基于码分多址复用的双重加密可逆信息隐藏

针对多数密文域可逆信息隐藏算法嵌入容量小、加密算法单一的问题，提出一种双重加密的方法，并利用码分多址复用（CDMA）的思想嵌入秘密信息。加密时将图像分块，先对像素块进行多粒度置乱加密，再对块中每个像素的中间2位用流密码加密。信息嵌入采用码分多址的思想，选取k个长为4的相互正交的矩阵嵌入k层秘密信息，利用矩阵的正交性实现秘密信息多层嵌入，在提高嵌入容量的同时保证了对像素点的较小改变。对不满足嵌入条件的像素块嵌入伪比特，可避免使用位置图。拥有信息提取密钥的合法接收者可以提取秘密信息；拥有图像解密密钥可以近似恢复原始图像；拥有两种密钥既可提取秘密信息又可无损恢复原始图像。实验结果表明，512×512灰度图像Lena在峰值信噪比（PSNR）大于36 dB时最大嵌入容量133 313 bit。所提算法增强了加密图像安全性，在保证可逆性的同时大大提高密文域可逆信息隐藏嵌入容量。

Double-level encryption reversible data hiding based on code division multiple access

Aiming at enhancing the embedded capacity and enriching the available encryption algorithm of reversible data hiding in encrypted domain, a new scheme was proposed by adopting double-level encryption methods and embedding the secret information based on Code Division Multiple Access (CDMA). The image was first divided into blocks and a multi-granularity encryption was introduced. The image was first divided into blocks, which were scrambled by introducing multi-granularity encryption, then 2 bits in the middle of each pixel in blocks were encrypted by a stream cipher. Based on the idea of CDMA, k mutually orthogonal matrices of 4 bits were selected to carry k-level secret information. The orthogonal matrices can guarantee the multi-level embedding and improve the embedding capacity. The pseudo bit was embedded into the blocks that cannot meet the embedding condition. The secret data could be extracted by using the extraction key; the original image could be approximately recovered by using the image decryption key; with both of the keys, the original image could be recovered losslessly. Experimental results show that, when the Peak Signal-to-Noise Ratio (PSNR) of gray Lena image of 512×512 pixels is higher than 36 dB, the maximum embedded capacity of the proposed scheme is 133313 bit. The proposed scheme improves the security of encrypted images and greatly enhances the embedded capacity of reversible information in ciphertext domain while ensuring the reversibility.