Study On Fragile Image Watermarking Based On Public Key Cryptography And Hash Function

As the rapid development of Internet and digital multimedia technologies, how to protect the integrity and authenticity of digital products becomes an urgent problem. The traditional digital signature technology can effectively solve this problem, but digital signatures are normally stored in the header of multimedia information, with the development of software technology, the digital signature is modified and deleted easily, consequently the failing authentication happens. The fragile watermark is directly embedded into the multimedia information, and provides an effective solution to the content authentication of multimedia information. The fragile image watermarking based on public key cryptography and hash function is studied in this paper. The main research results are as follows:(1) An improved fragile watermarking algorithm for JPEG image content authentication is proposed. First, the owner of the image and the detector use Diffie-Hellman key agreement scheme to get shared key, then the owner decodes the original image to obtain the quantized DCT coefficients. For each8x8quantized DCT coefficients block, the average value of the most seven bits of DCT coefficients is calculated. The shared key is added by these average values, and the sum is as the input of the hash function. Thus, the watermark bits are generated by XOR operation. Finally, the generated watermark is embedded into the quantified DCT coefficients by modifying the least significant bit, and the watermarked image is obtained. For tampering detection, the detector decodes the watermarked image to get the quantified DCT coefficients, then watermark is extracted from the least significant bit of these coefficients. Similarly to the watermark embedding process, the watermark is reconstructed by the image to be certified. The reconstructed watermark and the extracted watermark make comparison, and the result of image tampering detection is obtained. In this algorithm, the watermark embedding and extraction are performed in the compressed domain in order to avoid the quantization error coming from JPEG adverse quantization and re-quantization process. Based upon the sensitive dependence of the output on the input of the hash function, and the content-fragile watermark, the tampering area can be located accurately. Because the watermark generation and detection process need shared key, the illegal watermark forgery can be avoided, and the original of the image is confirmed.(2) A self-embedding watermarking algorithm based on RSA signature technique is designed. The private key is used to embed the watermark, and the public key is used to verify whether the image is tampered or not. Moreover, the tampered area can be recovered. The security shortage of transmiting the system key by a secret channel is avoided. In addition, The watermark is generated by the integer DCT transform coefficients, and embedded into the image which has been scrambled by a secure hash function. This scheme not only improves the security, but also has a better quality of the recovered image content.