Abstract
We apply a graphical model to develop a digital loyalty program protocol specifically tailored to small shops with no professional or third-party-provided infrastructure. The graphical model allows us to capture assumptions on the environment the protocol is running in, such as capabilities of agents, available channels and their security properties. Moreover, the model serves as a manual tool to quickly rule out insecure protocol designs and to focus on improving promising designs. We illustrate this by a step-wise improvement of a crude but commercially used protocol to finally derive a light-weight and scalable security protocol with proved security properties and many appealing features for practical use.