# Review on Semi-Fragile Watermarking Algorithms for Content Authentication of Digital Images
## 5 Overview on Semi-fragile Watermarking Algorithms
### Recoverable Semi-Fragile Watermarking Algorithms
#### 5.2.2 DWT Domain  
In this field, [PERSON] and [PERSON] and [PERSON] (2017) proposed a low-frequency semi-fragile watermark according to the properties of the DWT. The watermark is embedded into the high-frequency band through the HVS. This algorithm can accurately detect and locate malicious attacks and recover them. In addition, the algorithm can accept moderate modifications, like JPEG compression and AWGN, but does not involve any other types of non-malicious tampering. In [PERSON] et al. (2017), [PERSON] et al. proposed a watermarking algorithm combining a cat map with discrete wavelet decomposition approach to ensure security. This algorithm embeds the approximate coefficients of discrete wavelet decomposition, which contain main features of image blocks, in the detail coefficients of the other block, and establishes the association between the two blocks by the cat map. In this way, even if the image is tampered with, the original information can still be retained. In addition, the algorithm can resist moderate tampering, such as JPEG compression, resize, noise addition, and filtering. It can also detect and locate the tampered areas accurately, and recover most of the areas in the original watermarked image.  
As block-based semi-fragile watermarking technology is susceptible to local tamper attacks, and the detection accuracy of tampering is often limited to the size of the block, [PERSON] et al. [PERSON] et al. (2017) proposed a semi-fragile watermarking algorithm based on wavelet domain with two watermarks. The algorithm uses the random key to permute the wavelet coefficients, and then embeds the authentication watermark. If the key cannot be known, the attacker cannot tamper the watermarked image under the premise that the watermark cannot be destroyed. In addition, the algorithm not only has good robustness for common image processing operations, for malicious tampering it also has high detection accuracy and recovery ability. However, when the tampered areas of the host image exceed to 20%, the recovered image may have poor quality and even cannot be recovered.  
Since many existing semi-fragile watermarking algorithms cannot meet important requirements including robustness, security, tampering location and recovery at the same time, [PERSON] et al. [PERSON] et al. (2018) proposed a novel semi-fragile watermarking algorithm based on group quantization and double authentication. The authentication watermark is extracted from first-order statistical matrix of each image block and embedded into medium-frequency regions of other image blocks. Among them, the embedding process is based on a new group wavelet quantization approach. The algorithm increases the security by permuting the wavelet coefficients in a group randomly. To improve the robustness, the algorithm embeds the watermark into the maximum coefficient in the sub-group by the means of significant difference parity quantization. To improve the accuracy of tampering localization, the algorithm proposed a double authentication ring structure. Since the algorithm is a block-dependent watermarking algorithm, it can resist VQ and collage attacks. The algorithm has medium accuracy ability for tampering localization, but it also can recover the tampered areas. However, the quality of the recovered image needs to be improved further.