Systems resilient to cyber-attacks for mission assurance are difficult to develop, and the means of effectively evaluating them is even harder. We have developed a new architectural design and engineering environment, referred to as HARDEN (High AssuRance Design ENvironment), which supports an agile design methodology used to create secure and resilient systems. This new toolkit facilitates the quantitative analysis of a system's security posture by setting up a systematic approach of securing and analyzing embedded systems. HARDEN promotes the early co-design of functionality and security that now enables the development of mission assured systems.