A complete MPEG-4 stream consists of many elementary streams, which may be generated by different authors. In the scenario of this paper, each author signs his own authentic elementary stream independently, and then an untrusted distributor aggregates these signatures into only one. Based on the unique signature, a client is able to verify the received MPEG-4 stream with the certificates of all the authors other than the certificate of the distributor. In addition, each author cannot deny what he has signed even if he is willing to admit a signature on another ES. This aggregated signature scheme is efficient in terms of transmission overhead and verification time since only one signature is processed in the client side.