In order to improve the robustness and efficiency of medical image encryption algorithm based on chaos, a new robust medical image encryption algorithm based on rapid chaotic scrambling named RMIEF-CS was presented. Firstly, the presented algorithm utilized two low dimensional chaotic systems to generate chaotic sequence with an alternating iterative way, and the problem of chaotic convergence due to computer precision was solved. Secondly, the data stream of plaintext image was firstly scrambled using the generated chaotic sequence, and the ciphertext was scrambled once again using a new chaotic sequence to obtain the final ciphertext image. In the second scrambling procedure, a bidirectional ciphertext feedback mechanism was used to enhance the security and robustness of RMIEF-CS. Because the proposed algorithm used the simple low chaotic system to generate key sequence, and did not need the time-consuming sort operation, it had good time efficiency and could be suitable for images with any shape. The simulation experimental results show that the presented algorithm has better encryption performance, and can decrypt the approximate image to the original medical image even if the ciphertext image has been damaged. In addition, compared with the method based on even scrambling and chaotic mapping, the time consumption of RMIEF-CS is reduced to 1/6. The presented algorithm is suitable for transmitting the medical image with large amount of data in real-time.

Aiming at the problem of low efficiency of tampering detection and accuracy of location, a medical image tampering detection and recovering method based on reversible watermark and quad-tree decomposition was proposed. The algorithm has higher accuracy and faster tampering location speed by using the hierarchical structure of the quad-tree in the decomposition of the medical images. The method used the diagonal pixel mean in the block as the recovered feature value, which ensures the recovery quality of tampered image. The experimental results show that compared with the existed methods, the proposed algorithm reduces the comparing times for locating tampered region to about 6.7 in the 512×512 images and improves the tampering detection accuracy about 5%.

In order to solve the problem of privacy disclosure in medical image's interest of region, a new reversible visible watermarking based privacy detection algorithm was proposed. The method embedded a binary watermark image into the interest of region of original medical image to protect privacy, and used visual masking of Human Visible System (HSV) and pixel's mapping mechanism to dynamically adjust the visibility and transparence of watermark. In addition, a shrinking projection technology was utilized to solve the problem of potential overflow and underflow during the embedding procedure. Finally, a random key was introduced to enhance the embedded watermark's robustness. The experimental results show that the proposed method achieves better performance in visibility and transparence of watermark, and the number of additional information produced by embedding is only 65608bits. In addition, the deletion of watermark is difficult without knowing the correct key, and the quality difference between the watermarked image and the recovered image is less than 1dB.

In order to solve the shortage of the method based on fragile watermarking, a new chaotic system and fragile watermarking based image tamper detection algorithm was proposed. Firstly, the method used Arnold cat map k times to scramble the original image, and then chose the Least Significant Bit (LSB) plane of the scrambled image as the embedding position. The actual watermark information was formed by using exclusive-or operation between a random binary sequence produced by Logistic map and the original watermark, and the LSB replacement method was utilized to embed watermark. Finally, the watermarked image was obtained by utilizing Arnold cat map T-k times. The experimental results show that, the introduction of chaotic system to a great extend improves the security of the proposed scheme. In addition, the proposed method achieves superior tamper detection and localization accuracy under different common attacks.

Identity-based signatures are the groundwork of many cryptographic protocols. This paper analyzed GU KE et al.'s (GU KE, JIA WEIJIA, JIANG CHUNLIANG. Efficient and secure identity-based signature scheme. Journal of Software,2011,22(6):1350-1360) efficient identity-based signature scheme. Two equivalent signature generating algorithms were proposed and it was pointed out that Gu et al.'s scheme could not satisfy the basic security properties. In other words, any attacker could use the equivalent secret key and signature generating algorithms proposed in this paper to forge a valid secret key of a user and a valid signature on any message with respect to any identity in their scheme. Furthermore, the reason that the scheme is insecure was also analyzed and it was pointed out that designing a more efficient identity-based signature scheme than the classical one is almost impossible.

A new reversible data hiding algorithm for medical images was proposed. The hidden information was embedded into Region Of Interest (ROI) and non-interest respectively. In ROI, an adaptive integer transform scheme was employed to enhance the embedding capacity and control distortions. And in Region of Non-Interest (RONI), the classical Least Significant Bit (LSB) method was used to keep the marked image’s quality. The experimental results show that, compared with previous works, the performance of the proposed method has been significantly improved in terms of capacity and image quality. The proposed method’s embedding capacity is between 1.2bpp and 1.7bpp, while the Peak Signal-to-Noise Ratio (PSNR) can maintain the 43dB or so. Moreover, the proposed method with high run efficiency can be applied into the practical hospital information system.