas of the image where brightness is high or low and the highly textured areas. http://www.ijettjournal.org Page 55 International Journal of Engineering Trends and Technology (IJETT) – Volume 29 Number 1 - November 2015 In [34] Rashidy and Bahram proposed an approach to find out the best place for embedding modified secret data in the host image to achieve high level of security. Here genetic algorithm is used to find out the best starting point, scanning order and other options such that the PSNR of the stego image is maximized. In [35], the authors used a preprocessing operation in order to increase the security of the system as well as the resistance against steganlytic attack. This operation checks whether an image could be used or not for embedding. They first divide the cover image into blocks and the pixel in th center of the blocks is checked by calculating the differences between this pixel and all the surrounding pixels. According to the values they categorized the blocks into 5 categories. The ratio of each category to the total number of pixels in the images is calculated and according to them they determine whether that image is suitable or not. V. CONCLUSION This paper presented a discussion about some major and efficient image steganographic methods evolved in the past decade. As we analyze emerging technologies like soft computing can be incorporated with the field of image steganography to achieve better and acceptable results. 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