Manipulating Scrip Systems: Sybils and Collusion

TL;DR

This paper investigates how sybils and collusion affect scrip systems in peer-to-peer networks, revealing that while sybils generally harm social welfare, under certain conditions they can be beneficial, and collusion often improves overall agent welfare.

Contribution

It provides a game-theoretic analysis of sybils and collusion in scrip systems, highlighting their complex impacts and limitations of existing equilibrium models.

Findings

Sybils can sometimes improve overall social welfare.

Collusion tends to benefit all agents in scrip systems.

Existing models do not fully address sybils and collusion effects.

Abstract

Game-theoretic analyses of distributed and peer-to-peer systems typically use the Nash equilibrium solution concept, but this explicitly excludes the possibility of strategic behavior involving more than one agent. We examine the effects of two types of strategic behavior involving more than one agent, sybils and collusion, in the context of scrip systems where agents provide each other with service in exchange for scrip. Sybils make an agent more likely to be chosen to provide service, which generally makes it harder for agents without sybils to earn money and decreases social welfare. Surprisingly, in certain circumstances it is possible for sybils to make all agents better off. While collusion is generally bad, in the context of scrip systems it actually tends to make all agents better off, not merely those who collude. These results also provide insight into the effects of allowing agents to advertise and loan money. While many extensions of Nash equilibrium have been proposed that address collusion and other issues relevant to distributed and peer-to-peer systems, our results show that none of them adequately address the issues raised by sybils and collusion in scrip systems.