The Discrepancy Attack on Polyshard-ed Blockchains

TL;DR

This paper introduces the discrepancy attack on Polyshard, a sharding-based blockchain security framework, demonstrating how malicious nodes can undermine security undetectably by dispersing inconsistent blocks.

Contribution

The paper identifies a novel discrepancy attack on Polyshard, revealing vulnerabilities and limitations in its current security assumptions.

Findings

Discrepancy attack can undermine Polyshard security

Attack is undetectable with current Polyshard setup

Malicious nodes controlling subchains can disperse inconsistent blocks

Abstract

Sharding, i.e. splitting the miners or validators to form and run several subchains in parallel, is known as one of the main solutions to the scalability problem of blockchains. The drawback is that as the number of miners expanding each subchain becomes small, it becomes vulnerable to security attacks. To solve this problem, a framework, named as \textit{Polyshard}, has been proposed in which each validator verifies a coded combination of the blocks introduced by different subchains, thus helping to protect the security of all subchains. In this paper, we introduce an attack on Polyshard, called \textit{the discrepancy} attack, which is the result of malicious nodes controlling a few subchains and dispersing different blocks to different nodes. We show that this attack undermines the security of Polyshard and is undetectable in its current setting.