2. The method according to claim 1 , wherein the multimedia file comprises a distorted signal.
2. The method according to claim 1 , wherein the multimedia file comprises a distorted signal. 4. The method according to claim 2 , wherein if the multimedia file is a video file, the distorted signal is generated when the video file is captured in a movie theater using a camcorder and released as a movie, or when the video file is subjected to image brightness change, size change or rotation.