## 3 D2 WaVe: Design Rationale and System Architecture
### System Overview  
Aiming at a secure-by-design video data service infrastructure for assurance-and resilience-oriented ITS applications atop heterogeneous IoV networks, D2 WaVe combines the merits of digital watermarks with the security properties of blockchain to resist the threats mentioned above. Figure 4 demonstrates the system architecture consisting of two sub-frameworks: (i) a video frame authentication scheme by using a robust FIAE-GAN-based digital watermarking model; (ii) a decentralized security fabric atop blockchain and distributed data storage.  
In the IoV network, in-vehicle and surveillance cameras generate a large volume of video data. The raw video data can be transmitted to nearby edge computing platforms, which provide storage and computing resources. To mitigate video forgery attacks in IoV networks, an FIAE-GAN-based digital watermarking model can generate robust and invisible digital watermarks that data owners embed into specific video frames. As Figure 4 shows, watermark hiding and extraction are executed at edge computing platforms guaranteed by secure execution environments. After receiving raw video streams, data owners use the FIAE-GAN encoder to generate digital watermarks as authenticity proofs and embed them into frames of video streams. Then, data owners can publish watermarked frames on blockchain-based security fabric. At another end side, data users firstly securely retrieve video data from blockchain-based security fabric. Finally, data users use the FIAE-GAN decoder to extract digital watermarks from video streams and then verify whether the video data are genuine. Given the assumption that attackers cannot access FIAE-GAN-based encoders, it is hard to create valid digital watermarks that are embedded into fake  
Figure 4: The architecture of D2 WaVe.  
video frames. Therefore, FIAE-GAN-based digital watermark embedding and verification can improve authenticity during video data generation and use stages. As an essential contribution of this work, the detailed design of the FIAE-GAN-based digital watermarking model is further presented in Section 4.  
To address integrity attacks and availability issues in video data distribution and use stages, a blockchain-based security fabric [37] is adopted as the underlying network infrastructure for the IoV network. As the right part of Figure 4 shows, a decentralized fabric contains three sub-systems: (1) security services based on the microservice-oriented architecture (MoA); (2) an Ethereum blockchain network to support security services; and (3) a decentralized data storage (DDS) system atop IPFS network to save off-chain data. By encapsulating security functions into separate containerized microservices loosely coupled with the rest of the system, these security services work independently under a distributed network environment, improving data service availability. In addition, we use the DDS system as an off-chain storage solution to save raw IoV data and meta-information. Compared to centralized data storage solutions, DDS can reduce data loss risks and improve data services' resilience. Moreover, Ethereum blockchain ensures the integrity and traceability of the on-chain data saved by the distributed ledger. Thus, it provides a decentralized and trust-free platform for secure and auditable IoV data.