说明：  When outsourcing video contents into the cloud, it is not easy to achieve fine-grained access control especially in timedomain, as the owners of video contents are not able to control their data as on their own servers. The untrustworthy cloud servers further make this issue more challenging, because: 1) cloud servers may not be fully trusted by the owners to control the access of their video contents and 2) cloud servers may also be curious about the stored video contents. Thus, existing server-based access control methods (e.g., Access Control Lists) are not applicable for cloud-based video content sharing. A possible approach is to encrypt video contents and only authorized users are given decryption keys. In the above mentioned example in Fig. 1, all the video frames in t1 are encrypted by one key, while all the video frames in t2 are encrypted by another key.