Hardware obfuscating is a proactive design-for-trust technique against IC supply chain threats, i.e., IP piracy and overproduction. Many studies have evaluated numerous techniques for obfuscation purposes. Nevertheless, de-obfuscation attacks have demonstrated their insufficiency. This paper proposes a register-transfer (RT) level finite-state-machine (FSM) obfuscation technique called ReTrustFSM that allows designers to obfuscate at the earliest possible stage. ReTrustFSM combines three types of secrecy: explicit external secrecy via an external key, implicit external secrecy based on specific clock cycles, and internal secrecy through a concealed FSM transition function. So, the robustness of ReTrustFSM relies on the external key, the external primary input patterns, and the cycle accuracy of applying such external stimuli. Additionally, ReTrustFSM defines a cohesive relationship between the features of Boolean problems and the required time for de-obfuscation, ensuring a maximum execution time for oracle-guided de-obfuscation attacks. Various attacks are employed to test ReTrustFSM’s robustness, including structural and machine learning attacks, functional I/O queries (BMC), and FSM attacks. We have also analyzed the corruptibility and overhead of design-under-obfuscation. Our experimental results demonstrate the robustness of ReTrustFSM at acceptable overhead/corruption while resisting such threat models.