目的 常见的H.264熵编码加密是一种将加密方案与熵编码方式融合的视频安全算法，这种算法不仅加解密速度快且可保持数据格式不变，是一种兼容性的加密方案，但需要处理帧内所有数据。方法 针对熵编码加密过程中存在的不足，提出了一种基于关键帧选择与熵编码相结合的视频加密方法，通过获取一帧中所有8×8子宏块的运动矢量，构造一个用于表示每一帧内容变化情况的向量F，然后将其欧几里德范数与阈值K比较，如果大于或等于阈值，则确定当前帧是关键帧，则在下一帧熵编码时加密其运动矢量差与残差系数值，否则不参与任何加密运算。结果 通过控制阈值K的大小来决定关键帧的选择，并结合混沌序列加密关键帧数据来保证视频信息的安全性，与熵编码加密算法相比，加密数据平均减少数据量约为39.78%。结论 实验结果表明该加密方法在保持编码后数据总量不变的情况下，以较小的时间代价获得了良好的加密效果，且与常规的熵编码加密算法相比大幅度减少了加密数据量。

Objective Entropy coding is a new technology used in the H.264 video standard.In the video security communications,entropy-coding encryption is one of the important encryption algorithms. Commonly, the H.264 entropy-coding encryption is a fusion of encryption and entropy encoding video security algorithm,which is fast and the data format can remain unchanged. It is a compatibility encryption scheme,but needs to deal with all of the data frames. Method To deal with the problem of the entropy coding encryption process, we propose a video encryption method based on key frames selection combined with entropy coding,by capturing all 8×8 sub-macroblock motion vectors in one frame and, constructing the vector F to represent the change of the content of each frame. Then the Euclidean norm is compared with the threshold K; if greater than or equal to the threshold,the current frame is the key frame. Then encrypts the next frame when the coding motion-vector and the coefficient of the residual value,otherwise it does not participate in any encryption arithmetic. Result By controlling the threshold value K to determine the key frame selection,combines with the chaotic sequence to guarantee the security of videos. Compared with the entropy coding encryption algorithm, the encryption data is reduced about 39.78%. Conclusion The experiments show that the encryption method maintains the total data,obtains a good encryption effect with a smaller time cost,and significantly reduces the amount of the encrypted data. The operation only changes the entropy encoding data, and it has nothing to do with the other processes of video coding. Therefores, the encryption system has no effect on the encoded data quantity. This algorithm is a compatibility video encryption.