Image encryption based on CGH by conjugate symmetric extension

An image encryption method based on computer generated hologram (CGH) is proposed. The hologram including amplitude and phase information can be transmitted as a cryptographic image. In making a CGH, the amplitude and phase information are usually an image and random noise, respectively. In this paper, another image is used as phase information instead of random noise. Thus more information can be encrypted. The phase image may be pre-processed by using m-sequence modulation to increase quality of the reconstructed amplitude and phase images. The two images compose a complex amplitude function. The hologram is generated by using conjugate symmetric extension and Fourier transform. In the decryption, the amplitude image is reconstructed directly by inverse discrete Fourier transform (IDFT) of the hologram, while the phase image reconstruction requires the correct key. Because reconstruction can be made from any part of the hologram, holographic image encryption has the capability to resist interference. Experimental results show effectiveness of this method.