## 4 The Design of the QKD-Secured V2I System  
A high-level overview of the functional layers for the AirQKD QKD-secured V2I system is shown in Figure 3.  
The whole AirQKD-protected V2I system can operate with four different layers:  
1. Quantum light channel--the optical FSO-QKD transmit (TX) and receive (RX) assemblies are mounted on the rooftops of two buildings. These optical assemblies maintain a line of sight (LOS) to each other, enabling the transmission and reception of the light channels. The optical layer assemblies are engineered by the AirQKD partner, Fraunhofer Centre for Applied Photonics (FCAP), with support from other partners, including Heriot-Watt, Strathclyde, and Bristol universities. 2. TX and RX QKD controls--the optical assemblies are connected by cables to a custom-built control system. The control layer equipment is engineered by the AirQKD partner, Nu Quantum, with support from other partners. The cables provide power to the optical assemblies, control signals, interfaces to the various optical components, and software drivers for the QKD algorithms, interfacing with a version of the Centre for Quantum Photonics (CQP) Toolkit [36] (an open-source package for QKD applications originating from the University of Bristol). 3. QKD system controller and key management module (KMM)--the AirQKD partner OpenLightComm (OLC) has engineered the software for the system management and KMM. The architecture, control, management functions, and interfaces of QKD systems are part of an ongoing standardisation process from organisations such as the European Telecommunications Standards Institute (ETSI) and the International Telecommunication Union (ITU) [13]. OLC provides ETSI-compliant software interfaces at the management and control layer. Figure 3: Overview of the QKD-secured V2I system; see points 1 to 4 in Section 4. V2I key-consuming application--the V2I application interfaces with the KMM, requesting and consuming encryption keys through an ETSI-compliant remote procedure call (gRPC). These are handled within the application layer via a gateway from the AirQKD partner, Angoka. They then use their custom zero-trust authentication protocol (ZAP) [37] to secure the V2I network. The optical QKD assemblies are shown in Figure 4. The rooftop locations in the images are from the first testing phase in Glasgow, where FCAP has its facilities. Each optical assembly is 70 by 30 by 20 centimetres in size and over 10 kg in weight. They are mounted on a heavy-duty camera tripod. This allows for manual coarse aiming before locking the tripod in position. Internal electromechanical systems are used for the final focusing of the beams. Umbilical cable management connects the optical assembly to its control system (located inside the buildings). The power, data, and control cables are routed through a 10-metre protective sheath. The prototype optical assemblies are not entirely weatherproof; therefore, they have to be moved indoors in poor weather. Final production versions would be designed with full weatherproofing. The operational FSO-QKD system feeds the securely generated keys into the OLC KMM software, preparing them for use by the V2I application. One consideration is the degradation of the optical link due to poor atmospheric conditions, such as severe weather, or obstructions from smoke or objects. This would reduce or cut off the supply of symmetric keys and must be considered for deployed FSO-QKD systems. The KMM performs key caching and stores all the QKD keys generated until they are requested by the consuming application. The two principal partners in the design and operation of the V2I testing application were Angoka Limited and WMG at the University of Warwick. These two organisations were supported by the other AirQKD partners when necessary during the design, engineering, testing, and implementation of the application. In particular, OLC provided support on their KMM application and interface. WMG partnered in the AirQKD project due to its expertise in vehicular and ITS systems.