| Summary: | 博士 === 國立嘉義大學 === 資訊工程學系研究所 === 103 === Due to the increase of the computation power and the worldwide popularity of the Internet, more and more data of various kinds can be accessed and transmitted over the Internet. However, the convenient communication ways are accompanied by problems of information security and the usage of networking bandwidth. To ensure the confidentiality, integrity and availability of data transmission over the Internet, many different approaches have been proposed in attempt to make private communication secure, such as cryptography and data-hiding. Cryptography techniques can scramble secret information into an unreadable message. Data-hiding techniques can provide secure transmission by embedding secret information within covert carriers to avoid observation. In this dissertation, we aim to design not only data-hiding algorithms but also visual encryption schemes.
The research begins with developing novel algorithms for data hiding. Data hiding techniques can be classified into two categories, namely, reversible data hiding schemes and irreversible data hiding schemes. For irreversible data hiding schemes, only secret data can be extracted, while restoration of cover images is unavailable. Conversely, reversible data hiding schemes can extract the secret data and recover the original cover images in the decoder.
First we proposed a new data hiding scheme based on the search-order coding compression method of VQ indices to increase embedding capacity. The scheme uses SOC coding algorithm to split all of the VQ indices in the index table into SOC indices and original vector quantization code indices. The scheme can embed more than one bit of secret data to each SOC indices so that the payload capacity of embedded secret data increases. Next, instead of using one VQ index as indicator, the second proposed scheme uses only one bit to distinguish between indices of two clusters. Not only the indices in the first cluster but also those in the second and third clusters can hide the secret bits. The schemes reduce the bit rate (BR) and increase the hiding capacity.
In addition to developing data hiding algorithms, we also develop the techniques of visual cryptography. In 2006, Fang and Lin proposed a progressive visual secret sharing scheme. Unfortunately, their performance would reveal the information about the secret image on each share and result in serious security problem that each share can obviously reveal information of the secret image. We propose a new secure progressive visual secret sharing method by redesigning basis matrices to satisfy both contrast and security conditions.
Moreover, in conventional -threshold VSS, each share has the identical capability to restore the secret image. However, participants might have different levels of duty in the real world. From the viewpoint of the management, we would consider each participant may has a unique priority, and design the corresponding share with different weight in response to his/her significance in the decoding phase. A weighted threshold visual cryptography without pixel expansion is proposed to achieve the goal of assigning a different weight to the share of each participant in a different priority class. In addition, the proposed method also provides a progressive mechanism to share images at multiple resolutions.
All the proposed methods described in this dissertation have been extensively evaluated by experimental examinations and theoretical analyses. The evaluation results show that our methods are practical and superior to other previous methods.
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