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.-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.