| Abstract: | In this dissertation, sereral key problems in the field of steganographic image security are mainly studied and discussed: (1) A novel kind of image cryptosystem is studied. The encrypted information of secret image is embedded into an ordinary image, which can be transmitted publicly, and it will not cause attention of illegal eavesdropper. The following results are achieved: (a) The image hiding algorithm and its improved algorithm based on wavelet transform and vector quantization are proposed, and both algorithms embed the secret image after taking full advantage of wavelet transform(WT) to compress it. (b) Introducing the CDMA concept in communication theory, a novel kind of image cryptosystem—image hiding cryptosystem of image division multiple access is proposed. We encrypt key information of several secret images and then hide them into an image, which is an ordinary image. After receivers get the image, each receiver can restore corresponding secret image independently. Compared with other image encryption algorithms, our new image hiding cryptosystem can compress secret image data efficiently. Moreover, the security and practicability of our system is better. (2) Digital watermarking is the most important branch of information hiding, which has been the hotspot of international academia. Digital watermarking has been proposed as a solution to the problem of copyright protection of multimedia documents in networked environments. In this field, the following results are achieved: (a) A new frequency information hiding and watermarking algorithm based on WT and Discrete Cosine Transform(DCT) is presented. After compressed by WT, the information of secret image is embedded into DCT domain. The algorithm has good imperceptibility and security and is robust against JPEG compressing and cropping. The algorithm can be used to embed watermark into images, and the watermark may be one 256-color picture or more and more than one secret image can be hidden into a single carrier image. (b) A novel watermarking algorithm based on DCT and modular operation is proposed. The modular operation is used to modify the coefficient of DCT of image according the value of finary watermark image. The watermark value can be restored by the difference of modular operation of DCT coefficients. The algorithm is robust against some attack because of the novel and efficient embedding strategy. (c) A novel kind of digital watermarking scheme is named public key digital watermarking. This new scheme is derived from the conception of public cryptography system and the performance of Hadarmard Transform. There are two visually recognizable watermarks in our scheme, and two keys to permute the contents of two original watermarks. Next, we use Hadarmard Transform to get the hybrid coding of the two permuted watermarks. Then, we embed the hybrid coding into frequency domain of the image by blocked DCT modular scheme. After Inverse DCT, we get the watermarked image. Our digital watermarking scheme could let any their part (the person or organization besides the creator and the IPR authority) verify the ownership while the creator or owner also can verify his ownership. This means that our scheme increases the possibility of finding illegal copy while keeping the ability to protect the ownership. Experimental results demonstrate that the watermarks are robust for some attacks and JPEG lossy compression. Our watermarking scheme can protect ownership (beyond dispute). Meanwhile, it retains the robust capability. Compared with most present watermarking schemes, our scheme is more likely accepted as evidence in court. (3) Secure watermarking architecture: Watermarking, as a new technique mainly used for IPR protection and authentication, has become a research hotspot shortly after its appearance. Many effective watermarking algorithms have been proposed. But the mature of watermark technology is not enough for IPR protection. Confronted with current various attack methods, a secure watermarking architecture must be built to standardize the whole process from watermark insertion to detection. We improve the AQUARELLE system, which is a good example for showing how IMPRIMATUR (an architecture for IPR protection) is operated, and the DHWM watermarking protocol. Our improved digital watermarking architecture is more concise and more secure. Moreover, digital watermarking technology is analysed thoroughly from theory to realization and from watermarking algorithm design to attack. The current state of this technique is presented and several directions of development of digital watermarking at next stage are proposed. At last, the thesis is summarized and the future possible research is discussed. |
