A WATERMARKING TECHNIQUE USING SYNONYM SUBSTITUTION FOR INTEGRITY PROTECTION OF XML DOCUMENTS

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1 ICIC Express Letters ICIC International c 2010 ISSN X Volume 4, Number 1, February 2010 pp A WATERMARKING TECHNIQUE USING SYNONYM SUBSTITUTION FOR INTEGRITY PROTECTION OF XML DOCUMENTS Chin-Chen Chang 1, Yi-Hui Chen 2 and The Duc Kieu 3 1 Department of Information Engineering and Computer Science Feng Chia University Taichung , Taiwan ccc@cs.ccu.edu.tw 2 Research Center for Information Technology Innovation Academia Sinica Taipei 115, Taiwan chenyh@cs.ccu.edu.tw 3 School of Computer Science and Engineering International University Ho Chi Minh city, Vietnam ktduc@hcmiu.edu.vn Received May 2009; accepted August 2009 Abstract. Extensible Markup Language (XML) as the standard representation language is widely used for information exchange. Up to now, how to protect the integrity of XML documents becomes an important issue. XML signature is one of the methods for integrity protection. However, it needs significantly extra spaces for recording the protection information. In this paper, we propose a novel watermarking scheme which uses synonym substitution for embedding the watermarks. Without needing large extra spaces, the XML document can verify the integrity by itself. Keywords: XML, XML signature, Watermark, Integrity protection, Synonym substitution 1. Introduction. XML signature [2, 3, 9] and XML watermarking [11] are techniques to protect the integrity of XML documents. As for XML signature, it requires extra storage for recording the signature information. Another issue concerned for protecting the copyright and integrity is a watermarking technique, which is a data embedding scheme [1, 4, 6, 7, 10, 10, 11, 12] to treat the protection information as watermarks and embeds the watermarks into digital media without attaching any extra data. Later, the watermarks can be extracted to verify the copyright and integrity of the media. Up to now, a few schemes [6, 10, 12] were proposed for text documents because few spaces of them can be used to embed the protected information. Katzenbeisser et al. [7] proposed a watermarking scheme for HTML documents with STC (Switching-tree coding) technique, but extra spaces and tags are needed for embedding watermarks. Furthermore, the file size is significantly expanded because it must record the STC codes after data embedding. In 2003, Sui and Luo [9] modified the case of letters in HTML tags for embedding the secrets as steganography scheme. Zhao and Lu [12] adopted the same concept to design a watermarking scheme applied to tamperproof of HTML documents. In 2006, Yao et al. [11] extended Zhao and Lu s scheme to design a watermarking scheme for protecting the integrity of XML documents. Later on, Huang et al. [6] proposed a steganalytic method, called Tag-Mismatch analysis, to successfully detect the hidden information embedded by using Sui and Luo s scheme. 89

2 90 C.-C. CHANG, Y.-H. CHEN AND T.-D. KIEU In this paper, we propose a watermark technique for protecting the integrity of XML documents by using synonym substitution. It has advantage of no extra storages needed in the proposed scheme so that the size of transmitted data can be significantly reduced than that is done in XML signature. Additionally, the proposed scheme is a self-authentication scheme because the watermark is generated by the XML document itself. 2. Proposed Scheme. Before embedding, a bit sequence is generated by using a one-way hash function with secret key K to hash all the contents without including the structure of the XML document. Next, the generated bit sequence B is re-expressed as a decimal integer with Equation (1), where n is the length of the bit sequence and b j is the jth bit of the bit sequence. B = n b j 2 n j (1) j=1 In the embedding procedure, we look for the synonym terms for each of elements and attributes in the DTD structure and then use the notation to separate any two synonym terms for re-defining the DTD structure as shown in Figure 1. For example, the name element is as someone s signature to let users input the student s full name so the caption of such element can be replaced with full name or signature. Owing to re-define the DTD structure, the XML document is a valid document for senders and receivers while the elements and attributes are replaced with other synonym terms. Figure 1. An example of an XML document Before data embedding, the XML document is scanned in advance for checking the predefined DTD structure, if the element/attribute (E/A) has synonym terms, it is judged as embeddable; otherwise, it is un-embeddable. It is noted that each of the embeddable elements/attributes is treated as a single group. For example, the first element Students does not contain any synonym terms so that it is un-embeddable. When the encoder scans the first attribute id and checks the DTD structure, the attribute id is judged as embeddable because it has another synonym term student id in Table 1. As for the second element student in Figure 1, the element has one child element represented as name, full name or signature in Table 1, therefore, student s child element is judged as embeddable. By this way, after the encoder completely scans the whole XML document, eight elements are picked up. In the end, the encoder generates an embedding sequence depicted as i with a secret key K 1 for later embedding the watermark in such order. All the results are arranged as shown in Table 1, where 1 g(i) w and w is total number of elements. In this example, w is equal to 8. Additionally, g is a function for mapping the ith sequence to the element G g(i). Furthermore, element G g(i) has G g(i) synonym terms,

3 ICIC EXPRESS LETTERS, VOL.4, NO.1, where signifies the number of synonym terms of the element G g(i). Subsequently, we can evaluate the total hiding capacity H with Equation (2). <!DOCTYPE Students [ <!ELEMENT Students (student)+> <!ELEMENT student (name full_name signature)> <!ELEMENT name (first_name given_name, last_name surname family_name)> <!ELEMENT full_name (first_name given_name, last_name surname family_name)> <!ELEMENT signature (first_name given_name, last_name surname family_name)> <!ELEMENT first_name (#PCDATA)> <!ELEMENT last_name (#PCDATA)> <!ELEMENT surname (#PCDATA)> <!ATTLIST student (id student_id) ID#REQUIRED> <!ATTLIST student sex (M F) #REQUIRED> ]> Figure 2. An example of the DTD Figure 3. An example of an embedding XML document H = w G g(i) (2) i=1 To ensure the length of the watermark being able to successfully hide into the XML document, an adjustment mechanism is needed to be used while the value of watermark B is not identical to that of H. The adjustment mechanism is broken into two cases. At first, the H and B are transformed into two bit streams as R 1 and R 2, respectively. In the first case, if the length of R 1 is larger than that of R 2, copy the bits of R 2 to concatenate with the original R 2 sequentially bit by bit until size of R 1 and R 2 are equal. In the other case, while the length of R 2 is larger than that of R 1, R 2 is divided into several segments with the size of R 1 and use the XOR operation to get the final watermark. When the size of the final segment is not equal to the size of R 1, the XOR operation is ignored. For taking an example as shown in Figure 4, the values of H and B are 21 and 157, respectively. The transformed bit streams R 1 and R 2 are and , respectively. The length of R 1 is 5, therefore, R 2 is segmented into two segmentations as and 101. Afterward, those two segments are calculated by using the XOR operation and the blank

4 92 C.-C. CHANG, Y.-H. CHEN AND T.-D. KIEU Table 1. The embeddable list for Figure 6 places are ignored, which can be seen as in Figure 4, to get the final bit stream Finally, the bit stream can be re-expressed as an integer and then the integer is replaced with the original value of B Length of is Segment every bits Watermark Figure 4. An example for the adjustment mechanism Input the group Gg(i) according to the embedding sequence order i. The total number of groups: Temporary parameters: r and tmp For i = 1 to { r= tmp mod G g(i) +1; tmp = tmp / G g(i) ; Replace the caption of the original element or attribute with rth synonym term. } Figure 5. The embedding algorithm

5 ICIC EXPRESS LETTERS, VOL.4, NO.1, After the adjustment mechanism has been performed, the watermark can be embedded into the XML document by using synonym substitution. The embedding algorithm is described in Figure 5. In the verification procedure, the watermark w can be extracted by hashing the whole document except the document structure. Also, with the key K 1, the embeddable list and hiding capacity H can be retrieved as described in the embedding procedure. With the same adjustment mechanism, a new watermark w can be extracted. Additionally, the hidden data h prime is extracted according to the rank of synonym terms displayed in the group G g(i), where the extracted data is depicted as R g(i). Next, the extracted R g(i) can be used to calculate h with Equation (3). Finally, in comparison between h and w, if they are equal, the XML document is authentic; otherwise, it is inauthentic. w 1 h = ( G g(i) (R g(i+1) 1)) + (R g(1) 1). (3) i=1 3. Implementation and Discussions. In this section, a prototype was developed to evaluate the feasibility of the proposed method. For portability, we used Java to implement the prototype. In addition, all of the XML files are processed using Apache s Xalan. The proposed method is suited for the valid XML documents so each of the XML files must comply with its corresponding DTD structure or XML Schema as shown in Figure 2. Assume that the original XML document is shown in Figure 6(a) and the contents needed for protection are , F, Alice, Mandel, , M, Eric, and Jlu. The important contents can be concatenated with each other to generate a watermark as by using hash function with the secret key K. With the watermark embedding algorithm, the watermarked XML document is shown in Figure 6(b). After receiving the watermarked XML document, the document must be verified to identify whether the document structure is matched with the DTD or XML Schema. If the verification fails, the XML document is ignored; otherwise, receivers will collect the contents of the XML document to generate the watermark with the secret key K. Also, the hidden watermark can be extracted by using the extracting algorithm as Due to the generated watermark and the extracted watermark are the same, the XML document is judged as an authentic document. If some contents of the watermarked XML document are manipulated as shown in Figure 6(c), the generated watermark will be different from the extracted watermark so the document is verified as an inauthentic document. If the tags of such document are modified as shown in Figure 6(d), the extracted watermark will be different from the generated watermark. As a whole, either tags or contents of such document are corrupted, the document will be judged as an inauthentic document. 4. Conclusions. To authenticate the integrity of an XML document, XML signature methods require significantly extra space to describe the digital signature. Although Yao et al. proposed the XML-based watermarking technique by altering the case of letters in XML tags, as for a valid XML document, the parser might verify the watermarked document as inauthentic one because XML is a sensitive-case language. In this paper, a watermarking technique is proposed to protect the integrity of XML documents without needing an extra storage to record the protection information. Additionally, the proposed scheme is flexibly adjusted by using the adjustment mechanism if the hiding capacity and the watermark size are not equal.

94 C.-C. CHANG, Y.-H. CHEN AND T.-D. KIEU (a) An original XML document (b) A watermarked XML document (c) The corruption of attribute values (d) The corruption of element values Figure 6.

6 94 C.-C. CHANG, Y.-H. CHEN AND T.-D. KIEU (a) An original XML document (b) A watermarked XML document (c) The corruption of attribute values (d) The corruption of element values Figure 6. An XML watermark example REFERENCES [1] Y. H. Chen, C. C. Chang and C. C. Lin, Adaptive data embedding using VQ and clustering, International Journal of Innovational Computing and Information Control, vol.3, no.6(a), pp , [2] B. Dournaee, XML Security, McGraw-Hill Companies, Inc., [3] D. Eastlake, J. Reagle and D. Solo, XML-signature Syntax and Processing, W3C Recommendation, [4] G. R. Feng, L. Jiang and C. He, Orthogonal transformation to enhance the security of the still image watermarking system, IEICE Transactions on Fundamentals, vol.e87-a, no.4, pp , [5] B. Galbraith, W. Hankison, A. Hiotis, M. Janakiraman, D. V. Prasad, R. Trivedi and Whitney, Professional Web Services Security, Wrox Press Ltd., [6] H. J. Huang, X. M. Sun, G. Sun and J. W. Huang, Detection of steganographic information in tags of webpage, Proc. of the 2nd International Conference on Scalable Information Systems, ACM Press, pp , [7] S. Katzenbeisser and A. P. Petitcolas, Information Hiding Techniques for Steganography and Digital Watermarking, Boston, Artech House, [8] C. C. Lin, Y. H. Chen and C. C. Chang, LSB-based high-capacity data embedding scheme for images, International Journal of Innovational Computing and Information Control, to appear, [9] E. J. L. Lu and R. F. Chen, An XML multisignature scheme, Applied Mathematics and Computation, vol.149, no.1, 2004, pp [10] X. G. Sui and L. Luo, A new steganography method based on hypertext, Proc. of the 2004 Asia- Pacific Radio Science Conference, pp , [11] R. Yao, Q. Zhao and H. Lu, A novel watermark algorithm for integrity protection of XML documents, IJCSNS International Journal of Computer Science and Network Security, vol.6, no.2b, pp , [12] Q. Zhao and H. Lu, A PCA-based watermarking scheme for tamper-proof of web pages, Pattern Recognition, vol.38, no.8, pp , 2005.

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