## 1 Introduction
However, this method was applicable for encrypting a large amount of data. By using an independent component analysis, an easy process to derive an effective method for image security was proposed [11]; however, the proposed encryption algorithm can only be applied to square images. Recently, since the concept of chaos synchronization was introduced by [PERSON] and [PERSON] [12], research into chaos synchronization and its application in secure communication has become an important issue. Many approaches for controller design have been proposed in order to achieve synchronization for both continuous and discrete chaotic systems, such as backstepping design [13; 14], fuzzy control [15; 16], and discrete sliding mode control [17]. Among these proposed approaches for control design, the sliding mode control method is frequently used because it has an inherently good robustness, especially for matched disturbances. On the other hand, the advanced encryption standard (AES) is a well-known symmetric cryptosystem using a static key, and it has been widely realized and fully supported in both hardware and software environments. With a similar structure to AES, there are five different encryption methods, namely AES-CBC, AES-ECB, AES-CFB, AES-OFB, and AES-PCBC [18]. Unill now, no available cryptanalytic attacks against AES have been reported. Additionally, AES is also the most commonly used encryption method in Internet of Things (IoT) data transmission encryption [19], because of its high security property. However, in [20], a suite of algorithms was designed to successfully deduce the 128-bit AES key using cache access attacks. Furthermore, in such a symmetric encryption method, a static key was used. When this static key is stolen, information will be fully exposed. In addition, with the recent introduction of IBM's quantum computer with 53 qubits [21], such an ultra-high computing speed would cause the existing AES encryption to be cracked. Motivated by this, this study aimed to develop a modified chaos-based AES algorithm with dynamic synchronized random keys. Firstly, a control scheme was proposed to solve the problem of synchronization for master-slave chaotic systems. Under synchronization, the same random chaos signals could be simultaneously obtained at the transmitter and receiver in communication systems. Then, the dynamic and unpredictable features of synchronized chaotic signals were extended to design a synchronized dynamic key generator for improving AES cryptosystems. These random keys were dynamically updated and hidden. Unlike the traditional AES algorithm, these keys did not need to be saved in advance, so security could be promoted. To verify the security, the developed chaos-based AES (CAES) algorithm was applied to construct a novel image encryption algorithm. The statistical analysis, histogram, information entropy, and correlation indexes were provided and examined through the simulation experiment in order to demonstrate the efficiency and security of the presented CAES algorithm. This paper is organized as follows. Section 2 formulates chaos synchronization and the design of a dynamic chaos-based random key generator. The synchronization controller is proposed using discrete sliding mode control. Numerical simulations are given to illustrate the derived results. Section 3 introduces the structure of the improved CAES algorithm. Section 4 proposes and analyzes the performance of the image cryptosystem based on the proposed CAES. Finally, brief conclusions and future works are given in Section 5.