Abstract
Traditional secret sharing scheme which encrypts secret image based on mathematical calculation to construct two or more shares that often requires the complicated computation to extract secret. Later on, conventional visual cryptography scheme was developed to deal with the perplexed calculation in encryption and extraction of previous schemes. The stack-to-see technique can be used easily to reveal the secret by human visual system, which can shorten computation time. However, the expansion of image size and the noise-liked shares of previous schemes lead to the difficulty in transmission and storage. Moreover, the noise-liked shares are insecure which may attract the attention of eavesdroppers and also difficult to identify and manage. To prevent above problems and protect the security of secret information is an important issue. Thus, this study proposes two schemes of user-friendly and unexpanded secret sharing mechanism. The first scheme with two decoding options based on polynomial based image secret sharing (PISS) and Visual secret sharing (VSS) scheme. The scheme is to generate the user-friendly shares and offer a previewing result of the secret image according to the stacking operation, while it can retrieve a lossless secret image. To embed more confidential information, the second scheme uses a pre-defined codebook to encode two secret images into two meaningful shares without pixel expansion. According to the turning mechanism, two secret images can be embedded into two shares simultaneously. The decryption process allows the user to get two secrets via turning and stacking. A notable feature of our scheme is that the black pixel value of the secret image can be completely extracted and the vision quality of stacking results can be identified clearly.