The processor of a drone runs essential functions of sensing, communications, coordination, and control. This is the conventional view. But in today's cyber environment, the processor must also provide security to assure mission completion. We have been developing a secure processing architecture for mission assurance. A study on state-of-the-art secure processing technologies has revealed that no one-size-fits-all solution can fully meet our requirements. In fact, we have concluded that the provision of a secure processor as a mission assurance foundation must be holistic and should be approached from a systems perspective. We have thus applied a systems analysis approach to create a secure base for the system. This paper describes our journey of adapting and synergizing various secure processing technologies into a baseline asymmetric multicore processing architecture. We will also describe a functional and security co-design environment, created to customize and optimize the architecture in a design space consisting of hardware, software, performance, and assurance.