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Top1. Introduction
With the rapid developments occurring in mobile internet and cloud storage, privacy and security of personal data has gained significant attention nowadays. The cloud service provider or malicious attackers may access users’ sensitive information without authorization. A general approach to protect the data confidentiality is to encrypt the data before outsourcing (Xia, X. H. Wang, Sun, & Q. Wang, 2016; Xia et al., 2016). However, in some application scenarios, the cloud service provider or database manager needs to embed some additional messages, e.g., authentication or notation data, directly into an encrypted data for tamper detection or ownership declaration purposes. For example, patient’s information can be embedded into his/her encrypted medical image to avoid unwanted exposure of confidential information.
As a trend, the research on data hiding in the encrypted domain has gained increasing attention, which is primarily driven by the needs from the third-party computing platforms (e.g., cloud computing). Over the past few years, a considerable amount of schemes about data hiding in encrypted images or videos have been reported in the literature (Subramanyam, Emmanuel, & Kankanhalli, 2012; Xu, Wang, & Shi, 2014; Xu & Wang, 2015; Guo, Zheng, & Huang, 2015; Parah, Sheikh, Hafiz, & Bhat, 2014; Xu, Wang, Shi, & 2016; Xu, Wang, & Zhu, 2017). However, within these schemes, the cover medium has been distorted during the data embedding operation and cannot be restored into its original form after data extraction. In some sensitive scenarios, such permanent distortion is strictly forbidden. This implies that, for a legal receiver, the original plaintext content should be recovered without any error after image decryption and data extraction. To solve this problem, reversible data hiding (RDH) in the encrypted domain is preferred.
RDH is a technique that slightly alters digital media (e.g., images or videos) to embed secret data while the original digital media can be recovered without any error after the hidden messages have been extracted (Shi, Li, Zhang, Wu, & Ma, 2016). This specific data hiding technique has been found to be useful in some important and sensitive areas, i.e., military communication, medical science, law-enforcement, and error concealment (Xu, Wang, Shi, 2014; Xu & Wang, 2016). Three major approaches, i.e., lossless compression (Fridrich, Goljan, Du, & R, 2002), histogram modification (Ni, Shi, Ansari, & Su, 2006; X. L. Li, B. Li, Yang, & Zeng, 2013), difference expansion (Tian, 2003; Li, Yang, & Zeng, 2011), have already been developed for RDH. For more details of these methods and other RDH methods, refer to the latest review of recent research (Shi, Li, Zhang, Wu, & Ma, 2016).