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Top1. Introduction
Digital watermarking is a method of embedding an imperceptible watermark into a host digital media, like still images, audio signals, and video streams, such that it doesn’t perceptually degrade its quality. The goal of digital watermarking is to provide practical platforms for designing security systems to accomplish copyright protection, broadcast monitoring, tamper detection, digital fingerprinting, intellectual property protection etc. (Potdar, 2005; Bas, 2016; Lei, 2004). Watermarking systems for digital media involve two stages: namely (i) watermark embedding- to indicate copyright and (ii) watermark detection- to identify the owner (Swanson, 1998). Depending on the requirements and purpose, a practical and bonafide watermarking scheme is characterized by four properties (Cox, 2008) such as: robustness- the watermarking scheme must be capable to resist different kinds of attacks; imperceptibility- watermark is embedded in a way such that the degradation in the quality of digital media is imperceptible; capacity- it refers to the number of bits a watermark encodes within a unit of time or within a work, for an image, it would refer to the number of bits embedded within the image; and security- it refers to the fact that an un-authorized person should neither detect nor read the watermark. The watermark is viewed as a sort of a mark that uncovers the proprietor of authoritative data. The data needs to be implanted with watermarks in their media objects for copyright security, content confirmation, tamper detection and so on. A watermarking scheme securely implants an imperceptible watermark in host media information. The embedding procedure is guided by utilization of a key which chose the portions inside the data where the watermark would be implanted. Once the watermark is implanted, it may encounter a few cryptanalytic analyses by the assaulters. The watermarked data needs to be prepared carefully. The assaults can be accidental (like transmission errors) or purposeful (like malicious editing). Subsequently, the watermark must be exceptionally robust against all cryptanalytic attacks from the unintended parties. When the proprietor needs to check the authenticity of watermarks in the potentially assaulted watermarked data, then it does depend on the secret key that was utilized to embed the watermark at sender side. By using key, the embedded watermark can be obtained for ensuring the authenticity, copyright and malicious activities on the multimedia data. The digital watermarking schemes are broadly divided into fragile watermarking and robust watermarking. Fragile watermarking is aimed to detect the integrity, authenticity and modifications of digital media (Zhang & Shuozhong, 2009). Whereas, the robust watermarking prevents someone to remove the watermark to determine the owner of digital media even if it is altered (Wang, 2008). Most of the multimedia applications require imperceptible, secure and robust watermarking to protect the ownership of digital media (Cox, 1996). In the last decade, a lot of digital image watermarking schemes have been proposed in the literature for content authentication, rightful ownership, copyright protection etc. (Liu, 2006; Shieh, 2006; Tao & Eskicioglu, 2004; Wang & Chen, 2007; Mohammd, 2008; Bhatnagar & Raman, 2009; Rawat & Raman, 2011; Chang, 2011; Jamal, 2013; Aslantas & Dogru, 2014; Ansari, 2016; Zhang, 2017). Contemporarily, the attempts have been also made by the researchers to analyze and assess the security of individual watermarking schemes. It has been performed with intent to arrive and design more robust, secure and efficient schemes. Consequently, the customized attacks are framed by the cryptanalysts to unveil the inherent flaws and weaknesses. It has been found that some watermarking schemes are not secure and incompetent to withstand possible attacks, as exposed by the cryptanalysts in (Das, 2005; Chou, 2007; Ting, 2008; Ting, 2009; He & Zhang, 2012; Teng, 2013; Ling, 2011; Benrhouma, 2016;Caragata, 2016; Ahmad, 2011; Sharma, 2014; Ahmad, 2015a; Ahmad, 2015b; Sharma, 2016).