Authenticated Keyword Search on Large-Scale Graphs in Hybrid-Storage Blockchains

The widespread availability of Internet access and online services has led to the generation of numerous large-scale graphs in various real-world applications, such as online social networks and knowledge graphs. Keyword search stands out as a crucial task in the analysis and mining of these graphs. However, graph data owners tend to outsource storage and computation tasks to the cloud due to limited computing and storage resources. In this case, it is critical to ensure the integrity of the query results, as the cloud may have an incentive to return tampered results to serve its own interests. Currently, blockchain systems can store data efficiently and securely, creating a decentralized, tamper-proof digital platform. This functionality positions blockchain as a crucial complement and enhancement to traditional cloud storage solutions. Mainstream blockchains use a hybrid storage system to improve scalability, storing small meta-data on-chain and outsourcing raw data off-chain. While cryptographic proofs protect data integrity for queries, current schemes only support key-value data. This paper pioneers the study of authenticated keyword searches on graphs in hybrid-storage blockchains. The key challenge is to design an authenticated data structure (ADS) based on the graph data that can efficiently deal with keyword search queries. We propose Merkle Path DAG (MP-DAG), a novel ADS that aggregates the unqualified paths that will not appear in the result trees to efficiently handle authenticated keyword search queries on graphs. Furthermore, to reduce the ADS storage cost, we design an optimization scheme MP-DAG* by combining the similar subgraphs of MP-DAG. Experimental results demonstrate the performance of the proposed ADS and optimization measure.