Hyper-Chaotic Cryptosystem Using Model-free Synchronization

In this paper a hyper-chaotic cryptosystem is proposed based on two encryption processes: the image substitution permutation cryptography process followed by masking of the ciphered image. In the substitution-permutation cryptography process, the selective encryption using exclusive or operation is performed in the frequency domain, and the image is further encrypted in the spatial domain using the permutation encryption rule and keys generated by the Qi hyper-chaotic system. The ciphered image is changed into a time varying series which is fed into the hyper-chaotic transmitter as a driving signal. The unmasking process is proposed based on a model-free multivariable high order differential feedback controller used to synchronize the driven and the response systems of our scheme. Analysis and result have shown that the model-free controller offers a robust synchronization and the two encrypting processes for image increase the image security.