Knowledge Issues Raised in Modelling Trust in a Public Key Infrastructure

The paper describes a knowledge based system (KBS) for modelling trust in the Certification Authority (CA) of a Public Key Infrastructure (PKI). It was built using a graphical KBS toolkit, Istar, that allows the knowledge builder to easily model the important relationships between concepts of the domain. The knowledge base was initially built using published work and was subsequently extended by knowledge obtained from leading PKI experts. The first prototype system computes the trust in a CA by asking the user a series of questions about the CA's Certification Practice Statement. Examples of its use with two well known public CAs is discussed. An important issue raised and discussed in this paper is how to map symbols in the KB to the knowledge level of human trust and beliefs, for such an ill-defined area of knowledge as trust, and four main mappings have been identified. Another issue that emerged relates to the use of questionnaires during knowledge acquisition. The expert system is currently available online via the Istar Knowledge Server, and future work is discussed.