# Replay Attacks and Defenses Against Cross-shard Consensus in Sharded Distributed Ledgers

## TL;DR

This paper identifies vulnerabilities in cross-shard consensus protocols of distributed ledgers through replay attacks and introduces Byzcuit, a new protocol that effectively defends against these attacks with minimal performance impact.

## Contribution

The paper reveals replay attacks against cross-shard consensus and proposes Byzcuit, a novel protocol that is resilient to these attacks and performs efficiently.

## Key findings

- Replay attacks can double-spend resources in sharded ledgers.
- Byzcuit effectively defends against replay attacks.
- Byzcuit outperforms previous protocols in efficiency.

## Abstract

We present a family of replay attacks against sharded distributed ledgers, that target cross-shard consensus protocols, such as the recently proposed Chainspace and Omniledger. They allow an attacker, with network access only, to double-spend or lock resources with minimal efforts. The attacker can act independently without colluding with any nodes, and succeed even if all nodes are honest; most of the attacks can also exhibit themselves as faults under periods of asynchrony. These attacks are effective against both shard-led and client-led cross-shard consensus approaches. Finally, we present Byzcuit - a new cross-shard consensus protocol that is immune to those attacks. We implement a prototype of Byzcuit and evaluate it on a real cloud-based testbed, showing that our defenses impact performance minimally, and overall performance surpasses previous works.