Manipulation-Resistant Reputations Using Hitting Time

Abstract

Popular reputation systems for linked networks can be manipulated by spammers who strategically place links. In Sergey Brin and Lawrence Page, “The Anatomy of a Large-Scale Hypertextual Web Search Engine.” Computer Networks and ISDN Systems 30:1–7 (1998), 107–117, pages endorse others by placing links, and the global link structure is analyzed to determine the reputation of each page. Though this is meant to be a global measure, page $v$ can boost its own PageRank considerably using a simple self-endorsement strategy: placing outlinks to form short directed cycles. In contrast, we show that expected hitting time---the time to reach $v$ in a random walk---measures essentially the same quantity as PageRank, but does not depend on $v$'s outlinks. We develop a reputation system based on hitting time and show that it resists tampering by individuals or groups who strategically place outlinks. We also present an algorithm to efficiently compute hitting time for all nodes in a massive graph; conventional algorithms do not scale adequately.