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Secure Dissemination and Protection of Raster Data using Merkle Hash Tree based Cryptowatermarking

Due to rapid development in the Internet and other communication technologies, it becomes quite easy to copy and distribute multiple illegal copies of high value and sensitive data. Raster data is one of the high voluminous data and it requires huge efforts to sense and generate this data. Therefore, ownership protection as well as its integrity become one of the key problems in spatial information service. There are lot many schemes are available for watermarking and encryption individually, but if both are combined gives manifold advantages. This paper presents a cryptowatermarking scheme by combining watermarking and encryption to protect the copyright of raster data as well as to provide security dissemination level. We have proposed a scheme by employing double transposition, LSB substitution watermarking and Merkle Hash Tree for encryption and watermarking. It has been observed that the proposed scheme is not only robust against encryption attacks, but also has transparency, strongness, large data hiding capacity and correct extraction of watermark.


Introduction
Due to rapid growth in communication technology, it becomes extremely easy to digitise data and generate other multimedia contents. It becomes easy to distribute data to multiple recipients over the Internet. However, this cause misuse of all kinds of data which is in digital format. The sensing, acquisition and pre-processing of raster data are cost and manpower intensive tasks. Also, this data is of high volume and it is sensitive sometimes. It is not advisable to store the data at any untrusted server or disseminate it directly without enforcing any safeguarding mechanism. As the data is especially important in lots of analysis applications and not freely available, it may be duplicated and distributed illegally. Therefore, copyright protection is crucial and necessary in today's era. In most of the applications, cryptographic mechanisms such as encryption and digital signatures are used to achieve confidentiality, integrity, and nonrepudiation services.
All these listed security mechanisms generally help users to safeguard their data at storage and communication. Although, Watermarking is being used since years as a complementary security mechanism, it fails to secure data completely. It is mainly used for copyright protection to prove the ownership of data.
Security of raster data is required at storage and dissemination level. Watermarking and encryption techniques can be useful to do secure them at these levels. There are many different approaches/schemes are available for watermarking of raster data in spatial as well as frequency domain [1][2][3][4][5][6]. However, no one have paid attention towards security watermarked raster data at dissemination level. Attackers can use many simple manipulative attacks to tamper watermarked data. Standard algorithms AES, DES, and RSA [7] are very popular for many different types of data, but they may not be suitable for raster data due to its voluminous nature. Also, these well-known algorithms take large amount of processing time for such voluminous data. Cryptography and watermarking can be combined and can provide overall security for raster data.
In the scheme proposed by Xu et al. [8], watermarking and encryption are combined, and the watermark is embedded in the image at the time of decryption of the image. One of the major problems of this scheme is that the encrypted image does not carry watermark. Jiang and Xu [9] depicted simultaneous encryption and watermarking operations in their scheme which they proposed for remote sensing data. Enhanced Arnold scrambling is used for encryption which utilises only one key for complete image. The issue here is that If a key is known to attacker, the whole encryption system fails. Encryption and watermarking are integrated in the scheme proposed by Jiang et al. [10]. They have utilised orthogonal decomposition of remote sensing data. This scheme does not give proper output and the edges get blurred in the resultant image. Ding et al. [11] have proposed integrity authentication scheme for remote sensing images wherein they have utilised perceptual hash scheme based on deep learning concepts. PCA is used to extract the features and perceptual hash is calculated on it. This will help to detect local tampering in high resolution remote sensing images. In all existing systems, complete security analysis as well as watermark robustness analysis are not done. This paper presents the idea of combining cryptography and watermarking to secure raster data. Simple yet effective transposition cipher, LSB based watermarking and robust Merkle hash tree is combined to provide secure solution for safeguarding raster data.
The paper is organized as follows. Proposed cryptowatermarking scheme is presented in section 2. Section 3 describes error analysis measures. Experimental results are discussed in section 4. Conclusions are drawn in section 5.

Proposed Cryptowatermarking Scheme
Remote sensing images includes Multispectral, hyperspectral, active/passive microwave, LIDAR images and aerial photographs etc. Due to the enhanced computing power and large number of applications capable to use these data, its demand is increased dramatically. Digital watermarking and cryptography are complementary to each other and used separately from many years to achieve copyright protection and security, but when combined as Cryptowatermarking results in vary robust and secure solution to safeguard the precious data. Simple yet strong and secure solution is proposed to provide to achieve protection and secure dissemination of remote sensing data.
In the proposed Cryptowatermarking scheme, two stage transposition cipher, least significant bit (LSB) based watermarking on zigzag pattern matrix, Merkle hash tree-based indexing of the encrypted image and RSA digital signature are effectively combined to make it robust and strong. These cipher when used as individual are prone to attacks. However, proper cascading of these ciphers makes the overall system robust and strong and it is impossible to break the security provided by this scheme. It is a common practice in communication systems to use secure hash algorithms to verify the integrity of content transferred over insecure communication channel. The employment of secure hash algorithms enables user to securely disseminate the content over the Internet. Typically, algorithms such as Secure Hash Algorithm (SHA) and Message Digest (MD) have been used to check the content integrity after receiving the entire data from sender. The Merkle hash tree (MHT) can be used to verify content delivered from sender. MHT is a binary tree that organizes hashes hierarchically. Figure 6.1 illustrates MHT having 8 objects O1 to O8 representing leaf nodes of a tree. A node N contains hash value hN which is computed as: if N is a leaf node then hN = H(O1) otherwise hN = H(hN:lc || hN:rc). Here, N:lc and N:rc are left and right child of N respectively, and "||" represents concatenation of two binary strings. The complete tree is formed using iterative hashing. Finally, root of the MHT is signed by the data owner using public key digital signature scheme. Here, each Oi corresponds to 8x8 non overlapping block of encrypted watermarked raster data. // EKSR is selected in such a way that EKSR is relatively prime to M. Two stage transposition ciphers employed in the schemes provide good diffusion whereas the watermarking process itself provides good confusion and hence making the scheme robust. The encrypted and watermarked raster data is divided in to 8x8 blocks. All the pixels in 8x8 blocks are concatenated and represents one object. All the blocks are represented in object as concatenation of all pixels in corresponding blocks. They can be treated as leaf node of MHT ( figure 1) At receiver side, receiver first verifies the signature SG.EWR. Successful verification indicates tamperproof communication of encrypted watermarked data, else the sender is requested to send the data again. Decryption and watermark extraction are carried out as per the steps illustrated in Algorithm 2 and the required keys received from sender.

Evaluation Measures
Normalised correlation is used to indicated similarity between original watermark (Wk) and extracted watermark (Wk'). It is evaluated using equation 1. A Complete similarity is indicated by a coefficient with value one. Normalised correlation ranges between 0 to 1.
(1) Equation 2 shows the Entropy (E) of an image I where Pr(αi) is the probability of occurrence of the grey level αi and G is the total number of grey levels. Entropy is a measure indicating randomness of the pixels in the encrypted image. The value of entropy is 8 if every pixel has equal probability in encrypted image. It shows complete randomness in the output image. Low value makes the encrypted image susceptible to various statistical attacks. (2)

Results and Discussion
The proposed Cryptowatermarking system is evaluated using various remote sensing images of varying sizes. Figure 2 shows 6 such images 1 to 6 of sizes 1280x1280, 1600x1080, 800x800, 960x840, 760x640 and 640x640 respectively. The watermark of size 100x50 pixels is used for watermarking. In the experiment, different keyset for double transposition cipher is used as shown in Table 1. Encrypted watermarked raster data and decrypted raster data are shown in figure 3 and 4. The original watermark and extracted watermark are shown in figure 5. The corresponding normalized coefficient between original and extracted watermark is obtained as one. SHA 256 is used for hashing in Merkle Hash Tree and RSA digital signature is employed for signing the root of Merkle Hash Tree. Public modulus(n) is considered as "d94d889e88853dd89769a18015a0a2e6bf82bf356fe14f2 51fb4f5e2df0d9f9a94a68a30c428b39e3362fb3779a497e ceaea37100f264d7fb9fb1a97fbf621133de55fdcb9b1ad0d 7a31b379216d79252f5c527b9bc63d83d4ecf4d1d45cbf8 43e8474babc655e9bb6799cba77a47eafa838296474afc2 4beb9c825b73ebf549" and public exponent (e) is considered as 10001. Private exponent (d) was calculated using values of n and e.
Encryption and decryption results are highly dependent on the order of ciphers and proper usage of utilized key space. Cryptosystem is said to be secure if the image cannot be decrypted correctly even if there is a change in sequence of ciphers and keys. Keys used here are more sensitive they should be relatively prime to number of pixels in rows and columns. Decryption many are not successful if there is a small change in the keys used in encryption and decryption.
In cryptography, it is very crucial to design the algorithms which provides robustness against all the attacks like brute force and statistical attacks. One of the best ways to check the security of the image security algorithm is histogram analysis. When histogram of encrypted image is uniform, it becomes difficult to get information from image and thus launching statistical attacks. As Merkle hash tree, hashing and digital signature are used, it is difficult to launch an attack and compromise key or raster data.  Double transposition cipher and watermarking algorithm provides exceptionally good diffusion and confusion in the encrypted watermarked image. Histogram analysis can be used to check confusion and diffusion of the proposed scheme. Figure 6 shows the RGB histogram of both the original and encrypted watermarked raster data. From the figure, it is observed that the histogram of the encrypted watermarked data is nearly uniformly distributed, and very different from the histogram of the original data and hence there will be no clue from the encrypted watermarked image to launch statistical attack on resultant data in the proposed scheme. Entropy for encrypted watermarked data are shown in Table 2 and it is observed that it is nearly equal to 8 which shows that the scheme is robust against statistical attacks.
(a) (b)  Perceptual quality of the image can be checked using peak to signal noise ratio (PSNR). If its value is small, image quality is poor and vice versa. From table 3, it is observed that the proposed system results into small PSNR values thus resulting into true randomness in the encrypted image.

Conclusion
In this paper, we have proposed cryptowatermarking, a combination of watermarking and encryption to provide copyright protection for raster data and for secure dissemination copyrighted raster data. During transmission, Merkle Hash tree-based encryption and RSA digital signature helps in preventing attacks and data loss. Watermark can be efficiently extracted at receiver side which can be used to prove ownership of the data. The proposed scheme satisfies the security of encryption, the invisibility and robustness against various brute force and statistical attacks.