VTBC: Privatizing the Volume and Timing of Transactions for Blockchain Applications

Abstract

Existing privacy-preserving blockchain solutions have shown how to maintain the anonymity and confidentiality of the contents of blockchain transactions. However, due to blockchains needing to be stored and updated in a decentralized manner, metadata like the volume of transactions and the timestamp of each transaction can always be publicly observed, even with state-of-the-art solutions. Blockchain applications, especially ones with time-sensitive or volume-sensitive outcomes, may require this volume and timing information to be privatized. One example is not leaking the lateness of students' exam submissions because this could violate student privacy laws. In this paper, we propose VTBC, a blockchain system to privatize such volume and timing information for multi-party privacy-preserving blockchain applications through decoy blockchain transactions which a) do not contribute at all to the execution of the application and b) are indistinguishable from real (non-decoy) transactions. Even though the volume and timing metadata of all transactions must be public, volume and timing information for an application can be indirectly privatized (even after the application has been finalized) by carefully deciding when and how many decoy transactions are added to the blockchain. We demonstrate how these decoy transactions can be created without sacrificing the application's integrity, functionality, or verifiability, without making changes to the underlying blockchain's architecture, and always using the blockchain as the trusted timekeeper. We implemented our approach via a Dutch auction that supports decoy bid transactions and evaluated its performance on a private Ethereum blockchain network.