A method for image encryption by employing the diffraction imaging technique is proposed. Only a single diffraction intensity image is recorded without using interference equipment that makes the method much more efficient, and the transmision of the single ciphertext more convenient. Before encrypting, the redundant data of the original binary image are extracted as the secret key, then the original binary image is encrypted by using optical diffraction image encryption method and the diffraction intensity image is the ciphertext. During image decryption, the redundant datas serve as partial input plane support constraint in phase retrieval algorithm iterative operation, which is employed for making algorithm convergences fastly. Computer simulation results verify the validity of the proposed approach. Its robustness against occlusion and noise attacks are also analyzed.