Dynamics, robustness and fragility of trust

Trust is often conveyed through delegation, or through recommendation. This makes the trust authorities, who process and publish the trust recommendations, into an attractive target for attacks and spoofing. In some recent empiric studies, this was shown to lead to a remarkable phenomenon of *adverse selection*: a greater percentage of unreliable or malicious web merchants were found among those with certain types of trust certificates, then among those without. While such findings can be attributed to a lack of diligence in trust authorities, or even to conflicts of interest, our analysis of trust dynamics suggests that the public trust networks would probably remain vulnerable even if the trust authorities were perfectly diligent. The reason is that the main processes of trust building, under reasonable modeling assumptions, naturally lead to power-law distributions of trust: the rich get richer, the trusted attract more trust. The evolutionary processes governed by such distributions, ubiquitous in nature, are known to be robust with respect to random failures, but vulnerable to adaptive attacks. We recommend some ways to decrease this vulnerability, and suggest some ideas for exploration.