| Summary: | 博士 === 國立交通大學 === 資訊科學與工程研究所 === 103 === Steganography is a research topic to protect the privacy communication. The techniques can be divided to two categories: lossy steganography and lossless steganography. Lossy steganography hides message in the content of cover media and distorts the real content. On the other hand, lossless steganography hides message in the cover media and keep the real content intact. For some applications such as medical images, military images, or forensic images (provided by law), reversible property, where the cover image can be perfectly recovered from the stego image, is an important issue. Therefore, lossy steganography can be divided into two categories: reversible steganography and irreversible steganography. Reversible steganography can perfectly recover the cover media in the extraction process, whereas irreversible steganography cannot.
In this dissertation, we will propose two reversible and one lossless steganographic methods using different cover media. First, a difference expansion based reversible steganographic approach, which yields low distortion under the same embedding capacity, is proposed. An edge-oriented prediction method and a modified overflow/underflow prevention method are proposed to achieve the requirement. Experimental results have demonstrated that the proposed reversible steganographic method yields lower distortion than several well-known expansion-based reversible steganographic methods.
Second, a novel DCT-based reversible steganographic method for stereo images is proposed. Stereo images captured from a pair of CCDs simultaneously are widely used to create the illusion of 3D depth. Every pair of stereo images has many similar block pairs. In this method, we embed secret data in those similar block pairs. Experimental results have shown that the proposed method is undetectable under Chi-square analysis and outperforms other methods in terms of embedding capacity and image quality.
Finally, we propose a lossless steganographic method to embed message in a PowerPoint file. In the proposed method, we not only hide message naturally but also keep the content of the cover media intact. Furthermore, the proposed method can resist the format conversion attack. The experimental results demonstrate that the proposed method is undetectable under some visual and statistical attacks.
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