Public-key infrastructures are a prerequisite for security in distributed systems and for reliable electronic commerce. It is their goal to provide the authenticity of public keys. Formal models for public-key infrastructures (trust models) contribute decisively to a deeper understanding of the desirable design principles of these infrastructures. The trust model of the present paper is based on the modelling technique of coloured Petri nets. These are a special class of high-level Petri nets with an intuitively appealing graphical representation and a few, but powerful primitives. Elaborate and well tested software is available.