Paper 2021/302

Post-Quantum Verifiable Random Function from Symmetric Primitives in PoS Blockchain

Maxime Buser, Rafael Dowsley, Muhammed F. Esgin, Shabnam Kasra Kermanshahi, Veronika Kuchta, Joseph K. Liu, Raphael Phan, and Zhenfei Zhang

Abstract

Verifiable Random Functions (VRFs) play a key role in Proof-of-Stake blockchains such as Algorand to achieve highly scalable consensus, but currently deployed VRFs lack post-quantum security, which is crucial for future-readiness of blockchain systems. This work presents the first quantum-safe VRF scheme based on symmetric primitives. Our main proposal is a practical many-time quantum-safe VRF construction, X-VRF, based on the XMSS signature scheme. An innovation of our work is to use the state of the blockchain to counter the undesired stateful nature of XMSS by constructing a blockchain-empowered VRF. While increasing the usability of XMSS, our technique also enforces honest behavior when creating an X-VRF output so as to satisfy the fundamental uniqueness property of VRFs. We show how X-VRF can be used in the Algorand setting to extend it to a quantum-safe blockchain and provide four instances of X-VRF with different key life-time. Our extensive performance evaluation, analysis and implementation indicate the effectiveness of our proposed constructions in practice. Particularly, we demonstrate that X-VRF is the most efficient quantum-safe VRF with a maximum proof size of 3 KB and a possible TPS of 449 for a network of thousand nodes.