Modern Internet of Things (IoT) ecosystems, including automotive systems, smart televisions, medical devices, and industrial Fuel Dispensing Units (FDUs), rely on interconnected Electronic Control Units (ECUs) and peripheral microcontrollers.

These devices handle critical operations but feature vulnerable firmware, flash memory components, and unencrypted bus topologies. Compromising a single node can allow an attacker to pivot through the entire internal network, resulting in unauthorized data access, code injection, or physical failure.

To address these vulnerabilities, this paper introduces a zero-trust, closed-user-group (CUG) Public Key Infrastructure (PKI) tailored for multi-ECU embedded deployments. Our architecture treats an interconnected group of peripheral IoT microcontrollers and a central host processing unit as a self-contained "family." Mutual authentication is enforced at system boot and at periodic operational run-time intervals.

The framework utilizes a dedicated manufacturing provisioning system that employs hardware-secured Root Certificate Authorities (CAs) and specialized physical testing jigs to write cryptographic credentials directly to non-volatile flash memory during assembly. At runtime, node integrity is verified using a hybrid cryptographic workflow combining Advanced Encryption Standard (AES-256) symmetric encryption with Secure Hash Algorithm 2 (SHA-256) digital signatures.

To validate the security posture of this ecosystem, we present a multi-layered audit methodology covering the provisioning process, cryptographic data-flow verification, and physical CA infrastructure security. Experimental results show that the entire mutual authentication loop for a standard multi-node FDU deployment finish well within a strict 500-microsecond threshold. This demonstrates that the proposed zero-trust framework provides strong protection against Man-in-the-Middle (MITM) attacks, firmware tampering, and unauthorized node replacements without degrading real-time performance.