## 4 The System Proposal: An Introduction  
In our system proposal, each building, from the moment in which it is conceived to its demolition, is identified by means of a DID. Each building has a BIM model associated with it, therefore multiple files that can be represented through the so-called claims of the SSI model, becoming when necessary verifiable attestations. Not only engineers and architects contribute to associating files to a given building/DID but also appropriate IoT devices, located at strategic points such as the entrance to a construction site in the case of a building under construction. These devices, equipped with suitable RFID readers, take useful data from the RFID tags that detect and associate such data to the building and therefore to the specific DID. Hence a building under construction could be equipped with RFID readers that, placed at the entrance to the construction site, must detect the RFID tags associated with each material/product/item entering the construction site. These latter tags must be carefully prepared by the manufacturer in such a way that their memory contains detailed information on the technical characteristics with which they are associated. The building could also include a set of cameras that can be used to record crucial moments in the life cycle of a building, such as the installation of insulated panels on the roof, or the installation of windows and air conditioners. In this way, all the characteristics of the materials chosen during the design phase can be compared with those of the products that have actually entered and used on the construction site, and the certifications, for example the energy performance certificates, can thus be drawn up using information extracted both from the BIM files but also from the data collected from the RFID tags and cameras. The BIM files, all data collected by IoT devices, and all useful files produced by stakeholders support themselves in the various activities that they must conduct in the different stages of the life cycle of a building, such as design, construction, certification and maintenance. For example, temperature and humidity information of some components belonging to the building could be collected in real time, through appropriate sensors, providing a reliable reference for the maintenance personnel to evaluate and maintain the quality of such components making correct decisions. In addition, thanks to data collected by IoT devices, artificial intelligence could be applied to detect operational problems in buildings by monitoring data in real time, and to establish correlations between existing performance and potential malfunctions allowing predictive maintenance or component replacement first problems to occur. Exploiting the SSI concepts and the Blockchain technology the model proposal ensures audit and provenance of all information associated with a building. First, the model proposal assumes that all building information, which can be requested to the building owner even many years after its realization, is stored in off-chain databases. Each building refers to a precise database at the level of the municipality in which it is located, and refers to a digital wallet, which is a digital portable and secure repository, belonging to building DID. Then the proposal assumes that the hashes of all files stored on the databases/wallet are stored on the Blockchain, and associated with the DID of the building they refer to. Hence for example data extracted reading the RFID tags is automatically collected and stored in the digital wallet of the building DID and its hash is computed and automatically stored on-chain assigning it to that precise DID. The BIM files generated through specific software by a given stakeholder are also stored in a digital personal wallet belonging to the stakeholder DID. The stakeholders can share these files with other stakeholders through secure and protected digital channels.