Medical image cipher using hierarchical diffusion and non-sequential encryption

This paper presents a medical image cipher with hierarchical diffusion and non-sequential encryption mechanism. An improved permutation approach is developed, which can contribute to not only image shuffling but also lightweight pixel modification at the same time. Therefore, diffusion property of the proposed scheme is achieved from both the permutation and diffusion phases, and hierarchical diffusion effect is thus obtained. Besides, non-sequential visiting mechanism is developed to encrypt the plain pixels in secret order; in other words, pixel visiting order is exploited as extra encryption factor in addition to the traditional pixel location and gray value. With hyper-chaotic Lorenz system employed as key stream generator, a complete image encryption scheme is finally constructed. Experimental results and security analyses validate the effectiveness and superiority of the proposed cipher.