All transfiguration-phase-inversion based systems always concern security. Risks associated with existing centralized systems can be prevented with the decentralized and distributed design features of blockchain. Due to limited resources, traditional security and privacy architectures are inappropriate to address this deficiency, this chapter talks about the Blockchain-based security mechanism which allows access to smart city resources in a secure manner. The Authentication and Authorization for Constrained Environments framework-based authorization Blockchain and the Object Security Architecture for the Internet of Things object model make up the mechanism discussed. The Blockchain establishes a versatile and trustless authorization system, whereas the Object Security Architecture for the Internet of Things organizes approved clients’ multicast groups using a public ledger. Furthermore, a meteor-based application is being created to offer an interface that is user-friendly for diverse smart city technologies. Through this application, smart city resources, such as traffic lights, smart power meters, surveillance cameras and so on will be controllable by users. The authorization will be used to assess the performance of the discussed mechanism. The Ethereum network is used to implement blockchain. Node.js is used to develop the authentication mechanism and Raspberry Pi B+ for the development of a smart city. Furthermore, the mocha and chai frameworks are used to evaluate the system’s performance. Even though the average hand-shaking time increases with the number of clients, experimental results show that the authentication response time is less than 100 ms.