[论文解读] On Bitcoin and Red Balloons

Moshe Babaioff, Shahar Dobzinski+2 more|arXiv (Cornell University)|Nov 10, 2011

Blockchain Technology Applications and Security参考文献 7被引用 9

一句话总结

本文识别出比特币交易传播机制中一个关键的激励错配问题，即节点因奖励竞争而缺乏传播交易的动力。作者提出一种抗Sybil攻击的奖励机制，通过迭代剔除劣策略的方法，激励诚实传播，证明该机制可消除搭便车行为，同时保持低开销并抵御虚假身份攻击。

ABSTRACT

Many large decentralized systems rely on information propagation to ensure their proper function. We examine a common scenario in which only participants that are aware of the information can compete for some reward, and thus informed participants have an incentive not to propagate information to others. One recent example in which such tension arises is the 2009 DARPA Network Challenge (finding red balloons). We focus on another prominent example: Bitcoin, a decentralized electronic currency system. Bitcoin represents a radical new approach to monetary systems. It has been getting a large amount of public attention over the last year, both in policy discussions and in the popular press. Its cryptographic fundamentals have largely held up even as its usage has become increasingly widespread. We find, however, that it exhibits a fundamental problem of a different nature, based on how its incentives are structured. We propose a modification to the protocol that can eliminate this problem. Bitcoin relies on a peer-to-peer network to track transactions that are performed with the currency. For this purpose, every transaction a node learns about should be transmitted to its neighbors in the network. The current implemented protocol provides an incentive to nodes to not broadcast transactions they are aware of. Our solution is to augment the protocol with a scheme that rewards information propagation. Since clones are easy to create in the Bitcoin system, an important feature of our scheme is Sybil-proofness. We show that our proposed scheme succeeds in setting the correct incentives, that it is Sybil-proof, and that it requires only a small payment overhead, all this is achieved with iterated elimination of dominated strategies. We complement this result by showing that there are no reward schemes in which information propagation and no self-cloning is a dominant strategy.

研究动机与目标

• 识别并形式化比特币中因奖励竞争导致节点缺乏传播交易动力的激励问题。
• 设计一种激励机制，以促进比特币等去中心化系统中诚实的信息传播。
• 确保该机制具有抗Sybil攻击能力，防止攻击者通过创建虚假身份来利用系统。
• 通过迭代剔除劣策略实现低支付开销，同时保持强激励相容性。
• 证明在抗Sybil攻击方案中，无法实现传播的主导策略激励且无重复传播。

提出的方法

• 提出一种奖励方案，根据节点在网络传播树中的位置，为更早传播交易的节点分配更高奖励。
• 采用分层奖励结构，奖励取决于节点在树中的深度和分支因子，深度越深奖励越低。
• 应用迭代剔除劣策略方法，证明在所提机制下，诚实传播成为最优策略。
• 将网络建模为深度 H ≥ 3、分支因子 d ≥ 3 的树，并基于奖励份额和知晓数量定义效用函数。
• 证明任何选择向子节点传播且其链长低于当前值的节点，其策略支配不传播策略。
• 通过反证法证明主导策略机制的不可能性：表明激励相容性要求 r₂₃ > 0，但该要求与非重复传播的主导性相矛盾。

实验结果

研究问题

• RQ1 能否设计一种奖励机制，激励比特币中的节点传播交易，而无需依赖利他主义？
• RQ2 能否创建一种抗Sybil攻击的机制，防止虚假身份利用奖励系统？
• RQ3 该机制能否确保深度为3或以内的所有节点，其诚实传播为支配策略？
• RQ4 在去中心化系统中，实现有效信息传播所需的最小开销（以总奖励计）是多少？
• RQ5 是否存在根本性限制，导致无法在Sybil攻击网络中设计出信息传播的主导策略机制？

主要发现

• 所提出的奖励机制通过迭代剔除劣策略，成功激励诚实交易传播，确保传播为最优策略。
• 该机制具有抗Sybil攻击能力，可防止攻击者通过创建多个虚假身份获利。
• 总奖励开销有界且较小，该方案仅需7个种子节点和 2β⁻¹ + 6d·A(yₕ) + 6 个额外节点，即可确保策略主导性。
• 本文证明，在 H ≥ 3 的情况下，抗Sybil系统中无法实现信息传播与无重复的主导策略激励。
• 任何抗Sybil机制的必要条件是 r₂₃ > 0，即必须向深度2的节点授予授权深度3节点交易的奖励。
• 不可能性证明中的矛盾源于：要求 r₂₃ > 0 违反了非重复传播的主导性，表明无法实现主导策略。