CHAPTER 6. LSB based data hiding with double Encryption. 6.1 Introduction

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1 CHAPTER 6 LSB based data hiding with double Encryption 6.1 Introduction In image steganography, the amount of secret data that can be embedded depends on the method and the cover-image as capacity limitation is a matter in steganographic systems. The frequency domain based steganography has drawback that the amount of secret data that can be embedded is limited. On the other hand, the spatial domain (LSB) based steganography has the benefit over the frequency method (DCT) in that the hiding capacity is much more and the working mechanism of LSB based method is simple. So in order to form a steganography system that provides high capacity, the LSB approach can be replaced with frequency domain based method. The basic concept of Least Significant Bit Substitution includes the embedding of the secret data at the bits having minimum weightage so that it will not affect the value of original pixel (Sharda and Budhiraja, 2013). The spatial domain based method enables simple form of data embedding by directly manipulating the LSBs of the cover-image and is also prone to detection as it embeds data sequentially in all pixels. The statistical attack may detect the presence of secret data in LSB embedding by simply verifying the bits of the cover-image pixel (Westfeld and Pfitzmann, 2000; Fridrich et al., 2001; Patil et al., 2012). In case the stego-image is known, the attacker can simply extract the LSBs of the cover-image to estimate the secret message (Chandramouli et al., 2004). Due to the possible attacks LSB embedding is relatively insecure, at least in its primitive form. However, due to the advantageous features of such method, it is useful for applications where security is desired. It also forms a good foundation for more secure steganographic techniques. Different variations of LSB based embedding can be performed and one of such variations is encrypting the secret message before embedding into the cover-image (Pavani et al., 2013). 102

2 As in chapter 5, transform domain based steganographic method is combined with cryptographic method to enhance the security of the secret message. Such technique is not suitable for high rate of data embedding. Thus in order to achieve higher embedding capacity and also security, LSB method can be combined with the encryption technique. Spatial domain method tries to substitute redundant parts of a digital image with secret message and is based on the fact that data manipulation results in the smallest change in the value of the byte. This technique does not need any transformation or decomposition on the original images and is fit for real-time processing. Although spatial domain based steganographic process ensures simple form of embedding than the frequency domain based technique, however it is not simple as certain variations (encrypting the secret message before embedding) can make such embedding process complex. Employing the LSB technique for data hiding achieves both invisibility and reasonably high storage payload (Johnson and Katzenbeisser, 2000). According to Chandramouli and Memon, (2001) LSB based techniques pose challenge to a steganalyst as it is difficult to differentiate cover-images from stego-images because of the small changes that it makes. They suggested that in order to maintain the random looking appearance of the LSB, the secret message can be encrypted before it is embedded. Cryptography and LSB steganography techniques can be combined for ensuring security, robustness and capacity. The role of cryptography is to assure privacy whereas steganography assures secrecy. Such combining technique can result in a better protection system by providing different layers of security. Even though both methods provide security, to add multiple layers of security it is always a good practice to use cryptography and steganography together (Raphael and Sundaram, 2011). Chan and Cheng, (2004) introduced a data hiding scheme by simple LSB substitution with an optimal pixel adjustment process (OPAP). They described that by applying an optimal pixel adjustment process to the stego-image obtained by the simple LSB substitution method, the image quality of the stego-image can be greatly improved with low extra computational complexity. Song et al., (2011) stated that a secured communication protocol can be formed by that combines LSB steganography and cryptography techniques. Karim et al., (2011), introduced an approach of least 103

3 significant bit (LSB) based image steganography that enhances the existing LSB substitution techniques, to improve the security level of hidden information. The security concept hides secret information within the LSB of the pixel of the image, where a secret key encrypts the hidden information to protect it from unauthorized users. According to their simulation results the value of PSNR gives better result and it is difficult to extract the hidden information. According to Tyagi et al., (2012), LSB steganography along with cryptography may be some of the future solution for protection method against unauthorized access and data secrecy. They used symmetric encoding process for encrypting the secret message and then embedded the bits of the encrypted message into the LSBs of the cover-image. They further stated that detection process is one of the active fields where the problems are big. So with some variations in LSB method, the secrecy of the steganographic process can be further enhanced. Tomar et al., (2012) stated that combining LSB based steganography and cryptography forms an effective data hiding technique and provide better security of hidden data. They encrypted the secret data by RSA (Rivest-Shamir-Adleman) algorithm and then embedded the secret bits into each cover pixel by modifying the least significant bits (LSBs). The average PSNR value of their scheme is db. Phad et al., (2012), developed a security model for secret data that uses Advanced encryption standard (AES) based cryptography and LSB Steganography approach, where the encrypted secret message is embedded into image by using pixel value differencing (PVD) and K-bit least-significant-bit (LSB) substitution. According to them, such combining process provides two-tier security to secret data and enables more significant promotion in the terms of adaptability, capacity, and imperceptivity. They further stated that the experimental result shows the proposed approach obtains both larger capacity and higher image quality. In their case for embedding bits the average PSNR is 42 db for color image. The encryption process uses a key for changing the order or replacing the letters of the secret message and the use of a key in any processing itself provides a measure of security. As the size of the key is not fixed, thus it will be extremely difficult for the unauthorized person to predict the size of key and data. The LSB approach when 104

4 combined with the encryption process provides both capacity and security of the data to be protected. The objective of this chapter is to study the method that uses cryptography (Transposition and Substitution cipher with keys) together with LSB steganography. The joining of these three techniques can create a robust steganography-based communication system capable of withstanding attacks and detection process. The secret data and the key used for encryptions can pass multiple levels of security checks by assuring the integrity, authenticity and security which can make a reliable process for sensitive data. While both encryption methods assure that the message becomes secure with the usage of keys, the steganographic algorithm introduces an additional level of security. 6.2 Proposed Method To enhance the embedding capacity of image steganography, a framework is proposed by combining cryptography and LSB steganography that can attain minimal perceptual degradation and protect from unauthorized data access. Here the secret message undergoes double encryption before embedding it into the cover-image. The secret message is first encrypted by transposition cipher method using a key. Then the ciphertext is encrypted further using substitution cipher technique with another key. The double encrypted message is then embedded into the cover-image by using LSB substitution. Minimizing embedding impact and maximizing embedding capacity are the key factors of any steganography algorithm and LSB method comply with this requirement. The objective of such mechanism is to form a steganographic model that meets both security as well as capacity. For this reason, LSB steganography is enhanced using an extra level of security, which encrypts the secret message two times using two different methods before embedding it into the cover-image Transpose-Substitute Encryption The encryption process used in the proposed work is based on classical techniques i.e. substitution and transposition which are regarded as building blocks of cryptography 105

5 (Kahate, 2008). The proposed encryption technique is based on combining transposition and substitution cipher method in a way to create ciphertexts which are double encrypted. Transposition cipher encrypt secret message (plaintext) by changing the order of the letters of the message. In such cipher the plaintext remains the same, but the order of characters is shuffled around or the plaintext symbols are rearranged (i.e., transposed or permuted) to produce ciphertext. Transposition does not alter any of the bits in the plaintext, but instant moves the position around within it (Smith, 2001). In this encryption, the key defines the possible ordered arrangements or groupings of the letters of the plaintext. This ciphertext is the disguised form of the information and could be transmitted across a network or stored within a file system with the objective of providing confidentiality (Stallings, 2003). A transposition cipher, also sometimes called a permutation cipher, for which applying encoding to plaintext produces ciphertext with the same symbols as the plaintext with a fixed period, but rearranged in different positions (Mao, 2004). A permutation cipher is a transposition cipher in which the key is a permutation, where the plaintext is grouped randomly. The larger the size of the permutation the more secure the cipher-text will be. On the other hand, instead of changing the order of characters as in transposition cipher, the substitution cipher replaces the character of the message with other character so as to make the message unintelligible (Friedman, 1967). In such encryption method, the letters or characters of the plain text are substituted by another letter in a way to make it meaningless. A substitution cipher is an encryption scheme that uses only substitution transformations where the method is to replace the character and the key is the number of characters to replace it. The two other techniques related with transposition and substitution encryption method for obscuring the plaintext message are diffusion and confusion (Smith, 2001). Diffusion changes the plaintext by spreading it over the ciphertext and the simplest way to cause diffusion is through transposition. Confusion obscures the relationship between the plaintext and the ciphertext and is done through substitution. 106

6 Figure 6.1: Block diagram of Transpose-Substitute Encryption process Although the two techniques differ in their way of encrypting the message but they can work together. In the proposed method the secret message is initially encrypted using transposition cipher and then another round of encryption is performed using substitution cipher technique as shown in figure 6.1. In the first phase of the encryption, the transposition cipher is used which rearranges of the letters of the secret message based on the key (i.e. it permutes the secret message). Such arrangement is done with K columns and K rows and thus forming a block of size K K, where K is the key that forms the permutation of the plaintext within the block. The transposition method operates by arranging the letters of the plaintext (secret message) into the blocks row-wise and then the letters are read-off in column-wise to form the ciphertext. Thus the plaintext is broken into segments of size of the permutation and the letters within that segment are permuted according to this key. Both the length of the rows and the subsequent arrangement of the columns are defined by the key. Once the message exactly fits the first block, then the next block is used to fill the remaining plaintext and so on. The key determines the number of rows and columns, and the number of letters in the message (based on the key) determines the number of blocks. The arrangement of plaintext in the blocks is performed using the spaces between the texts, so in encryption process the spaces are placed in the sequence it comes. Such enables in decoding process to retrieve the 107

7 exact plaintext. Here is an example of the first encryption step i.e., the transposition encryption, in which the letters are written in the block row-wise, where the key is 7 that forms blocks of 7X7 and the number of blocks is formed by the number of characters as in table 6.1. Table 6.1: Plain Text entered into the two blocks in row-wise B Y C O M B I N I N G S T E G A N O G R A P H Y A N D C R Y P T O G R A P H Y W E C A N A C H I E V E B E T T E R S E C U R I T Y O F D A T A A N D I N F O R M A T I O N If the following text, written below is used as plaintext: BY COMBINING STEGANOGRAPHY AND CRYPTOGRAPHY WE CAN ACHIEVE BETTER SECURITY OF DATA AND INFORMATION Then it is placed row-wise into the block. To form the ciphertext, it is read off in columns to get the following: BITRNTYYNEAD IGP GWCNAHCREOGNYRA M O YPCBSGAPHANVER NR ERIDDMA TA ACBSYTITHEE ANIITCO FOETUFAON Thus it forms a different text (ciphertext) which is not understandable. In the second encryption phase, the ciphertext generated by transposition technique is further encrypted by using substitution cipher technique. In this process each letter is substituted by an arbitrary letter to form second state of ciphertext. In such replacement of letter another key is used that defines how much to shift. Thus the ciphertext formed as a result of substitution encryption produces different ciphertext depending on the specific key being used. If the substitution cipher s key is 17, then the ciphertext formed by encrypting the transposed cipher is given below: 108

8 SZKIEKPPEVRUF ZXG XNTERYTIVFXEPIR D F PGTSJXRGYREMVI EI VIZUUDR KR RTSJPKZKYVV REZZKTF WFVKLWRFE Such ciphertext is the substituted form of the transposed cipher which is the final ciphertext formed by combining transposition and substitution encryption technique. Decrypting the cipher-text: The decryption of the ciphertext for combined transposition and substitution technique is performed in the reverse order as shown in figure 6.1. Firstly the ciphertext is decrypted using substitution decryption process based on key that outputs the encrypted form of the transposition cipher. Then the ciphertext is arranged into the blocks in the order of columns and then it is read in row-wise. The order of the permutation of column and row is defined by same key as used in encryption process. For each of the blocks, all of the possible permutations of the columns are considered. The letters are arranged column-wise and are read off row-by-row to get the original plaintext. Thus the decryption process also requires two keys one each for transposition and substitution cipher to get back the original plaintext (secret message) Data embedding procedure In this data hiding process the bits of the double encrypted message is embedded in to the LSBs of the cover-image. As image comprises of pixel contribution from red, green and blue components and each pixel has numbers from the color components (for 24-bit bitmap image each of red, green and blue plane has 8 bit). At 8 bit of the color number, if least significant bits are changed, the visual system cannot detect changes in pixel and thus it is possible to replace message bits with image pixels bit. For example if the following pixel value is considered and the secret information is stored in the least significant bit, at the worst situation the pixel changes to and examinations shows that such alteration is not perceptible. If the LSB in a byte of an image is manipulated, either one is added or subtracted from the value it represents. So, the proposed method uses LSB based process to embed the encrypted data into the cover-image. 109

9 In order to hide the encrypted message, data is first converted into byte format and stored in a byte array. The secret message is embedded into the LSB position of each pixel. If the original pixel consist of bits as follows: (r7 r6 r5 r4 r3 r2 r1 r0, g7 g6 g5 g4 g3 g2 g1 g0, b7 b6 b5 b4 b3 b2 b1 b0) In addition, the encrypted character (bytes) consists of bits: (c7 c6 c5 c4 c3 c2 c1 c0). Then the first secret bit is replaced with the LSB of the selected pixel s blue plane, next secret bit with the green plane s LSB and then with the LSB of red plane (r7 r6 r5 r4 r3 r2 r1 c2, g7 g6 g5 g4 g3 g2 g1 c1, b7 b6 b5 b4 b3 b2 b1 c0). The pixels of the cover-image are selected sequentially and the embedding process operates in the pixels by first inserting the secret bit in the LSB of the blue plane, then green and finally the red plane. Such method of manipulating the secrets bits into LSBs also increases the security because the embedding is done in reverse order. Once the insertion in the first pixel is over, then data embedding takes place in the rest of the pixels in the similar way. In this way entire secret message is inserted into the pixels of the cover-image. If a pixel with value (225,107,100) is used to embed message character a, then the new pixel is formed as shown below: Original pixel = ( , , ) a = (ASCII value 97) New pixel = ( , , ) New pixel = (224, 106,101) Here it is noticed that at worst case, the pixel value of (225,107,100) is changed to a new pixel value of (224, 106,101) and such manipulation does not affect the overall quality and statistics of the cover-image. The embedding process operates over the image, and embeds the message character into cover-image pixel by pixel at a time. Once all the message characters are embedded into the cover-image, a target character is inserted into the pixels of the cover-image immediately after the last character of the secret message. The target character can be a special symbol and is also known as 110

10 terminator character which is the last character that is embedded and after its embedding, insertion process stops i.e., the target character signifies the termination of the embedding process. The binary representation of the target character is embedded into the pixels in the same way as the message bits are embedded. This helps the decoding process to stop extracting of data from stego-image after getting the target character, which signifies the end of the message. Algorithm to embed secret message: Step 1: Read the cover image and text message which is to be hidden in the cover image. Step 2: Encrypt the message using transposition cipher. Step 3: Perform the second round of encryption using substitution cipher. Step 4: Convert encrypted message into binary. Step 5: Calculate LSBs of each pixels of cover image. Step 6: Replace LSBs of cover image with each bit of secret message sequentially. Step 7: Repeat step 6 until the entire secret bits are embedded. Then place the binary representation of the target character at the end of the embedding. Step 8: Write stego image Message extraction process The embedding process stated above gives a picture to what extent it is possible to extract the secret message bits from the LSBs of the pixels. Using the same sequence as in the embedding process, the set of pixels storing the secret message bits are extracted sequentially from the stego-image. The LSBs of the selected pixels are extracted and lined up to reconstruct the secret message bits. As in the embedding process a target (terminator) character is inserted into the stego-image which is the last character to signify the end of embedding of data. When the binary representation of the target character is found the extraction process stops. The purpose of putting this character is that, in the process of retrieving the message the extraction algorithm may take extra bits and thus the target character will terminate the extraction process. The bits of the LSB are retrieved and placed in the array. Then content of the array 111

11 converts into decimal value that is actually ASCII value of encrypted message. Each 8 bit from the array is converted into character. Thus the message that is retrieved from the stego-image is actually encrypted form of the original message. The retrieved message is firstly decrypted using substitution decryption method based on the key to get back the encrypted message and then further decrypted using the transposition method. In decrypting the message using transposition cipher method, the ciphertext is written column-wise in blocks of same size as in encrypting method and is read out row-wise in a order depending on the key. If the decryption keys do not match with the keys used in the encryption methods, the original plaintext (secret message) will not retrieve. Algorithm to extract secret message: Step 1: Read the stego image. Step 2: Calculate LSBs of each pixel of stego image. Step 3: Retrieve bits & convert each 8 bit into character. Step 4: Repeat Step 2 and 3 until the binary representation of the target character is found. Step 5: Perform substitution decryption technique to the retrieved characters. Step 6: Again decrypt the message using transposition decryption method. Step 7: The retrieved characters are placed sequentially to get back the original secret message. 6.3 Results and Discussion In order to examine the performance of the proposed steganographic technique, an evaluation scheme for steganographic system is conducted. As steganographic systems have two fundamental characteristics - imperceptibility and the hiding capacity which are investigated in order to evaluate the system that determines the superiority of a steganography technique. The secret data are embedded in the LSBs of the cover-image as mentioned in the proposed algorithm. The issues relating to the experimental approach is considered, which is used to measure the impact of such 112

12 embedding. The experiments have been conducted using the MATLAB R2009b program on Windows 7 platform. Various performance parameters like PSNR, MSE, payload size, time computation etc. have been used to evaluate the performance using various images which include both well known standard and natural images. Initially, the experimental analysis presents a comparison of the proposed method with OPAP (Chan and Cheng, 2004) to demonstrate its performance. Fourteen well-known images each with size , namely Lena, Baboon, Peppers, Jet, Tank, Airplane, Truck, Elaine, Couple, Boat, Man, and Tiffany, Barbara, House, are used as cover-images in the experiments. The secret messages consist of the texts of this article that are encrypted before embedding into the cover-images. The experiments are based on embedding and bits of secret message into the cover images. Some of the cover-images and the resulting stego-images formed are shown in figure 6.2 and figure 6.3. (a)cover-image: Airplane (c) Stego-image: Steg-airplane (b) Cover-image: Boat (d) Stego-image: Steg-boat Figure 6.2: (a)-(b) Cover-images and (c)-(d) their resulting Stego-images 113

13 (a) Cover-image: Barbara (c) Stego-image: Steg-barbara (b) Cover-image: House (d) Stego-image: Steg-house Figure 6.3: (a)-(b) Cover-images and (c)-(d) their resulting Stego-images The difference between the resulting cover-images and stego-images are indistinguishable as seen in figure 6.2 and figure 6.3. Thus it can pass the perceptibility test as there is no visual distortion between stego-image and the original cover image. The more the outlet of algorithm is similar to the original image, the more perceptibility of algorithm is achieved therefore the recognition of hidden data will be more difficult. The generated stego-images show that they contain no artifacts that can be identified by human eyes. 114

14 Table 6.2: Comparison of PSNR values of OPAP (Chan and Cheng, 2004) and the proposed method obtained by embedding encrypted bits Cover-image OPAP (Chan and Cheng, 2004) Proposed Method PSNR MSE PSNR MSE Airplane Boat Baboon Barbara Couple Elaine House Jet Lena Man Peppers Tank Tiffany Truck Zelda The PSNR and MSE values of the proposed scheme and OPAP (Chan and Cheng, 2004) based on embedding bits secret data into the fourteen cover images is presented in table 6.2. Referring to the table, it is seen that the proposed method presents better PSNR values than OPAP. 115

15 Figure 6.4: Shows the results of PSNR values of the proposed method in comparison with OPAP (Chan and Cheng, 2004) for embedding encrypted bits A larger PSNR value indicates the fact that the discrepancy between the cover image and the stego-image is not visible to the human eye and also it can override some of the statistical measures of detecting the secret data. As seen in figure 6.4, the proposed method acquires an average PSNR value 55 db and OPAP has 50 db for the fourteen images each with size Thus in evaluating Objective Quality performance the proposed method performs better than OPAP. It is observed that the PSNR values of the proposed method increased by 5 db. This is because the distortion of the cover-image can be greatly improved by the proposed scheme. In order to evaluate further and to analyze image quality and perceptibility of the proposed method in comparison to OPAP with regard to increasing capacity, the next experiment is based on embedding bits of data in the same standard cover images. The results of the embedding are shown in table 6.3 where the secret data is increased by two times of the earlier experiment. 116

16 Table 6.3: Comparison of PSNR values of OPAP (Chan and Cheng, 2004) and the proposed method obtained by embedding encrypted bits Cover-image OPAP (Chan and Proposed Cheng, 2004) Method PSNR MSE PSNR MSE Airplane Boat Baboon Barbara Couple Elaine House Jet Lena Man Peppers Tank Tiffany Truck Zelda From the table 6.3, it is seen that by increasing the embedding capacity the proposed method still exhibits better PSNR values in comparison to OPAP. For a given attack, PSNR is used as quality metrics can be used in determining the desired visual quality for a given robustness. 117

17 Figure 6.5: Shows the results of PSNR values of the proposed method in comparison with OPAP (Chan and Cheng, 2004) for embedding encrypted bits For embedding encrypted bits, the proposed method shows an average PSNR value of 52.2 db which is 6 db more compared to OPAP as shown in figure 6.5. Thus, in deciding whether an image is stego or not, the quality measure plays a vital role. The results indicate that the quality of the stego-image is improved using such approach. From table 6.2, it is observed that for all images when encrypted bits is embedded, the proposed scheme exhibits PSNR value ranging from 55.2 to 55.4 db while OPAP shows value ranging from 50.0 to In table 6.3, it is observed that when the secret data is doubled i.e. when encrypted bits is embedded, the proposed method shows PSNR value ranging from 52.1 to 52.3 db and it is 45.2 to 45.7 in case of OPAP. It is also observed that incase of OPAP, PSNR values decreases from 50 db to 45 db and the proposed method decreases from 55 db to 52 db when the hiding capacity is increased from bits to bits. Thus the decrease in PSNR value is high in case of OPAP compared to the proposed method. It can also be stated that the proposed method can afford higher capacity without major changes and degradation of stego-images compared to OPAP. By comparing table 6.2 and table 6.3, it is observed that there is a tradeoff between capacity and PSNR, when the capacity is increased PSNR value decreases. However, steganographic capacity and imperceptibility are at odds with each other. Thus, it is not possible to simultaneously maximize the capacity and imperceptibility of a steganographic system for a given cover-image. Consequently, steganographic systems must achieve a balance among these requirements. 118

18 Furthermore, in order to compute the performance of the proposed method, some color images are taken as cover-medium to embed three set of random sized encrypted messages. In this evaluation several criteria such as the size of stego-image and cover-image, size of the embedded and extracted message, the quality of the images (PSNR and MSE) and the computational time (embedding and extracting time) are considered. In this case, the size of the secret message and images has been considered to investigate the impact of embedding data on the final stego-image. The results of the embedding process are listed in table 6.4. (a) Cover-image: Desert (b) Stego-image: Stegdesert1 (c) Stego image: Stegdesert2 (d) Stego-image: Stegdesert3 Figure 6.6: (a) Cover-image: Desert and (c)-(d) its resulting Stego-images 119

19 (a) Cover-image: Park (b) Stego-image: Stegpark1 (c) Stego image: Stegpark2 (d) Stego-image: Stegpark3 Figure 6.7: (a) Cover-image: Park and (c)-(d) its resulting Stego-images From the figure 6.6 and 6.7 it is observed that the cover-images and its resulting stego-images are indistinguishable to the naked eyes. Based on this distinction, there appears no visual distortion on the stego-images. This is because of the fact that the secret bits are embedded by replacing the LSB s of an image. In an image changing the LSBs does not result in image distortion and thus the resulting stego-image is identical to the cover-image. If a steganographic algorithm leave a trace during embedding than it can be detected through statistical analysis. For an algorithm to be statistically undetectable, it should not form any visible distortion to the resulting image. 120

20 Table 6.4: Results of embedding variable sized encrypted messages into some color images takes as cover-medium Cover Size of the Size of the Time Stego Size of Time Size of the MSE PSNR image Cover Secret required to image Stego required to Extracted (in %) (in db) Image Message embed the Image extract the Message (in KB) (in KB) message (in KB) message (in KB) (in Seconds) (in Seconds) Bud Stegbud Stegbud Stegbud Desert Stegdesert Stegdesert Stegdesert Scene Stegscene Stegscene Stegscene Park Stegpark Stegpark Stegpark

21 Different sets of message ranging from 1.05 K.B to 260 K.B that are encrypted firstly by transposition cipher and then using substitution cipher method are used to embed into the LSB of the cover-images namely bud, desert, scene and park which are of different dimensions. Table 6.4, presents the results of embedding, evaluated using different parameters. It is observed that the variation in PSNR values obtained with increasing values of embedded capacity. The PSNR value lowers slightly with the increase in message capacity. The PSNR values obtained are better in regard to the fact that the color images should show PSNR values larger than 40dB. In the proposed method the PSNR values ranges from 51 db to 59 db. The MSE is inversely proportional to PSNR, an increase in PSNR leads to decrease in MSE value and vice versa. A good steganographic method should have lower MSE value. Figure 6.8: Shows the PSNR values of four cover images with secret message of different sizes As expected, Table 6.4 shows that the increase of payload affects the value of PSNR. In the experimental work it is found that by increasing the payload, PSNR value drops down. Thus it is necessary to make a compromise between payload and PSNR. Table 6.4 shows the average results for each group of colored images and figure 6.8 shows the 122

22 results in graph to help study results more easily. If the set of images is changed, results will change accordingly. As it is clear when the value of correlation factor is increased, value of MSE increases and ultimately decreases the value of PSNR. From the table 6.4, it is also seen that the size of the stego-image is same as the size of the cover-image as the proposed embedding procedure follows lossless compression which maintains the original image data exactly. Lossless compression attains low reduction of bits compared to lossy and thus the image size remains the same. LSB method tries to substitute redundant parts of a digital image with secret message and does not need any transformation or decomposition on the original images. The basic concept of such scheme includes the embedding of the secret data at the bits which having minimum weighting so that it will not affect the value of original pixel. The proposed method also enables high embedding of secret data as seen in the table 6.4, where 260 K.B of encrypted data is hidden in the cover-image park which also shows a good PSNR value of 51 db. Spatial domain steganographic techniques offer lossless compression to embed secret data into the LSB of an image and thus they are preferred for high capacity data hiding. Such technique does not make any serious perceptual change in image as they involve in removing redundant parts of information of the coverimage for data hiding. The amount of secret data that is embedded is retrieved fully without any error and without any data loss as in table 6.4. It is also from the fact that the proposed method only changes the LSB of the cover-image which embeds secret messages in a subset of the LSB plane of the image by altering least significant bit in a certain layer of the image file. This is also from the fact that lossless compression attains very low compression and thus the original secret message remains intact and is extracted successfully. The processing time for embedding and extraction technique using different cover images is presented in Table 6.4. It shows that for embedding 1.05 K.B of secret data the time to embed is seconds while time required in extracting the message is seconds. On the other hand for embedding 260 K.B of secret data the time to embed is seconds and the extracting time is seconds. The execution time of 123

23 extracting process is longer compared to embedding and is in direct ratio with the size of embedded files. The reason for such increase in time processing is that more time required in extracting secret message of larger size and finding the binary representation of the target character. As in the embedding process a target character is inserted as the last character to facilitate in the extraction process. Figure 6.9: Shows the computation time for embedding the secret message into four cover-images The embedding time increases gradually with the increase in the size of the secret message as in figure 6.9. As for the execution time, it is based on the size of the message. With increasing payload the computation time increases. Basically, the processing time depends on the specifications of the computer that used to run the program. Even though, the processing times for embedding and extraction are acceptable. 6.4 Chapter Summary In this chapter LSB based steganography is combined with double-encryption technique to enhance the embedding capacity of image steganography and provide an imperceptible stego-image. As Steganography pay attention to the degree of invisibility and 124

24 cryptography ensures security of the message. Thus, the combining model will result a steganographic image by performing cryptographic functionality and, preserving its steganographic nature. Hiding data using LSB modification alone is not highly secure. The combination of these two methods will enhance the security of the data embedded. The secret message undergoes double encryption firstly using transposition cipher method and then with substitution method before embedding into the cover-image file. Then encrypted message is embedded in the cover-image by using least-significant-bit (LSB) technique that enables high capacity of data embedding. The embedding process is also different which operates in the pixels by first inserting the secret bit in the LSB of the blue plane, then green and finally the red plane. Once all the message characters are embedded into the cover-image, the target character is inserted in the pixel of the coverimage immediately next to the one containing the last input character of the message. The main security lies in the encryption method where the secret message which is embedded undergoes double encryption process and both the encryption process is controlled by two different keys. In examining the performance using various parameters, it is seen that the proposed steganographic technique shows good results. 125