  With the fast development of digital technologies, we are running into a
digital world. The relationship among people and the connections among things
become more and more complex, and new challenges arise. To tackle these
challenges, trust-a soft security mechanism-is considered as a promising
technology. Thus, in this survey, we do a comprehensive study on the trust and
trust modelling for the future digital world. We revisit the definitions and
properties of trust, and analysis the trust theories and discuss their impact
on digital trust modelling. We analyze the digital world and its corresponding
environment where people, things, and infrastructure connect with each other.
We detail the challenges that require trust in these digital scenarios. Under
our analysis of trust and the digital world, we define different types of trust
relationships and find out the factors that are needed to ensure a fully
representative model. Next, to meet the challenges of digital trust modelling,
comprehensive trust model evaluation criteria are proposed, and potential
securities and privacy issues of trust modelling are analyzed. Finally, we
provide a wide-ranging analysis of different methodologies, mathematical
theories, and how they can be applied to trust modelling.
