A smart infrastructure with many sensors becomes a complex hybrid system. Excluding external adversaries from infiltrating into the infrastructure is a key problem. With a formalized adversary model, we propose a decentralized architecture for smart infrastructure security. First, it effectively embeds information from the physical features of infrastructures into security. Second, our mechanism is managed through a decentralized blockchain architecture. Third, our approach is especially scalable to ever growing smart infrastructures. We prototyped an authentication blockchain for smart grids with a high penetration of distributed energy resources (DERs). Our implementation demonstrates the effectiveness of our mechanism compared to existing approaches.