Research On The Image Secret Sharing Methods

The secure protection of secret image is an important research area ofinformation security. Image secret sharing (ISS) mainly resolves the problems oflosing and modifying of the secret image information in storage and transmission,and improves the security of the secret image’s protection. Currently, the research ofimage secret sharing techniques can be divided into two categories: visualcryptography sharing (VCS) and polynomial-based ISS (PISS). VCS is suitable forthe situation where no computation resources are available, and the perfectrevealing of the secret image is not required. PISS needs computer to reveal theperfect secret image. This dissertation mainly addresses the researches of ISSmethods by improving efficiency and visual quality. It first improves theperformances of PISS and VCS, respectively. Then the combining problem of VCSand PISS is researched. All research contents are as follows.(1) Aiming at large size and low visual quality of the stego image inpolynomial-based image secret sharing and hiding (PISSH) method, the dissertationproposes a PISSH method based on the largest PSNR estimation. The PSNR value ofstego image is estimated by different information hiding methods, theoretically.Then a hiding method of shadow image is proposed based on the largest PSNRestimation. It can jointly optimize the size and visual quality of stego image. Thesize of stego image can be chosen self-adaptively. The better visual quality of thestego images can be achieved with the cost of larger size. According to the problemof verifying the validity of the stego image, a piecewise authentication algorithm isproposed, which can verify the validity of the stego image effectively.(2) Aiming at the large size expansion of shadow image in traditional visualcryptography sharing, and distortion of the revealed image, the dissertationresearches the method of aspect ratio invariant visual cryptography sharing(ARIVCS). It first realizes shadow image without size expansion by compressingthe secret image before sharing. Then optional size expansion of shadow image isachieved using the resizing process of the secret image. In order to improving thevisual quality of the revealed image in details, the edge secret pixels have priorities to be retained in the revealed image. Finally, the dissertation proposes two ARIVCSmethods based on deterministic VCS and probabilistic VCS, respectively. It solvesthe problem of degraded visual quality in VCS when the size expansion is smallerthan1.(3) A single image secret sharing method cannot simultaneously realize fast andperfect revealing the secret image. The dissertation researches the method of visualcryptography and polynomial-combined image secret sharing (VCPISS). In therevealing process, it can reveal a vague previewed image by stacking shadowimages, also a perfect image by computation. Two shortcomings of a literatureVCPISS method are analyzed: low visual quality of the previewed image bystacking shadow images, and lossy reconstructed secret image by computation. Animproved VCPISS method of lossless reconstruction is then proposed. In order toimprove the visual quality of the previewed image by stacking shadow image, aVCPISS method is proposed based on gray VCS (GVCS). The visual quality of thepreviewed image can be largely improved with a small cost of size expansion. Toevaluate the visual quality of the previewed image more reasonable, a newdefinition of the contrast is proposed. Under the new definition, the visual quality ofthe revealed images in GVCS can be compared effectively.(4) Aiming at revealing secret image with different visual quality in differentthresholds and decoding conditions, the dissertation researches the method ofdouble-threshold image secret sharing (DTISS), and proposes two DTISS methodsbased on VCS and GVCS, respectively. In the VCS-based DTISS method, secretimage information is embedded into shadow images of VCS by choosing differenttypes of share matrixes. All pair-wise different share matrixes are divided intodifferent types. Therefore, larger information can be embedded into the shadowimage, and get smaller size of shadow image without degrading visual quality. Inthe GVCS-based DTISS method, except embedding information by choosingdifferent types of share matrixes, each shadow pixel embeds gray-valuedinformation. Hence it greatly improves the storage of the embedded information inshadows. Therefore, the GVCS-based DTISS method can greatly reduce the sizeexpansion of shadow image with a small cost of visual quality.In conclusion, this dissertation has researched the methods of visualcryptography and polynomial-based image secret sharing, and the combination of these two techniques. It achieves good performances in two key problems: thesharing efficiency and the visual quality of the revealed image.