# Review on Semi-Fragile Watermarking Algorithms for Content Authentication of Digital Images
## 2 Basic Principles and Characteristics of Semi-Fragile Watermark  
Semi-fragile watermarking algorithms for image authentication focus on the ability of detection, location and recovery from tamper attacks carried out on images. Because the semi-fragile watermark has a certain degree of fragility, the algorithm can realize the image authentication according to whether the watermarking data is tampered with. When the host image suffers attacks, the watermarking information will make a corresponding change as well. However, for normal image-processing operations, the semi-fragile watermark also has a certain degree of robustness, so that the normal image processing operations and malicious tampering operations can be distinguished. The framework of the semi-fragile watermarking authentication system contains the watermark embedding process and the watermark detection process. The watermark embedding process is shown in Figure 1, and the watermark detection process is shown in Figure 2 (the dashed part may not be needed in some cases).  
In the embedding process, the watermark can be the information that is determined by the contents of the original image, or the information that is irrelevant to the original image.  
In the detection process, the watermarking information in the tested image is extracted and compared with the original watermark. If the two are consistent, we consider that the image has not been tampered with; if the two are inconsistent, we consider that the image has been tampered with. Then the algorithm enters the tamper detection and recovery process. In addition, when the watermarking information is generated using image content features, in the authentication process, there is no need to provide the original watermarking information. Only the comparison between the extracted watermarking information and image content features is needed.  
At present, a lot of semi-fragile watermarking algorithms for content authentication have appeared. In general, to achieve good performance, a semi-fragile watermarking algorithm must meet the following characteristics [5]:  
* Balance between robustness and fragility. On the one hand, after experiencing malicious tampering, such as collage attack, constant average attack, and statistical analysis attack, a watermark should be able to make an alarm response to show its fragility; on the other hand, after experiencing reasonable image-processing operations, such as JPEG compression, brightness adjustment, affine transform, filtering, and noise addition, a watermark should be able to accept them to show its robustness. Therefore, the watermark embedded into the semi-fragile watermarking algorithms must balance the robustness and the fragility well.
* Invisibility. After embedding watermark into the image, the quality of the image cannot have obvious degeneration, and the embedded watermark needs to be invisible to ensure that the original image is not damaged.
* Blind detection. Sometimes people cannot find the original image in many applications, and even if sometimes the original image is provided by a third party, people still cannot believe it. So this requires that content authentication algorithms can achieve the detection process without original images.
* Localization and recoverability.  
Figure 1: Watermark embedding process.  
Figure 2: Watermark detection process.  
If the image has been maliciously tampered with, the algorithm should be able to accurately determine and locate the tampered regions, and then recover it using the recovery data.
* Security. Attacks on the security of watermarks are intentional, and the next step is to threaten the robustness of the watermark. Semi-fragile watermarking algorithms should be able to resist malicious tampering, and cannot be easily detected, imitated, tampered with or fabricated by unauthorized parties.
* Versatility. Versatility, which is also known as ease of use. If a watermarking algorithm has good performance, it should be applicable to a variety of file formats and media formats.