PrivHChain: Monitoring the Supply Chain of Controlled Substances with Privacy-Preserving Hierarchical Blockchain

With rapidly increasing drug abuse across the world, it is imperative to monitor their supply chain with sufficient transparency. Blockchain is a common solution for achieving transparency in supply chain monitoring, but it does not have sufficient throughput for large-scale supply chains. It is challenging to achieve throughput and privacy simultaneously because complex dependencies among the supply chain events and the need for aggregation both make the application of ZKP challenging. We present PrivHChain, a privacy-preserving hierarchical blockchain that preserves transaction privacy even against blockchain peers while allowing them to verify record consistencies. This is enabled by novel modeling of supply chain events which makes it possible to use novel efficient zero-knowledge protocol schemes to verify the complex dependencies. Novel aggregation techniques are proposed to enable the proof aggregation, and the proofs are used to design monitoring protocols. PrivHChain is implemented and validated with extensive experiments and simulations. The results indicate that (i) the extra overhead of encryption and ZKP schemes is acceptable or negligible, and (ii) the throughput is improved by up to 5 times in simulations even with all the encryption/ZKP schemes.