Private Data Access in Blockchain Systems Employing Coded Sharding

In present blockchain systems, privacy of transactions is maintained by keeping the identity of accounts anonymous. The associated pseudonyms are ephemeral in nature, and can not be easily traced back to the real identity. An alternate infallible approach is to make use of private information retrieval (PIR) protocols that enable users to fetch details of transactions without revealing which transactions they seek. In this paper, we formalize this approach for blockchain systems that employ coded sharding. We present a PIR protocol for private data access, in particular private balance-checking, in blockchain systems when data is stored using generalized Reed-Solomon codes. Our protocol can be readily applied to the PolyShard scheme that has been recently proposed as a method to build truly scalable blockchain system.