Abstract

Digital identity is the foundation for all of contemporary digital life—banking and e-commerce, telemedicine and smart infrastructure. Legacy identity systems, however, which centralize sensitive user data in a handful of providers, put individuals and institutions at risk of catastrophic breaches, dark consent models, and vendor lock-in. Blockchain technology presents a different paradigm: its distributed ledger stores user registrations, verifiable credentials, and access transactions securely; smart contracts autonomously execute credential issuance, verification, and revocation without intermediaries; and decentralized storage networks such as IPFS provide personal data to be off-chain but tamper-evident through cryptographic hashes. This survey is a critical analysis of nine notable contributions in four different categories: foundation architecture taxonomies that classify blockchain networks and consensus protocols; end-to-end proofs of concept on Ethereum/IPFS demonstrating consent-based KYC workflows and one-time-password login sequences; theoretical explorations of self-sovereign identity (SSI) comparing DID architectures with minimal principles of user control and minimal disclosure; and specialized extensions including context-aware access rules, NFT-based academic credentials, and hierarchical DID architectures optimized for resource-constrained IoT deployments. We include real-case studies such as Estonia's e-Residency smart IDs, MIT's blockchain diplomas, time-geofence smart contracts managing city parking permits, and authentication of industrial sensors in private Practical Byzantine Fault Tolerance (pBFT) gateways to ground our analysis in real-world deployments. By combining architectural patterns, privacy-enhancing modules like zero-knowledge proofs, and performance benchmarks, we identify common design trade-offs and the open issues of global scalability, cross-chain interoperability, human-centric key recovery, and regulatory compliance with "right to be forgotten" requirements. To enable future innovation, we suggest a modular reference architecture of user-centric agent SDKs, specialized Layer-2 DID registries, hybrid consensus networks, decentralized storage, and trust-minimized oracle networks. This multi-layered architecture outlines a path to a truly unified, self-sovereign digital identity ecosystem—one that is secure, privacy-preserving, and universally interoperable.