Cryptology ePrint Archive: Report 2021/1545

Securing Proof-of-Stake Nakamoto Consensus Under Bandwidth Constraint

Joachim Neu and Srivatsan Sridhar and Lei Yang and David Tse and Mohammad Alizadeh

Abstract: 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.

Category / Keywords: cryptographic protocols / blockchain, consensus

Date: received 24 Nov 2021

Contact author: jneu at stanford edu, svatsan at stanford edu, leiy at csail mit edu, dntse at stanford edu, alizadeh at csail mit edu

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Version: 20211129:121439 (All versions of this report)

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