As digital infrastructures become increasingly decentralized and interconnected, protecting sensitive data and ensuring reliable user authentication have become key challenges. Centralized identity and data management systems often suffer from critical vulnerabilities, including single points of failure, limited transparency, and increased exposure to cyberattacks. This research aims to develop secure and efficient blockchain-based systems tailored to data protection and decentralized authentication. By leveraging blockchain’s inherent properties—immutability, transparency, and distributed consensus—we propose a modular architecture that addresses key security and scalability challenges in modern digital ecosystems. The work focuses on two main aspects: (1) Data protection, through secure, verifiable, and distributed storage leveraging permissioned or hybrid blockchain networks; and (2) Authentication, using decentralized identity frameworks (DIDs), attribute-based models, and advanced cryptographic techniques such as zero-knowledge proofs and selective disclosure. To ensure feasibility in resource-constrained environments such as IoT and edge computing, lightweight consensus mechanisms—like Proof of Authentication (PoAh)—are explored to minimize energy consumption and latency while preserving trust. The paper also examines the integration of blockchain with existing authentication standards like OpenID Connect and Verifiable Credentials, and discusses practical implementation using tools such as Solidity, Truffle Suite, and MetaMask on Ethereum-compatible platforms. This work contributes to the advancement of trustworthy digital systems that enhance data privacy, support secure identity verification, and overcome the limitations of centralized architectures.