This paper deals with the evaluation of trust in a network of public-key infrastructures. We consider the PGP web of trust or the network of cross-certificates of trust anchors for website authentication. Different trust models have been proposed to interconnect the various PKI components in order to propagate the trust between them. In this paper we provide a simple model for trust and reputation management in PKI architectures, and a new polynomial algorithm using linear algebra to assess trust relationships in a network using different trust evaluation schemes.