Given a network of nodes with certain communication and computation capacities, what is the maximum rate at which a blockchain can run securely? We study this question for proof-of-work (PoW) and proof-of-stake (PoS) longest chain protocols under a 'bounded bandwidth' model which captures queuing and processing delays due to high block rate relative to capacity, bursty release of adversarial blocks, and in PoS, spamming due to equivocations. We demonstrate that security of both PoW and PoS longest chain, when operating at capacity, requires carefully designed scheduling policies that correctly prioritize which blocks are processed first, as we show attack strategies tailored to such policies. In PoS, we show an attack exploiting equivocations, which highlights that the throughput of the PoS longest chain protocol with a broad class of scheduling policies must decrease as the desired security error probability decreases. At the same time, through an improved analysis method, our work is the first to identify block production rates under which PoW longest chain is secure in the bounded bandwidth setting. We also present the first PoS longest chain protocol, SaPoS, which is secure with a block production rate independent of the security error probability, by using an 'equivocation removal' policy to prevent equivocation spamming.

翻译：给定一个具有特定通信和计算能力的节点网络，区块链在保证安全的前提下所能达到的最大运行速率是多少？我们针对工作量证明（PoW）和权益证明（PoS）最长链协议，在“有限带宽”模型下研究该问题。该模型捕捉了因高区块率相对于容量、恶意区块的突发性释放以及PoS中因欺诈性分叉导致的垃圾信息所产生的排队和处理延迟。我们证明，当PoW和PoS的最长链在容量限制下运行时，其安全性需要精心设计的调度策略来正确优先处理哪些区块——我们展示了针对此类策略的攻击方式。在PoS中，我们提出了一种利用欺诈性分叉的攻击，表明采用广泛调度策略类别的PoS最长链协议的吞吐量必须随着期望安全错误概率的降低而下降。同时，通过一种改进的分析方法，我们的工作首次识别了在有限带宽环境下PoW最长链保持安全的区块生成速率。我们还提出了首个安全的PoS最长链协议SaPoS，该协议通过采用“欺诈性分叉移除”策略来防止欺诈性分叉垃圾信息，使区块生成速率独立于安全错误概率。