Method and system for CAS key assignment in digital broadcast service

A method for assigning conditional access system (CAS) keys to a terminal to thus reduce the number of entitlement management messages (EMMs) required to send contents to a digital broadcast subscriber. To this end, a broadcast center causes transmission of the CAS-related key to limit access to the digital content, over a mobile communication network used for a mobile communication, and causes transmission of the digital content over a digital broadcasting network used for digital broadcasts. Since the CAS-related key is transmitted to the digital broadcast subscriber's terminal over the mobile communication network, it is possible to effectively utilize the frequency bandwidth of the digital broadcasting network for the digital broadcasting. Furthermore, the security of the CAS-related key can be strengthened by sending the CAS-related key to the terminal over the mobile communication network.