Blockchain systems often rely on rationality assumptions for their security, expecting that nodes are motivated to maximize their profits. These systems thus design their protocols to incentivize nodes to execute the honest protocol but fail to consider out-of-band collusion. Existing works analyzing rationality assumptions are limited in their scope, either by focusing on a specific protocol or relying on non-existing financial instruments. We propose a general rational attack on rationality by leveraging an external channel that incentivizes nodes to collude against the honest protocol. Our approach involves an attacker creating an out-of-band bribery smart contract to motivate nodes to double-spend their transactions in exchange for shares in the attacker's profits. We provide a game theory model to prove that any rational node is incentivized to follow the malicious protocol. We discuss our approach to attacking the Bitcoin and Ethereum blockchains, demonstrating that irrational behavior can be rational in real-world blockchain systems when analyzing rationality in a larger ecosystem. We conclude that rational assumptions only appear to make the system more secure and offer a false sense of security under the flawed analysis.

翻译：区块链系统通常依赖理性假设来保证安全性，预期节点会以最大化自身利益为行动动机。这些系统通过设计协议激励节点执行诚实协议，但未考虑带外合谋行为。现有分析理性假设的工作存在局限性，要么聚焦于特定协议，要么依赖不存在的金融工具。我们提出一种利用外部通道破坏理性假设的通用理性攻击方法，通过激励节点合谋对抗诚实协议。攻击者创建带外贿赂智能合约，以分享攻击者利润为条件，诱使节点实施双花交易。我们构建博弈论模型证明：任何理性节点都有动机遵循恶意协议。通过分析对比特币和以太坊区块链的攻击案例，我们展示了在更大生态系统框架下分析理性时，非理性行为在现实区块链系统中可能具有理性特征。最终结论表明：在存在缺陷的分析框架下，理性假设只是看似增强了系统安全性，实则提供了虚假的安全感。