由于图像、音频、视频等媒体数据具有数据量大，实时性要求高等特点，这对于加密算法也提出了相应的要求，而且由于采用传统的基于数论的加密算法对数据完全加密不能满足实时性要求，因此产生了对部分加密算法的研究，但这些算法也不能完全保证数据格式不变和压缩比不变等更高的要求；如果将压缩编码和加密过程相结合，则既能满足实时性要求，又能保持数据格式不变。鉴于四叉树结构常被应用于图像编码中，为此提出了两种四叉树置乱算法，并将其用于四叉树编码、小波零树编码、基于四叉树的分形编码等图像和视频编码中。理论分析和实验结果表明，由于这些算法将加密与编码过程相结合，因此具有以下优点：具有较高的安全性，且加密效果好；不仅能保持编码格式不变，而且便于对数据的直接操作；既能保持压缩比不变，又不增加额外数据；具有较快速度，能满足实时性要求等。

For multimedia data is often voluminous and requires real time operations, multimedia encryption algorithms should meet extra requirement compared with textual data encryption algorithms. Traditional algorithms such as DES, RSA, can't meet some real time requirement. Some selective encryption algorithms can't completely keep the file format and compression ratio unchanged. In order to meet such requirement, the algorithms should combined encryption process with encoding process. Due to the wide application of quad tree structure in image or video encoding, two types of quadtree permutation algorithms are proposed here. They are used in quadtree based image encoding, wavelet zerotree encoding (EZW, SPIHT), and quadtree based fractal image or video encoding. The encryption algorithms combine encryption with compression, and have some advantages. Firstly, it is secure, and the encryption result is too confused to be understood. Secondly, it keeps the file format unchanged, and thus it is easy to operate compressed and encrypted data directly. Thirdly, it keeps the compression ratio unchanged, that is to say that the encryption operations do not produce extra data. Lastly, it is fast and can meet real time requirement.