This letter presents a hybrid conditional access system (CAS) for advanced terrestrial digital multimedia broadcasting (AT-DMB). The proposed architecture is characterized by its use of a unified CAS channel and various communication networks for CAS message transmissions. We implement a prototype CAS based on the hybrid architecture, which improves the CAS message transmission efficiency greatly compared to the existing T-DMB CAS standard and supports various AT-DMB interlayer services more easily and efficiently.