New Extended Visual Cryptography Schemes with Clearer Shadow Images

• Main Authors: Yao-Yu Yang, 楊燿宇
• Other Authors: Ching-Nung Yang
• Format: Others
• Language: en_US
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/32595310299373298691

碩士 === 國立東華大學 === 資訊工程學系 === 97 === Cryptography schemes usually use complex operation, encoding, and translation to achieve the information hiding. In 1994, Naor and Shamir proposed a so-called Visual cryptography scheme (VCS) which can decode a secret information through human visual system. VCS encodes a visual information (pictures, text, etc.) into several noise-like shadow images. One can photocopy shadow images on transparencies and stack them on an overhead projector to visually decode through the human visual system without computation. This distinctive staking-to-see property of VCS attracts the attention of research so that many VCSs have been recently proposed. Noise-like shadows of VCS, however, were suspect to censors and difficult for identification and management; as a result, the extended VCS (EVCS) with meaningful images on shadows have been proposed to address this problem of suspicious shadow images. In this thesis, we adopt the digital halftoning technology on shadow images and arrange the black dots on shadows according to the specific patterns of a digital halftone image to retain a high-quality cover image shown on the shadow. We propose two types of EVCS. One is the extension of Naor et al.’s (2, 2)-EVCS and has clearer shadow images. The second scheme enhances the visual quality of the shadow quite substantially, while the halftone cover image remains on a reconstructed image and diminishes the clarity of a secret. However, it still holds the perfect secrecy.