Research On Digital Watermarking Algorithm Based On DCT Domain

With the rapid development of Internet, the communication of multimedia becomes more and more convenient. But because of the digital media is very easy to be illegal copied, information security becomes increasingly outstanding. How to efficiently prevent the illegal copying of data and identify intellectual property rights of multimedia product is a pressing problem. In this background, the digital watermark technology emerged at the proper moment, which plays an important role in information security and rights protection.Digital Watermarking technology will hide the symbolic information in multimedia carrier, which should not influence the quality of the multimedia carrier obviously and should not be perceived by people's perceptual system, so that it can prove the ownership of copyright (infringing act) and integrity the authentication of digital products in copyright disputes. Digital watermarking technology as an effectual means about digital media copyright protection, data authentication, data hiding and so on has gotten the attention of scholars both at home and abroad.This paper mainly researches the digital watermark technology applied in digital image. At first, the research background and current development status of digital watermark are introduced. Furthermore, it makes a brief analysis about principle, classification, typical algorithms and standards of digital watermark. This paper is a study of static images, with a deep research about the several digital watermarking algorithms based on DCT are proposed in the paper. And the main research contents are as follows:1.This paper proposes a multi-bits watermarking scheme in which carrying more information of watermark using multi-pseudo-random sequences. In this scheme, the 8×8 bock based DCT is performed to the host gray level image and the coefficients of a block in the transformed domain represent different frequency component of the host image. Select the DCT coefficient on position (4,4) of every block to form a dataset and permute it to a vector X with length N. Then generate orthogonal sequences with length N randomly according to a private key and select M sequences to add to vector X with certain intensity. The way of selecting is determined by the watermark. This scheme uses the comparison of correlations between the host data and pseudo-random sequences to carry the watermark, therefore providing a payload significantly more than the number of added sequences and enhancing the performance of watermarking. Especially when N is large, L is much larger than M. The watermark is embedded into the middle frequency component of DCT domain, taking into account the robustness and imperceptibility. This scheme have tested the robustness and imperceptibility with different parameters and the results show that the robustness is mainly related to system parameter A and M have little effect to it; the embedding capacity is determined by M; the imperceptibility is related A and M. In the practical application, we can adjust these two parameters according to the actual requirement2.JPEG is a common image format and it is significance to locate the tampered area accurately in the JPEG image. In the process of JPEG encoding, the host image is partitioned to blocks sized 8×8 and DCT is performed to them, then quantify the DCT coefficients to integers with different quantify steps. In order to reduce the amount of watermark information, we do not embed the Hash values of every block to the host image directly. Instead, we calculate the Hash values of the block's principal content first, then re-group the Hash values and sum them with modulus 2, and finally we get the genuine watermark. The bits of watermark are consistent to the blocks of JPEG image, so one bit is embedded into one block and good imperceptibility is guaranteed. When authentication is performed, we comprise the calculated and the extracted watermark of the image to be authenticated and estimate the tamper rate by the overall match. Then we determine if the block is tampered or not according to the damaged degree of the watermark information of the block. This scheme can locate all tampered blocks accurately when the tamper rate is less than 1%. We can't locate the tampered area when the stego-image is re-encoded with different quantization factor or quantization matrix, or added by high level noise because these operations are equivalent to tamper the image with large range. Theoretical analysis to the ability of tamper location and design of a fragile watermarking scheme that can locate more extensive tampered area accurately are the future works.3.This paper proposes a semi-fragile watermarking scheme based on DCT and DWT that can resist JPEG compression. In this scheme, we first get the host image by quantization and inverse quantization operations to the original image. Then perform DWT to it and just select the high four bits of the LL3 sub-band as a watermark. In the next step, we perform 8×8 block based DCT to the host image and embed the watermark into the low frequency component selected by private key of DCT domain integrating the embedding and JPEG encoding process. On the extracting end, we extract the high four bits of watermark according the parity of the DCT coefficients. This scheme can detect the watermark without the original image which is a completely blind watermarking algorithm. The experimental results show that the extracted watermark is clear and is very close to the original watermark when the compression quality factor is 50, and the watermark can be extracted successfully also when the quality factor is 40. The result illustrates that this scheme is robust to JPEG compression. This scheme is also robust to noising and cropping attack, and can locate the tampered area to a certain extent.