  There has been tremendous interest in the development of formal trust models
and metrics through the use of analytics (e.g., Belief Theory and Bayesian
models), logics (e.g., Epistemic and Subjective Logic) and other mathematical
models. The choice of trust metric will depend on context, circumstance and
user requirements and there is no single best metric for use in all
circumstances. Where different users require different trust metrics to be
employed the trust score calculations should still be based on all available
trust evidence. Trust is normally computed using past experiences but, in
practice (especially in centralised systems), the validity and accuracy of
these experiences are taken for granted. In this paper, we provide a formal
framework and practical blockchain-based implementation that allows independent
trust providers to implement different trust metrics in a distributed manner
while still allowing all trust providers to base their calculations on a common
set of trust evidence. Further, our design allows experiences to be provably
linked to interactions without the need for a central authority. This leads to
the notion of evidence-based trust with provable interactions. Leveraging
blockchain allows the trust providers to offer their services in a competitive
manner, charging fees while users are provided with payments for recording
experiences. Performance details of the blockchain implementation are provided.
