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，其背后有工作量证明矿工的巨大算力支持。权益证明链具有高能效、快速最终性，但面临若干安全问题：易受不可罚没的长程安全攻击、低活性和鲁棒性，以及从低代币估值启动的困难。我们证明，这些安全问题是任何无外部可信源的PoS链所固有的，并提出了一种新协议“Babylon”，该协议将现成的PoS协议通过检查点锚定到比特币上，以解决这些问题。一个不可能性结果证明了Babylon的最优性。Babylon的一个用例是缩短权益提取延迟：我们的实验结果表明，使用Babylon可将该延迟从现有PoS链的数周缩短至不到5小时，而将检查点发布到比特币上的年交易成本低于1万美元。