Sybil-proof accounting mechanisms with transitive trust

For the design of distributed work systems like P2P filesharing networks it is essential to provide incentives for agents to work for each other rather than free ride. Several mechanisms have been proposed to achieve this goal, including currency systems, credit networks, and accounting mechanisms. It has proven particularly challenging to provide robustness to sybil attacks, i.e., attacks where an agent creates and controls multiple false identities. In this paper, we consider accounting mechanisms for domains in which (1) transactions cannot be bound to reports, (2) transactions are bilateral and private, and (3) agents can only form trust links upon successful work interactions. Our results reveal the trade-off one must make in designing such mechanisms. We show that accounting mechanisms with a strong form of transitive trust cannot be robust against strongly beneficial sybil attacks. However, we also present a mechanism that strikes a balance, providing a weaker form of transitive trust while also being robust against the strongest form of sybil attacks. On the one hand, our results highlight the role of strong social ties in providing robustness against sybil attacks (such as those leveraged in credit networks using bilateral IOUs), and on the other hand our results show what kind of robustness properties are possible and impossible in domains where such pre-existing trust relations do not exist.