I. Abstract: In the past two decades, reversible data hiding (RDH), also referred to as lossless o invertible data hiding, has gradually become a very active research area in the field of data hiding. This has been verified by more and more papers on increasingly widespread subjects in the field of RDH research that have been published these days. In this paper, the various RDH algorithms and researches have been classified into the following six categories: 1) RDH into image spatial domain; 2)

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... RDH into image compressed domain (e.g., JPEG); 3) RDH suitable for image semi-fragile authentication; 4) RDH with image contrast enhancement; 5) RDH into encrypted images, which is expected to have wide application in the cloud computation; and 6) RDH into video and into audio. For each of these six categories, the history of technical developments, the current state of the arts, and the possible future researches are presented and discussed. It is expected that the RDH technology and its applications in the real word will continue to move ahead. II. Introduction: Data hiding has received much attention fromthe research community in the past more than two decades. By this technique, it can embed secret data into a cover medium, and later enable the intended user to extract the embedded data from the marked medium for various purposes. However, for most data hiding methods, the cover medium has been distorted during the data embedding operation and hence cannot be restored into its original form after data extraction. In some sensitive scenarios, such permanent distortion is strictly forbidden and the exact recovery of the original cover medium is required. To solved this issue, reversible data hiding (RDH) also called lossless or invertible data hiding, is proposed losslessly recover both the embedded data and the cover medium. That is, with the RDH, besides the embedded data, the cover medium can be exactly recovered from the marked data as well. The first RDH algorithm is the one proposed by Barton in a US patent in 1997. He proposes to embed the authentication information into a digital medium, and enable legitimate users to extract the embedded authentication information for verifying the authenticity of the received data. II.1. Conventional techniques: A. Digital Watermarking: A watermarking system is usually divided into three distinct steps, embedding, attack, and detection. In embedding, an algorithm accepts the host and the data to be embedded, and produces a watermarked signal. Then the watermarked digital signal is transmitted or stored, usually transmitted to another person. Digital watermarking is the process of embedding information into digital multimedia content such that the information (which we call the watermark) can later be extracted or detected for a variety of purposes including copy prevention and control. Digital watermarking has become an active and important area of research, and development and commercialization