为提高光学图像加密系统的安全性, 利用双光楔联合菲涅耳变换相关器和矢量分解设计了一种非线性光学图像加密系统。通过矢量分解将原始图像分解为两个相位模板, 其中一个相位模板f1(x)放置于双光楔联合菲涅耳变换相关器物窗口实现图像加密; 携带另一相位模板f2(x)信息的光束与解密系统输出的携带f1(x)信息的光束相干叠加得到解密图像。数值模拟了加密系统的加、解密过程, 对于灰度图像和二值图像, 当光楔楔角为1.8°和相位模板f1(x)与密钥k(x)的中心间距为18 mm时, 解密图像与原始图像的相关系数分别为0.812 7和0.810 9; 分析了密钥模板相位分布错误对解密效果的影响, 验证了加密方法的可行性。模拟分析表明, 密钥k(x)的位置和光楔楔角作为附加的密钥参量, 有效扩展了加密系统密钥空间, 并能抵御唯密文攻击、已知明文攻击和选择明文攻击。

To improve the security of optical image encryption system, a nonlinear optical image encryption system based on double-optical-wedge joint Fresnel transform correlator and vector decomposition was proposed. The original image was decomposed into two phase masks. One of them, f1(x), was located at the reference window to realize the image encryption; the wave containing the information of the other phase mask, f2(x), interferenced with the decrypted output wave containing the information of the mask f1(x) to obtain the decrypted image. The encryption and decryption processes were numerically simulated. For both grayscale and binary images, the correlation coefficient between the decrypted and original image is 0.812 7 and 0.810 9, respectively, when the wedge angle is 1.8° and the distance between the centers of the mask f1(x) and the key code k(x) is 18 mm. The feasibility of the encryption method was verified by the simulation of the decryption result using incorrect key code with phase distribution error. Analysis results show that the position of the key code k(x) and the wedge angel are proved to be the additional key parameters and can effectively expand the key space. The encryption system is able to defend ciphertext-only attack, known-plaintext attack and chosen-plaintext attack.