# Review on Semi-Fragile Watermarking Algorithms for Content Authentication of Digital Images
## 1 Introduction  
The 21 st century is an era with a large amount of information. People can get almost any information they want through the network, and can store, create, edit, and distribute the information freely using powerful computer software. While it brings convenience to us, the related issues are gradually exposed, especially the issue of information security. Using various editing software for multimedia information, such as Adobe Photoshop, Meitu Xiu Xiu, Image Doctor Software, and so on, original digital products can be copied by pirates without losses, and spread through the Internet, which seriously affects the enthusiasm of creators and causes a negative impact on market health. To achieve certain purposes, pirates can tamper with relevant digital products without trace, which seriously threatens the authenticity and integrity of the information. If the tampered contents involve national security, court evidence, historical documents or other important data, adverse social impacts or considerable political and economic losses may be caused. Therefore, authenticity and necessary integrity protection of multimedia information have become an urgent problem. The intellectual property rights of digital products can be protected effectively by digital watermarks [1], which has attracted widespread attention of international academic circles and become a research hotspot [2]. The robust watermark, fragile watermark, and semi-fragile watermark are three branches of digital watermarks. Among them, the robust watermark is mainly used for copyright protection. Although it has good transparency and robustness, it cannot determine whether the content of a digital product is hampered with and cannot locate the tampered areas. Fragile watermarks andsemi-fragile watermarks are primarily used to certificate the integrity and authenticity of image data. The fragile watermark is usually used to achieve accurate authentication. It regards the digital image as an entirety and does not allow any tampering. Even if there is only a one-bit change, the digital image cannot pass the certification system. The semi-fragile watermark, combining the advantages both of the robust watermark and the fragile watermark, is mainly used for fuzzy authentication of digital images [3]. Under the premise that the image content is basically unchanged, the image is allowed to have a certain degree of distortion. In other words, the common image processing operations can be distinguished from malicious tampering. In addition, semi-fragile watermarking technology can locate the tampered areas as well, or even restore them. In many practical applications, people need to confirm whether the image has been tampered, which areas have been tampered and how to restore them. Thus, the application of robust watermarking technology has been limited. For example, in the legal field, photographs that are the basis for judging the cases must be authentic and complete; in the medical field, 3D medical images play a very important role in healthcare. With the improvement of Internet technology, these images can be transmitted and exchanged in a variety of ways, so it is crucial to ensure the integrity and authenticity of 3D medical images. [PERSON] et al. [4] proposed a virtual infrastructure-less Cloud solution for secure management of 3D medical images, combining with the secure watermarking technique to generate a lossless dynamic and adaptive compression engine, and the scheme performs well in terms of invisibility, complexity, compression ratio, adaptability, scalability, and energy efficiency. Furthermore, in the process of image storage and spread, due to the large amount of data, people often choose to compress an image, which limits the application of fragile watermarking technology. Therefore, the semi-fragile watermark has a greater advantage and a wider application in digital image authentication. However, there are few review articles on the semi-fragile watermark for image authentication, so we have written this paper.