eHealth provides great relief for patients and physicians. This means, patients autonomously monitor their condition via IoT medical devices and make these data available to physicians for analyses. This requires a data platform that takes care of data acquisition, management, and provisioning. As health data are highly sensitive, there are major concerns regarding data security with respect to confidentiality, integrity, and authenticity. To this end, we present a blueprint for constructing a trustworthy health data platform called SEAL. It provides a lightweight attribute-based authentication mechanism for IoT devices to validate all involved data sources, there is a fine-grained data provisioning system to enable data provision according to actual requirements, and a verification procedure ensures that data cannot be manipulated.