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Supervisory control and data-acquisition (SCADA) systems are control systems for many national infrastructures. In the past, SCADA systems were designed without security functionality because of the closed operating environment. However, the security of SCADA systems has become an issue with connection to open networks becoming more common. Any damage to the SCADA system can have a widespread negative effect to society. In this paper, we review constraints and security requirements for SCADA systems and then investigate whether the existing key-management protocols for the SCADA systems satisfy these requirements. Afterward, we propose an advanced key-management architecture fitted for secure SCADA communications. The contributions of our work are two-fold. First, our scheme supports both message broadcasting and secure communication. Second, by evenly spreading much of the total amount of computation across high power nodes (MTU or SUB-MTU), our protocol avoids any potential performance bottleneck of the system while keeping the burden on low power nodes (RTU) at minimal.