The Internet of Things (IoT) connects billions of devices and generates massive amounts of data. However, IoT systems face major security challenges, especially regarding authentication and authorization. Centralized approaches have single points of failure and limited scalability. Blockchain technology offers a decentralized solution, but public blockchains like Bitcoin and Ethereum have limitations for IoT devices due to computational overhead and latency. This paper proposes IoT-optimized two-layer blockchain architecture for authentication and authorization. The first layer is a consortium blockchain optimized for IoT devices. The second layer is a public blockchain for decentralized identity and access control. Cross-chain protocols connect the two layers securely. Linear secret sharing schemes (LSSS) implement fine-grained, attribute-based access control policies in smart contracts. Experiments on Ethereum and Hyperledger Fabric validate the approach’s efficiency and security compared to centralized methods. The two-layer model significantly reduces overhead for IoT devices while retaining decentralization benefits. The architecture provides resilient, scalable access control suitable for distributed IoT networks.