Law enforcement officers nowadays are usually equipped with some camera device which captures real time visuals on their duties. The footages would be used as evidence and they carry legal effects. However in the near future, civilians are encouraged to use their smart phone in recording footages in critical scenes; how could the video or pictures recorded by a normal cell phone from the hand of a civilian (that can be anybody) carried the same legal effect when it comes to authentication? In this paper we explore the possibility of an image processing technology which can hidden a secret message in each video frame for verification purpose-to prove that the footage is indeed taken by or belong to a particular user. In our design, the algorithm must be lightweight enough to be embedded into the use of a cell phone or compact camera device, the secrecy must be secured known to only the authenticator, and the watermarking process must be simple. Such digital water-marking technique is presented in this paper, backed with simulation experiments.