为了保证数据的安全性，在网络中传输的图像数据常需要进行加密处理。在这一过程中，不但要考虑加密算法的安全性，还需要考虑加密算法对码率的影响，以及图像传输的实时性要求。提出了一种基于二次置乱的图像加密算法，该算法将图像按一定的大小划分成若干宏块，对每一行的宏块进行置乱。然后在离散余弦变换的基础上，对量化以后每个8×8矩阵随机选取前任意数量的交流（AC）系数并进行置乱。理论分析和实验结果表明，该算法既保证了网络传输过程中图像内容的安全性，又在码率、运算复杂度上有很好的性能，平均码率增长6.478％，满足网络传输的实时性要求。同时，该算法满足格式兼容性并具有一定的容错能力，适合作为基于网络传输的图像加密算法。

Encryption algorithm is used to ensure the transmission safety of multimedia data. Not only the security of algorithm, but also the compression efficiency and encryption complexity should be taken into consideration when design an encryption scheme. A new encryption scheme for image transmission is proposed in this paper, which is based on duple permutation. Firstly the original image is divided into several macro blocks in the light of certain size, and macro block within each row are permuted. Then, the first random S of AC coefficients in each 8×8 matrix are permuted after DCT （discrete cosine transform)and quantization. Theoretical analysis and experimental results indicate that the scheme proposed in this paper is secure and fast. It has little adverse impact on the compact ratio, 6.478％ by average. Moreover, it is format compatible and robust to transmission errors. All the attributes mentioned above make this algorithm suitable to be the encryption approach for image transmission.