Securing Proof-of-Stake Nakamoto Consensus Under Bandwidth Constraint

Satoshi Nakamoto's Proof-of-Work (PoW) longest chain (LC) protocol was a breakthrough for Internet-scale open-participation consensus. Many Proof-of-Stake (PoS) variants of Nakamoto's protocol such as Ouroboros or Snow White aim to preserve the advantages of LC by mimicking PoW LC closely, while mitigating downsides of PoW by using PoS for Sybil resistance. Previous works have proven these PoS LC protocols secure assuming all network messages are delivered within a bounded delay. However, this assumption is not compatible with PoS when considering bandwidth constraints in the underlying communication network. This is because PoS enables the adversary to reuse block production opportunities and spam the network with equivocating blocks, which is impossible in PoW. The bandwidth constraint necessitates that nodes choose carefully which blocks to spend their limited download budget on. We show that 'download along the longest header chain', a natural download rule for PoW LC, emulated by PoS variants, is insecure for PoS LC. Instead, we propose 'download towards the freshest block' and prove that PoS LC with this download rule is secure in bandwidth constrained networks. Our result can be viewed as a first step towards the co-design of consensus and network layer protocols.