With the absence of physical evidence, the concept of trust plays a crucial role in the proliferation and popularisation of online services. In fact, trust is the inherent quality that binds together all involved entities and provides the underlying confidence that allows them to interact in an online setting. The concept of Federated Identity Management (FIM) has been introduced with the aim of allowing users to access online services in a secure and privacy-friendly way and has gained considerable popularities in recent years. Being a technology targeted for online services, FIM is also bound by a set of trust requirements. Even though there have been numerous studies on the mathematical representation, modelling and analysis of trust issues in online services, a comprehensive study focusing on the mathematical modelling and analysis of trust issues in FIM is still absent. In this paper we aim to address this issue by presenting a mathematical framework to model trust issues in FIM. We show how our framework can help to represent complex trust issues in a convenient way and how it can be used to analyse and calculate trust among different entities qualitatively as well as quantitatively.