Visual cryptography is a technique that applies the human visual system to decode encrypted information, such as text, image and number, without any sophisticated devices and computing capabilities. Therefore, compared with the traditional cryptography, it is apparent that it saves a large amount of time and money on devices and computations. Also, visual cryptography provides the convenience for humans to carry out decryption with a portal card which is significant to the business application. In the past decade, visual cryptography has been thoroughly researched not only on its contrast and subpixel expansion, but also on its applications.;The main contribution of this thesis is the security of visual cryptography related to the dishonest shareholders. This is the first known work concerning this variety of potentially secure problem. In the previous papers, the shareholders are inherently honest. However, in the real world, it is impossible to guarantee that every shareholder would be honest forever (e.g., because of the interest of business or military, some shareholders might change to be the traitors). Therefore, a new method based on visual authentication [16] is proposed and the improvement is also made. In this thesis, we also review the previous papers on different fields of the visual cryptography.
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