Selective medical image encryption based on 3D Lorenz and Logistic system

Medical images contain very sensitive and confidential information about the patient and are huge in terms of size. Therefore, there is a need to develop encryption schemes that reduce computational time without compromising the security level. Selective image encryption is one such technique that can reduce time and complexity. In this paper, we propose a selective image encryption scheme that can be used to encrypt medical images. The medical image is first decomposed into non-overlapping blocks, and then the variance of each block is calculated and compared to a preset threshold value. The blocks for which variance is greater than the threshold value are considered significant and encrypted. The pixels within each significant block are permutated with a hybrid sequence (which is different for each block) obtained from Logistic maps. The diffusion operation is performed using a different hybrid sequence obtained from a 1-D Logistic map and a 3-D Lorenz map, which also is different for every block. Finally, the cipher image is formed by concatenating the encrypted significant and insignificant blocks. The proposed encryption scheme has been tested on several grayscale and color medical images to validate its performance in terms of security provided and computational time. It has been found resilient to differential cryptanalysis, as the NPCR and UACI values are greater than 99.60% and 33.20%, respectively besides having a large key space of the order of 10(108). The PSNR values for all encrypted images are less than 12 dB even though the images are selectively encrypted, and the entropy of the Region of Interest (ROI), i.e., encrypted blocks, is nearly equal to 8, which signifies a better security of our system. The computational time required to encrypt an image of sizes 225 x 225 and 512 x 512 is 0.06 and 0.10 s respectively making it suit-able for real time applications. The experimental results show that the proposed encryption scheme provides high security with less complexity and computational time