MTU: The Multifunction Tree Unit for Accelerating Zero-Knowledge Proofs

TL;DR

This paper introduces the Multifunction Tree Unit (MTU), a hardware accelerator that significantly speeds up zero-knowledge proof computations involving binary tree structures, outperforming CPUs and traditional traversal methods.

Contribution

The paper presents the MTU hardware accelerator and a hybrid traversal strategy that enhances performance and scalability for tree-based ZKP workloads.

Findings

MTU achieves up to 1478x speedup over CPU.

Hybrid traversal outperforms BFS by up to 3x.

Provides practical guidance for hardware design in ZKP applications.

Abstract

Zero-Knowledge Proofs (ZKPs) are critical for privacy-preserving techniques and verifiable computation. Many ZKP protocols rely on key kernels such as the SumCheck protocol and Merkle Tree commitments to enable their key security properties. These kernels exhibit balanced binary tree computational patterns, which enable efficient hardware acceleration. Although prior work has investigated accelerating these kernels as part of an overarching ZKP protocol, exploiting this common tree pattern remains relatively underexplored. We conduct a systematic evaluation of these tree-based workloads under different traversal strategies, analyzing performance on multi-threaded CPUs and the Multifunction Tree Unit (MTU) hardware accelerator. We introduce a hardware-friendly Hybrid Traversal for binary tree that improves parallelism and scalability while significantly reducing memory traffic on hardware. Our results show that MTU achieves up to $1478\times$ speedup over CPU at DDR-level bandwidth and that our hybrid traversal outperforms breadth-first search by up to $3\times$. These findings offer practical guidance for designing efficient hardware accelerators for ZKP workloads with binary tree structures.