## 3 Design of the Improved AES Algorithm  
AES encryption is a popular symmetric-key cryptosystem. The main architecture of the AES encryption algorithm is given in Figure 6. In order to generate a uniform distribution of cipher text, a series of linked operations were included in AES cryptosystem design. Some of the linked operations were performed by substituting the inputs using some output feedback (substitutions). The left operations involved shuffling many bits around (permutations). AES runs computations using bytes. Therefore, the AES separated 128 bits of information block into 16 bytes. These 16 bytes were then formatted as a matrix form with four columns and four rows in order to process the operations. The operation rounds in the AES cryptosystem were dependent on the key length we selected. Here, 10 rounds, 12 rounds, and 14 rounds, were used for the key lengths of 128-bit, 192-bit, and 256-bit, respectively, as shown in Figure 6. Using different round keys, \(K_{R}\), derived from the original static AES key, each of these rounds was performed, where \(R\) was the round.  
Figure 4: Time responses of the switching surface (\(s(k)\)) and controller (\(u(k)\)).  
Figure 5: The dynamic random key generator with SHA256.  
For AES symmetric-key cryptosystems, the keys represented a shared secret in the communication systems that could guarantee private information transmission. However, compared with asymmetric key encryption, one of the main weaknesses of symmetric key cryptosystems was the requirement of the secret key [25]. Once this static secret key is stolen, all information will be exposed. Therefore, to solve this problem, in this paper, a new CAES encryption method was proposed based on synchronized chaotic dynamic keys, as shown in Figure 7.  
In this proposed improved chaos-based AES architecture, the dynamic random states of synchronized chaos systems were used as the secure keys. Obviously, this improved design could provide synchronized random states to a random key generator and then generate dynamic random keys with 256 bits for AES to promote its security. By introducing a designed controller to the slave chaotic system, the slave states could be synchronized with the master states. Furthermore, to promote the security of practical applications, the synchronization controller (\(u(k)\)) was decomposed into \(u_{m}(k)\) and \(u_{s}(k)\), calculated from the master and slave chaotic systems, respectively. The controller was then realized at the receiver side with \(u(k)=F(u_{m}(k),u_{s}(k))\). Even if hackers had these parameters in the synchronization controller, they would still not be able to obtain the states of the master and slave systems separately, and they would not be able to reorganize the synchronization  
Figure 6: Advanced encryption standard (AES) architecture.  
Figure 7: Improved chaos-based AES architecture.  
controller. Therefore, the security of the dynamic keys and the encryption system would be ensured. After achieving synchronization, the same random chaos signals could be simultaneously obtained, and the synchronized dynamic random keys at both sides of transmitter and receiver could be constructed. The static keys in the traditional AES were replaced by synchronized random signals, and did not need to be given in advance or be communicated through public channels. As a result, the disadvantage of key storage could be eliminated and the security of the encryption could be improved because of the synchronized dynamic random keys.