  Zero Trust security has recently gained attention in enterprise network
security. One of its key ideas is making network-level access decisions based
on trust scores. However, score-based access control in the enterprise domain
still lacks essential elements in our understanding, and in this paper, we
contribute with respect to three crucial aspects. First, we provide a
comprehensive list of 29 trust attributes that can be used to calculate a trust
score. By introducing a novel mathematical approach, we demonstrate how to
quantify these attributes. Second, we describe a dynamic risk-based method to
calculate the trust threshold the trust score must meet for permitted access.
Third, we introduce a novel trust algorithm based on Subjective Logic that
incorporates the first two contributions and offers fine-grained decision
possibilities. We discuss how this algorithm shows a higher expressiveness
compared to a lightweight additive trust algorithm. Performance-wise, a
prototype of the Subjective Logic-based approach showed similar calculation
times for making an access decision as the additive approach. In addition, the
dynamic threshold calculation showed only 7% increased decision-making times
compared to a static threshold.
