离散分数阶随机变换与加权直方图交叉置乱的双图像加密算法

为了实现对两幅图像进行同步加密，降低传输负载并提高密文的抗明文攻击能力，提出了离散分数阶随机变换与加权像素混沌置乱的双图像加密算法。将2个分阶参数引入到Tent映射中，设计了新的Tent映射；根据明文像素值，构建加权像素直方图模型，联合位外部密钥，生成改进的Tent映射的初值；再利用初值对分数阶Tent映射进行迭代，输出2组随机序列，对2幅明文进行位置交叉混淆，获取2个置乱密文；基于DWT(discrete wavelet transform)技术，对2个置乱密文进行稀疏表示；根据混沌序列，定义随机循环矩阵，联合稀疏表示，获取2个置乱密文对应的测量矩阵。根据随机掩码与调制相位掩码，建立数据融合模型，将2个测量矩阵组合为复合矩阵；基于离散分数阶随机变换，对复合图像进行扩散，获取密文。测试数据显示:与已有的多图像加密方案相比，该算法的抗明文攻击能力与用户响应值更理想，密文的NPCR、UACI值分别达到了99.83%、34.57%。该算法具有较高的加密安全性，能够有效抵御网络中的外来攻击，确保图像安全传输。

Double-image encryption algorithm based on discrete fractionalrandom transform and weighted histogram cross permutation

In order to achieve the synchronously encryption of two images, reduce the transmission load and improve the ability of anti-plaintext attack, a double-image encryption algorithm was proposed by discrete fractional random transform and weighted pixel chaos scrambling. A new Tent map was designed by introducing two order parameters into the Tent mapping. Then the weighted histogram model was constructed according to the plain pixel value to generate the initial value of new tent map by combining with the 256 bits external key. Two sets of random sequences were outputted by using the initial value to iterate the fractional order Tent map to confuse the two pixel positions for obtaining two scrambling cipher. The sparse representation of two scrambling cipher was done by discrete wavelet transform (DWT) technology. The random circular matrix was defined according to the chaotic sequence, and the measurement matrix of two scrambling cipher was obtained by jointing sparse representation. The image fusion model was established based on random mask and modulation phase mask to take two measurement matrices into composite image. The composite image was diffused based on discrete fractional random transform to obtain the cipher. Experimental results show that the proposed algorithm has stronger ability to resist plaintext attack which NPCR、UACI of cipher is up to 99.83%, 34.57%, and higher user response in comparison with current multi-image encryption scheme. This algorithm has high encryption security which can effectively resist external attacks in the network and ensure the safe transmission of images.