A brownfield security gateway is configured to support a trusted execution environment (TEE) that employs cryptographic and physical security—which forms a trusted cyber physical system—to protect sensitive transmissions on route to a controllable device. The gateway may be implemented with a System on Chip (SoC) that utilizes an application layer gateway to filter content within a transmission. When the application layer gateway authorizes the transmission, the transmission is forwarded to a trusted peripheral device that is configured with communication transport protocols, and the trusted peripheral device transfers the transmission to the controllable device. The trusted peripheral device and the controllable device are physically protected by, for example, protected distribution systems. Accordingly, the trusted peripheral device functions as a gateway between the SoC and the controllable device.