# A Hybrid Watermarking Approach for DICOM Images Security
## 2 Related Works  
Various image watermarking methods have been proposed in the literature to achieve image authentication. In this section, we present some of them. The authors in [4] ([PERSON] et al.) proposed a fully reversible watermarking method combining the advantages of three approaches: reversible, zero, and RONI watermarking. The method is based on the separation of the original image into ROI and RONI parts. The ROI of the original image and the watermark are combined to generate a'secret share' using a zero-watermarking approach. The'secret share' is embedded as a watermark into the RONI part of the original image using a reversible watermarking method called reversible contrast mapping (RCM). Further, the original ROI and the watermarked RONI are concatenated to generate a watermarked image. The imperceptibility ratio in terms of SSIM reached 0.99, and the PSNR was around 81. The authors in [15] ([PERSON] et al.) proposed a reversible tamper detection and recovery watermarking method. The watermarking method applied the concept of the separation of ROI and RONI. The separation of ROI and RONI is done manually by using a rectangle to define the ROI of the medical image. Four rectangles are used to form a pyramid shape for the ROI, and six rectangles are used to define areas in the RONI. The ROI is divided into blocks of 8 \(\times\) 8 pixels and RONI into blocks of 6 \(\times\) 6 pixels. A one-to-one mapping technique is used to assign a unique integer to each block in the ROI and RONI. Each block in RONI corresponds to a block in ROI. The number of blocks in the ROI must not exceed the number of blocks in the RONI to ensure that the original LSBs that will be removed from the ROI for watermark embedding can be stored in the RONI without any storage issue. [PERSON] et al. [16] presented a reversible region of non-interest (RONI) watermarking to verify the authenticity and integrity of DICOM images. This method is based on the separation of ROI and RONI also. The watermark is generated from the ROI by creating a hash value of the whole image, using SHA-256. After that, the watermark is embedded in the least significant bits (LSB) of the RONI. This method is reversible. If the image has not been altered, the watermark will be extracted and the original image will be recovered and the SHA-256 of the recovered image will be compared with the extracted watermark for authentication purposes. Experimental results showed that the PSNR was equal to 32 dB. In [17], [PERSON] et al. described a fragile reversible watermarking method of radiographic images for integrity and authenticity control for telemedicine applications. Quantum noise is computed using half of the wavelet coefficients of the anatomical object. The method uses a classification process to differentiate the image's black background from the anatomical object which is the region of interest. From the image background, histogram shifting HS is applied to pixels; and from the anatomical object, HS is applied to wavelet detail coefficients, masking the watermark into the image quantum noise. The experimental results showed that the PSNR was equal to 76.5 dB and the SSIM was equal to 0.99. [PERSON] et al. [18] described a recovery algorithm to confirm the integrity and authenticity of medical images. The whole image is divided into two regions, ROI and RONI. The sparse coding of the EPR data and the reshaped ROI are jointly embedded in the transform domain of the RONI using singular value decomposition (SVD). In the first part of the sparse coding, the patient information was saved along with the image, while the second part was used for verifying the authenticity. The hidden sparse coded ROI can be extracted to reconstruct the altered image. This technique can be used for several purposes such as EPR data hiding, authentication of the ROI, and retrieving the manipulated area. Experiment results showed that the PSNR was equal to 69.82 dB and the NC was equal to 0.80. [PERSON] et al.