原子跨链交换
Atomic Cross-Chain Swaps
摘要：
原子跨链交换是一种分布式协调任务，多方在多个区块链上交换资产，例如，用比特币交换以太币。一个原子交换协议保证（1）如果所有各方都遵守协议，那么所有的交换都会发生，（2）如果一些联盟偏离协议，那么没有一个符合协议的方最终会变得更糟，（3）没有联盟有偏离协议的动机。跨链交换被建模为有向图${\cal D}$，其顶点是各方，其弧是提议的资产转移。对于任意一对$({\cal D},L)$，其中${\cal D} = (V,A)$是一个强连通有向图，$L \subset V$是${\cal D}$的反馈顶点集，我们给出了一个${\cal D}$的原子跨链交换协议，该协议使用一种哈希时间锁契约形式，其中$L$中的顶点生成哈希锁秘密.我们表明，没有这样的协议是可能的，如果${\cal D}$不是强连接，或者如果${\cal D}$是强连接，但$L$是不是一个反馈顶点集。该协议具有时间复杂度$O(diam({\cal D}))$和空间复杂度（存储在所有区块链上的比特）$O(|A|^2)$。
英文摘要：
An atomic cross-chain swap is a distributed coordination task where multiple parties exchange assets across multiple blockchains, for example, trading bitcoin for ether. An atomic swap protocol guarantees (1) if all parties conform to the protocol, then all swaps take place, (2) if some coalition deviates from the protocol, then no conforming party ends up worse off, and (3) no coalition has an incentive to deviate from the protocol. A cross-chain swap is modeled as a directed graph ${\cal D}$, whose vertexes are parties and whose arcs are proposed asset transfers. For any pair $({\cal D},L)$, where ${\cal D} = (V,A)$ is a strongly-connected directed graph and $L \subset V$ a feedback vertex set for ${\cal D}$, we give an atomic cross-chain swap protocol for ${\cal D}$, using a form of hashed timelock contracts, where the vertexes in $L$ generate the hashlocked secrets. We show that no such protocol is possible if ${\cal D}$ is not strongly connected, or if ${\cal D}$ is strongly connected but $L$ is not a feedback vertex set. The protocol has time complexity $O(diam({\cal D}))$ and space complexity (bits stored on all blockchains) $O(|A|^2)$.
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