| Summary: | 博士 === 國立成功大學 === 電機工程學系碩博士班 === 91 === Watermarking is a potential method for copyright protection and authentication of multimedia data in the internet. The watermarking process can be viewed as a communication task, where the watermark acting like information is embedded into a host image acting like noise in a communication channel which susceptible to all kinds of attacks acting like jamming. In this thesis, two novel watermarking schemes using frequency embedding and phase embedding respectively are proposed. In the first proposed (frequency embedding) scheme, a secret bit is embedded into two rather than one DCT coefficients of a block. This is analogous to the FSK as opposed to the ASK modulation in digital communication. High variance block selection (HVBS) is employed to enhance robustness. In order to solve the synchronization problem caused by HVBS, local block selection and a novel technique called two states constraint is implemented. In order to demonstrate the effectness of the proposed scheme, simulations under various conditions were conducted. The empirical results show that our proposed scheme can sustain most common attacks including JPEG compression, rotating, resizing, cropping, painting, noising and blurring etc. The comparison between using HVBS and without shows that HVBS with the two state constraint can effectively enhance robustness without the sacrifice of imperceptibility. The enhancement is much more significant for host images with fluctuation characteristics than those with smooth characteristics. The comparison between ASK and FSK shows that the extracted watermark from the latter deteriorate gracefully and that from the former is blown out as the strength of a JPEG attack is beyond a threshold. For other common attacks, the ASK and FSK perform about the same except for the blurring attack for which the extracted watermark from the latter can be clearly identified while that from the former is blown out.
In the second proposed (phase embedding) scheme, amplitude boost (AB) and low amplitude block selection (LABS) are used to achieve superior performance in terms of robustness and imperceptibility. AB is hired to increase the robustness while LABS is employed to improve the imperceptibility. In order to demonstrate the effectiveness of the proposed scheme, simulations under various conditions were conducted. The empirical results show that our proposed scheme can sustain most common attacks including JPEG compression, rotating, resizing, cropping, painting, noising and blurring etc. The comparison between using the scheme with AB and LABS properly combined and that without shows that the former can effectively enhance robustness without the sacrifice of imperceptibility. The enhancement is much more significant for host images with smooth characteristics than those with fluctuation characteristics.
Steganography and copyright protection are closed related in the internet environment. In this thesis a public-key steganography system is proposed to offer safe exchange of a stego-image in the internet. In the proposed system, the secret image is first compressed by the quadtree partition vector quantization (QPVQ) coding into the indices bits stream which is then embedded into the Discrete Cosine Transform (DCT) of the cover image. In the embedding process, adaptive quantization and appropriate DCT coefficients are used to enhance the imperceptibility of the stago-image. On the other hand, The Rivest, Shamir and Adleman (RSA) algorithm is hired to provide the mechanism of a public-key system for secure communication in the internet. Our proposed system has two advantages compared with other data hiding techniques. One is the safety and convenience offered by the RSA system to exchange data in the internet. The other is the stego-image can pass through high quality Joint Photograph Experts Group (JPEG) compression without noticeable degradation.
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