The Internet of Things builds a digital world where all assets including people, machines, human-machine interfaces (HMIs), processes, services, etc. are internetworked. Digital Twins provide 1-to-1 digital replica of physical assets and enable their networking and their goal-directed and standardized communication. The combination of assets and their corresponding Digital Twins are the “things”, the nodes of the IoT. With the rapid development of IoT and its pertinent technologies, more and more things will be mutually interconnected over the Internet. Hence, the security issues that come with it are gradually becoming sharper and sharper. In this paper, we focus on the topic of the secured communication among things - or more precisely their Digital Twins - and propose an approach related to OAuth2, asymmetrically encryption and digital signing to secure the communication within the newly developed Smart Systems Service Infrastructure (S³I).