This paper proposes a novel form of reversible data hiding using two marked images by employing the adaptive coefficient-shifting (ACS) algorithm. The proposed ACS algorithm consists of three parts: the minimum-preserved scheme, the minimum-preserved with squeezing scheme, and the base-value embedding scheme. More specifically, each input block of a host image can be encoded to two stego-blocks according to three predetermined rules by the above three schemes. Simulations validate that the proposed method not only completely recovers the host medium but also losslessly extracts the hidden message. The proposed method can handle various kinds of images without any occurrence of overflow/underflow. Moreover, the payload and peak signal-to-noise ratio (PSNR) performance of the proposed method is superior to that of the conventional invertible data hiding schemes. Furthermore, the number of shadows required by the proposed method is less than that required by the approaches which are based upon secret image sharing with reversible steganography. 1. Introduction Due to ubiquitous broadband services and high-speed networks provided by Internet service providers (ISPs), along with mass production of high-capacity and low-cost multimedia devices, individuals and organizations can easily share their information on the Internet. Moreover, thanks to the portability and mobility provided by such wireless communications as intelligent mobile systems, wireless fidelity (Wi-Fi), and worldwide interoperability microaccess (WiMax), people can exchange/retrieve resources anywhere and anytime. Preventing data from being eavesdropped, tampered with, and falsified during transmission has become an important goal. In addition to the use of cryptographic systems, one can use data hiding to achieve this result. Primary applications of data hiding can be found in proof of ownership, content authentication, copyright protection, and covert communications. In general, data hiding can be divided into two categories: digital watermarking and steganography [1, 2]. In digital watermarking, the embedded message (or watermark) is often related to the medium and conveys additional information about the medium. Robust performance is a key feature of the watermarking schemes [3–5]. In steganography, the hidden message often has nothing to do with the host media; however, both hiding capacity and perceived quality are the two areas of concern pursued by the authors [6–8]. One of the major issues of the steganographic approach is that the marked images are susceptible to manipulation. In
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