Hybrid image encryption based on digital pre-encryption and optical single random phase encoding

In this paper, the optical image encryption scheme based on the double random phase encoding system is modified by introducing a nonlinear digital image pre-encryption coupled with a real to complex conversion. It consists in performing the bit-wise XOR operation recursively between successive pixels of an input image together with chaotic scrambling in the spatial domain. The resulting real-valued pre-encrypted image is halved into two equal parts, one being considered as the real part and the other one as an imaginary part. The complex image thus constructed by concatenating the two previous parts, passes into the second stage of the double random phase encoding where it will be multiplied by a random phase mask and then transformed into a frequency domain by the two-dimensional Fourier transform or any of its derivatives to obtain the encrypted image. The advantage of halving is to save the same information and reduce the size of encrypted image to store or transmit a single complex image instead of double as in all existing based double random phase encoding methods. Results of computer simulations prove the effectiveness of the proposed method toward different attacks and confirm its security when compared to existing works, especially in terms of key sensitivity and histogram analysis.