RISC: a robust image symmetric cryptosystem

In recent years, AES is known as the excellent choice between the existing symmetric cryptosystems to encrypt binary or text data. However, this standard encryption algorithm is unsuitable for images regarding the particular features of these kinds of data such as high correlation, high redundancy, and voluminosity. Based on this fact, we have designed a new cryptosystem taking into account the aforementioned characteristics which provides high performance and security level. Actually, the cryposystem adopts a new structure named Outer-Inner structure. This new structure consists of two phases: Outer phase and Inner phase. In the former phase, the image is treated as a single block. The purpose of this phase is to address the correlation and the redundancy issue of images. In order to ensure the efficiency and to avoid the problem of treating an image as a single block where several image encryption algorithms suffer from it, this phase only involves lightweight operations. In the later phase the image is treated as a set of fixed blocks. The aim of this phase is to address the voluminosity issue. In fact, during this phase each block can be encrypted independently through an iterative function. The experiment results show that 7 rounds are sufficient to reach an acceptable security level.