Key Laboratory of All Optical Network and Advanced Telecommunication Network of Ministry of Education, Institute of Lightwave Technology, Beijing Jiaotong University, Beijing 100044, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 61975009, 61827818, 61775015)
Received Date:25 July 2019
Accepted Date:19 November 2019
Published Online:05 February 2020
Abstract:In order to ensure the secure and effective transmission of image information, a new method of optical image encryption using the multimode fiber (MMF) specklegram based compressive sensing combined with the double random phase encoding (DRPE) is proposed in this paper. The specklegrams obtained from the facet of the multimode fiber are used as the measurement matrix of compressive sensing (CS), and the compression and the first-stage encryption of the image are completed by compressive sensing, in which the specklegram also functions as the first secret key. Then, the second-stage encryption is implemented by using the double random phase encoding technology, in which the random phase mask acts as the second secret key. All of the specklegrams used in this paper are obtained from the facet of a 5 m-long and 105-μm-diameter-MMF and offset launching technique. Then the fiber specklegrams are proposed in several steps to provide the measurement matrix in CS. By performing an encryption and decryption test on a standard Lena image of 256 × 256 size, it is found that the decrypted image and the original image are visually consistent, and the encryption is also realized in the process of compression, which indicates the method proposed in this paper is feasible. Furthermore, the comparison studies of the performances of specklegram based measurement matrix and some classic measurement matrices show that the decrypted image quality using the specklegram matrix is better. And at the same time, comparing with the high hardware implementation complexity and high cost of other measurement matrices, specklegram based matrix can be easily realized by simple optical device, and the corresponding secret key can be easily changed by the working wavelength, which is helpful for enlarging the secret key space. It is further proved that the encryption method be able to effectively resist the statistical analysis attacks, cropping attacks and noise interference, and also have high sensitivity to the secret key, which shows good robustness and high security. Therefore, the image encryption method combined with the specklegram matrix based compression sensing with the optical DRPE can obtain good encryption effect and has a great secret key space, which may provide a good candidate scheme for the pure optical realization of image encryption. Keywords:multimode fiber specklegram/ compressive sensing/ optical image encryption/ double random phase encoding
式中$M \times N$为原始图像的大小, $g\left( {m, n} \right)$为解密图像, $f\left( {m, n} \right)$为原始图像, k表示图像像素灰度值的位数, 通常灰度图像的k为8, 也即有256个灰度级. 图5中特别给出了在不同压缩比情况下采用光斑矩阵和高斯矩阵时对应的解密图像, 另外也给出了使用其他测量矩阵时对应解密图像的PSNR随压缩比的变化曲线. 可以看出使用光斑测量矩阵时解密图像质量更好. 图 5 光斑矩阵和高斯矩阵对比分析 (a)?(d)使用光斑矩阵在压缩比为0.3, 0.5, 0.7, 0.9时的解密图像; (a')?(d')使用高斯矩阵在压缩比为0.3, 0.5, 0.7, 0.9时的解密图像; (e)使用不同测量矩阵时对应解密图像的PSNR随压缩比的变化 Figure5. Comparative analysis of specklegram matrix and Gaussian matrix: (a)?(d) The decrypted image using specklegram matrix at compression ratio of 0.3, 0.5, 0.7, 0.9; (a')?(d') the decrypted image using Gaussian matrix at compression ratio of 0.3, 0.5, 0.7, 0.9; (e) comparison between the PSNRs of the decrypted images varying with the compression ratio when using different measurement matrices.
表3加密图像像素相关系数 Table3.Correlation coefficient of encrypted image pixels.
24.3.抗噪声分析 -->
4.3.抗噪声分析
考虑到密文图像在现实通信环境中容易受到噪声干扰, 也对本方法的抗噪声性能进行了分析, 分别对密文图像添加均值为0, 方差为0.1到0.9的高斯白噪声后进行解密, 图8(a)—(d)给出了密文图像中噪声方差为0, 0.1, 0.3和0.5时的解密图像, 图8(e)为噪声方差和解密图像PSNR的关系图, 可以看出加噪声后的解密图像与不加噪声后的解密图像质量基本相同, 由此可见该方案具有抗噪声干扰的鲁棒性. 图 8 抗噪声分析 (a)?(d)在密文图像中分别加入方差为0, 0.1, 0.3和0.5的噪声时的解密图像; (e)密文图像中加入噪声后的解密图像PSNR随相应噪声方差的变化 Figure8. Anti-noise analysis: (a)?(d) Decrypted images with noise of 0, 0.1, 0.3 and 0.5 variances added to ciphertext image respectively; (e) curves of relationship between noise variance and the PSNR of decrypted image with noise in ciphertext mage
24.4.抗剪切分析 -->
4.4.抗剪切分析
还对本方法的抗剪切能力进行了分析, 分别对密文图像从水平、垂直、中心和边角4个方向进行不同程度的剪切攻击, 图9给出了受到剪切攻击的密文图像及其对应的解密图像. 可以看出, 解密图像中依然能够分辨出主要的图像信息, 表明本方法能够有效抵抗剪切攻击. 图 9 水平、垂直、中心和边角方向不同程度的剪切攻击和剪切攻击后的解密图像 (a)垂直剪切10%; (b)垂直剪切50%; (c)水平剪切10%; (d)水平剪切50%; (e)中心剪切; (f)边角剪切; (a')垂直剪切10%解密图; (b')垂直剪切50%解密图; (c')水平剪切10%解密图; (d')水平剪切50%解密图; (e')中心剪切解密图; (f')边角剪切解密图 Figure9. Cropping attack of different degrees in horizontal, vertical, central, corner directions and decrypted image after cropping attack: (a) Vertical cropping 10%; (b) vertical cropping 50%; (c) horizontal cropping 10%; (d) horizontal cropping 50%; (e) central cropping; (f) corner cropping; (a') decrypted image after vertical cropping 10%; (b') decrypted image after vertical cropping 50%; (c') decrypted image after horizontal cropping 10%; (d') decrypted image after horizontal cropping 50%; (e') decrypted image after central cropping; (f') decrypted image after corner cropping.
24.5.光斑密钥敏感性分析 -->
4.5.光斑密钥敏感性分析
密钥敏感性也是加密方法的一个重要性能指标. 图10给出了原始光斑密钥及对应的解密图像, 以及修改后的光斑密钥及对应的解密图像, 图中两个光斑是利用不同的工作波长(波长差为0.1 nm)获得的, 可以发现从解密图像(图10(d))中分辨不出原始图像的内容. 为了比较解密图像和原始图像之间的差异, 引入了均方误差(mean squared error, MSE), 定义为 图 10 光斑密钥敏感性分析 (a)原始的光斑密钥; (b)修改后的光斑密钥; (c)与(a)相对应的解密图像; (d)与(b)相对应的解密图像; (e)使用1550?1551.9 nm (间隔为0.1 nm)工作波长产生的光斑进行解密的MSE曲线; (f)对应于(e)中使用的实验测得的不同工作波长光斑 Figure10. Specklegram key sensitivity analysis: (a) Original specklegram key; (b) modified specklegram key; (c) decrypted image corresponding to (a); (d) decrypted image corresponding to (b); (e) MSE curve for decryption using specklegram generated at different wavelengths; (f) the corresponding specklegram at 1550?1551.9 nm with a wavelength interval of 0.1 nm.