A recent survey of self-embedding fragile watermarking scheme for image authentication with recovery capability <s> Segmented blok size <s> Abstract The original hierarchical watermarking scheme for image tamper detection and recovery has simple computation and high performance precision which can achieve 2 × 2 subblock. However, four-scanning and blind attacks have been proposed recently on this scheme. We generalize these attacks and analyze the cause of security flaws. We think that it is promising to improve the original scheme's security and keep its merit at the same time. In order to defeat these attacks, we develop an improved method to generate a block mapping sequence by sorting the chaotic sequence and add the chaotic encryption and permutation based on the exact content of each subblock to be the post-processing of the 3-tuple watermark. Our method uses the simple watermarking scheme and satisfies the performance requirements of fragile watermarking, such as high-precision tamper detection, localization and recovery. Theoretical analysis and computer simulation demonstrate that our proposed scheme is much more secure and can overcome possible attacks. <s> BIB001 </s> A recent survey of self-embedding fragile watermarking scheme for image authentication with recovery capability <s> Segmented blok size <s> Abstract To take invisibility and restoration quality into account, this paper proposes an alterable-capacity watermarking scheme. For each block of size 8 × 8 pixels, the alterable-length code is generated based on the roughness of it. The alterable-length watermark generated by the alterable-length code is divided into three parts and embedded in other three blocks based on the secret key. The authenticity of each block is determined by comparing the watermark reconstructed by the block content and the corresponding extracted watermark. To improve the quality of recovered images, two copies of the significant-code of each block are embedded in different blocks and the image inpainting method is adopted to recover the tampered blocks whose significant-code embedded in other blocks is destroyed. The alterable-payload watermark preserves adequate information of image blocks especially for texture images with as few bits as possible and takes into account invisibility, security and restoration quality. Experimental results demonstrate that the proposed scheme improves the quality of watermarked and reconstructed images and is resilient to the known forgery attacks. <s> BIB002 </s> A recent survey of self-embedding fragile watermarking scheme for image authentication with recovery capability <s> Segmented blok size <s> A fragile self-recovery watermarking scheme with simple and effective tamper detection capability is proposed in this paper. In conventional fragile watermark for tamper detection and recovery, the parity section of the watermark is used for tamper detection and the restoration section of the watermark is used for image recovery, separately. In addition, to provide second chance for block recovery in case one copy is destroyed, Lee and Lin proposed dual watermarking scheme in which two copies of restoration watermark are embedded. In the proposed new scheme, fragile watermark with one parity section and two restoration sections are embedded, too. In addition to the second chance for image restoration, the two restoration sections as well as the parity section are all used for tamper detection. Experimental results show that the tamper detection capability is superior to other techniques. <s> BIB003 </s> A recent survey of self-embedding fragile watermarking scheme for image authentication with recovery capability <s> Segmented blok size <s> The paper proposes a modified self-embedding watermarking scheme of the Fan Chen & Hongjie He [4] that improves the quality of the recovered image. The watermarking bits are of variable length that best approximates 5 MSB of the pixel values of a 2 × 2 block of original image compared to average intensity of the block. There are 7 different schemes that help to calculate the watermarking bits for a 2 × 2 block, including the average intensity of the block. That is, when the watermarking bits of a 2 × 2 block survive an attack, the greatest deviation of the recovered block from the original block is not greater than the deviation when the block is recovered using its average intensity. Thus the PSNR between the original and recovered image is relatively higher compared with the one when image recovery used only the average intensity of the block. The experimental results confirm this fact. <s> BIB004
Many researchers have conducted research in the modification of block size based on fragile watermarking. The watermark embedding process is shown in Fig. 2 . In this procedure, most of the researchers divided the original image into blocks of uniform size (n × n pixel). So far, they have investigated experiments with blocks of different size, i.e. 2 × 2 pixels  , 3 × 3 pixels BIB003 , 4 × 4 pixels BIB001 and 8 × 8 pixels  . These blocks were used to generate more watermarks, as authentication and restoration bits. In 2016, Singh et al.  developed a scheme using a small non-overlapping block sized 2 × 2 to improve the accuracy of localisation and effectively remove the blocking artefacts as illustrated at Fig. 4 . The original image X of size m × n is divided into non-overlapping blocks of size 2 × 2  . This argument was also expressed by Sreenivas et al. BIB004 , stating that small block sizes generally allow for better tamper detection. This also allows better block encoding when textured blocks are smooth. Meanwhile, to localise tampering, the original image is divided into blocks of size 3 × 3 BIB003 . Xiao's method BIB001 divides an image into 4 × 4 non-overlapping blocks, generates authentication watermarks for the blocks by comparison and parity check among average intensities and embeds them into corresponding blocks. The recovery information of another block is embedded into its mapping block. Huo et al.'s scheme BIB002 proposes the recovery of discrete cosine transform (DCT-based) alternate self-recovery fragile watermarking scheme. The image was divided into non-overlapping sub-blocks with 8 × 8 sizes and sub-blocks classified into different types according to block variance. Experiments in  also show that the scheme can detect and localise damage by 8 × 8 pixels and can recover 40% tampered image. Chetan et al.  calculated the tamper detection accuracy and values were recorded for blocks of some various sizes. The smaller the block size is, the more accurate the tamper localisation is.