Abstract
With the popular of internet and the increasing of recorder equipment, the digital media can distribute more easy and faster. In order to avoid the pirator from copying digital multimedia without the real owner's grant, several technologies are developed to deal with this problem. However, traditional cryptography, the most widespread protected method, only can guarantee the security of transmission. Once decrypted, the data is no longer protected. For decrypted media, there is a strong demand to prevent the malicious attacker from distributing the decrypted media arbitrarily. Digital watermarking is a novel technique that can overcome above disadvantage. Whether the media is decrypted or not, we can easily judge the legitimacy by checking the extracted information.The unique property of the watermark is that watermark should not be separated from the embedded media. That means that the embedded watermark should survive in the circumstance where media may locate. In limited resource, compression is an inevitable processing for media. That is why the watermarking system should be designed from the viewpoint of compression. In numerous compression approaches, JPEG and MPEG2, both DCT-based coding, are the most popular compression standards in this world. In this thesis, we will develop a watermarking scheme which aims for above two compression standards.We scheme to propose a robust watermarking system which can be adjusted according to image and video compression and quality. At first, the embedded domain such as DCT coefficients is partitioned into several ranges with each range representing a watermark bit. Embedding procedure is to let original coefficient we want to embed falls in the certain point of the region that can correctly represent watermark. The length of every range has very huge impact on the ability in resisting compression and media quality. In our system, the user can choose the compression intensity they want their watermark can survive. For the situation without harming the robustness of watermark under the chosen requirement of compression, we then optimize the quality of the media. In order to achieve this goal, a table which can completely describe the shifted quantity of every value after compression is created. By using this table, a searching algorithm is proposed to determine the length of every range. In addition, for general semantic watermarks, we also develop a method which improves the media quality by reducing the number of watermark bits we practically need to embed.In the proposed system, two modes, the basic mode and advanced mode, are provided. In the basic mode, the users need only to choose their watermark and press the embedding button. This basic mode can satisfy a great part of applications. If someone needs improving performance, the advanced mode can be used by utilizing the asymmetrical and symmetrical key management in different secure requirement.A series of experiments were conducted and the results show that our system can fit theoretical anticipating under JPEG and MPEG2 compression. Under the same parameters, we also test the survival ratio of watermark under several attacks except for compression. The experimental results also demonstrate that our system is feasible and effective.