Secret image sharing with authentication-chaining and dynamic embedding

A popular technique to share a secret image among n participants is to divide it first into some shadow images and then embed the shadows in n”cover” images. The resulting “stego” images, which contain the embedded data, are distributed among intended recipients. In order not to attract any attacker’s attention, it is important to apply a suitable embedding such that high quality stego images are produced. Moreover, to ensure the integrity of stego data, a robust authentication mechanism which can detect tampering with high probability should be implemented. Recently, a series of papers ( Lin and Tsai, 2004; Yang et al., 2007; Chang et al., 2008; Yang and Ciou, 2009) have considered polynomial-based secret image sharing with steganography and authentication. The embedding technique employed in all these papers is static, i.e. hidden bits are embedded in predetermined fixed-size blocks of each cover image. It is therefore possible that all the hidden data is replaced in only a subset of blocks of cover images while other blocks remain intact. As for authentication, the best of these schemes detects a tampered stego block with probability 15/16, however, since this is obtained at the cost of using 4 authentication bits per block, the visual quality of stego images is seriously degraded. In this paper, we propose a novel polynomial-based secret image sharing scheme with two achievements. First, a new embedding is proposed so that the block size is determined dynamically according to the size of hidden data and therefore, all the capacity of cover images is used for data hiding. Second, we introduce a new authentication-chaining method which achieves 15/16 as its tamper-detection ability while using only 2 authentication bits. Experimental results are provided to confirm the theory.