Trust*: Using Local Guarantees to Extend the Reach of Trust

Abstract

This is work done with Stephen Clarke, who’s just there lurking, and Hannan Xiao, who was here yesterday but can’t be here today. The motivation is that we often want to do business, or conduct transactions, with strangers, people we haven’t done business with before. Reputation systems don’t really work, or at least reputation systems for giving reputations to strangers don’t work. Now most people say that that’s because you don’t know the people giving the reputations, I want to make the slightly stronger claim that even if you did know all the people who gave all the reputations, and you trusted them all, that still wouldn’t help. This is basically because trust isn’t transitive. The fact that Alice trusts Bob, and that Bob trusts Carol, isn’t enough to ensure that it’s appropriate for Alice to trust Carol, because the fact that Carol gives Bob a good service doesn’t mean that Carol is going to give Alice a good service. It might be that Bob is a regular customer, it might be that there’s some other reason why Carol’s giving Bob a good service. Perhaps she fancies him. The assumption that we’re going to make, for the purpose of this talk, is that local trust management is a more tractable problem than the one that we started with here. If you have people that you do business with all the time, then there are various systems that more or less allow you to do get a particular service in such a way that it’s either cheap or reliable depending on which has more utility in that circumstance.