Paper 2020/827

Encrypted Blockchain Databases

Daniel Adkins, Archita Agarwal, Seny Kamara, and Tarik Moataz

Abstract

Blockchain databases are storage systems that combine properties of blockchains and databases like decentralization, tamper-proofness, low query latency and support for complex queries. Blockchain databases are an emerging and important class of blockchain technology that is critical to the development of non-trivial smart contracts, distributed applications and decentralized marketplaces. In this work, we consider the problem of designing end-to-end encrypted blockchain databases to support the development of decentralized applications that need to store and query sensitive data. In particular, we show how to design what we call blockchain encrypted multi-maps (EMM) which can be used to instantiate various kinds of NoSQL blockchain databases like key-value stores or document databases. We propose three blockchain EMM constructions, each of which achieves different tradeoffs between query, add and delete efficiency. All of our constructions are legacy-friendly in the sense that they can be implemented on top of any existing blockchain. This is particularly challenging since blockchains do not support data deletion. We implemented our schemes on the Algorand blockchain and evaluated their concrete efficiency empirically. Our experiments show that they are practical.