Abstract: A security mechanism is presented for the networkbased, largescale, operational video surveillance system: virtual private network (VPN) and session initiation protocol (SIP) are used to build up a flexible closedusergroup to provide authentication and access control; dynamickey based media stream encryption solution helps to avoid exposing raw video data in networks; simple network management protocol version 3 （SNMPv 3）based management subsystem is offered to ensure the security of management information.

Key words: video surveillance, security, virtual private network, session initiation protocol