Bitcoin is the most secure blockchain in the world, supported by the immense hash power of its Proof-of-Work miners. Proof-of-Stake chains are energy-efficient, have fast finality but face several security issues: susceptibility to non-slashable long-range safety attacks, low liveness resilience and difficulty to bootstrap from low token valuation. We show that these security issues are inherent in any PoS chain without an external trusted source, and propose a new protocol, Babylon, where an off-the-shelf PoS protocol checkpoints onto Bitcoin to resolve these issues. An impossibility result justifies the optimality of Babylon. A use case of Babylon is to reduce the stake withdrawal delay: our experimental results show that this delay can be reduced from weeks in existing PoS chains to less than 5 hours using Babylon, at a transaction cost of less than 10K USD per annum for posting the checkpoints onto Bitcoin.

翻译：比特币是世界上最安全的区块链，由其工作量证明矿工的巨大算力支撑。权益证明链能效高、最终确认速度快，但面临若干安全问题：易受不可罚没的长程安全攻击、活性韧性低、以及从低代币估值启动困难。我们证明，这些安全问题是任何缺乏外部可信源的PoS链所固有的，并提出一种新协议Babylon，该协议将现成的PoS协议锚定到比特币上以解决这些问题。一项不可能性结果论证了Babylon的最优性。Babylon的一个应用场景是减少权益提取延迟：我们的实验结果表明，使用Babylon可将该延迟从现有PoS链的周级缩短至5小时以内，而将检查点发布到比特币上的年化交易成本低于1万美元。