## 3 Security Implications and Fundamental Approaches of Blockchain-Based Data Storage Methods
### Authentication Methods  
Authentication is a key element of data security in blockchain-based systems, especially in sectors managing sensitive data such as healthcare, finance, and government [40; 45; 107; 118; 126; 105]. Blockchain enhances security and privacy by providing decentralized, tamper-proof authentication [115; 144; 56; 145]. On-chain methods ensure immutability, off-chain solutions offer scalability, and hybrid models combine both for flexible, secure authentication [16; 27; 32; 37; 77; 146]. In the next section, decentralized authentication methods using blockchain are explored.  
Decentralized Authentication, Applications, and Security Enhancements  
Blockchain-based authentication removes central authority dependence and secures identity data. By leveraging blockchain's immutability and transparency, authentication records are securely stored, preventing unauthorized modifications and ensuring data integrity [1; 144; 104].  
Decentralized authentication systems record each verification process immutably on the blockchain, reinforcing identity protection and preventing data tampering. Self-Sovereign Identity (SSI) solutions enable users to manage their credentials independently, leveraging blockchain's distributed nature to prevent unauthorized modifications [1; 104; 105]. Authentication processes rely on asymmetric encryption, where users sign credentials with a private key, and verification occurs via a public key before being recorded on-chain, ensuring both security and integrity [102; 53; 102].  
Blockchain authentication seamlessly adapts to different data storage models, balancing security, scalability, and efficiency. On-chain storage offers high security and ensures data integrity, making it ideal for critical identity records. However, it presents challenges in scalability and cost, particularly for large identity datasets [16; 37; 101]. Off-chain storage mitigates these issues by storing identity data externally while keeping cryptographic references on-chain, ensuring cost-effective scalability [10; 77; 89]. Hybrid approaches integrate both strategies, maintaining detailed records off-chain (e.g., in IPFS) while keeping verification data on-chain, ensuring transparency, security, and efficiency [16; 32; 37].  
Decentralized authentication plays a crucial role in sensitive sectors such as healthcare and finance. In healthcare, patients securely share identity credentials with authorized professionals, ensuring privacy and controlled access [1; 48; 147; 148; 149]. Hybrid models enhance both security and scalability by maintaining access logs on-chain while managing sensitive health records off-chain. In the financial sector, blockchain authentication strengthens fraud-resistant identity verification and transaction security [139; 43; 100].  
A key advancement in blockchain authentication is the Zero-Knowledge Proof (ZKP) technique, which enables identity verification without exposing personal data, significantly improving privacy in authentication processes [40; 56; 132]. In hybrid models, ZKP can validate credentials off-chain while storing only verification proofs on-chain, enhancing both security and efficiency [109; 115; 119].  
By providing decentralized, secure, and privacy-preserving authentication, blockchain solutions drive trust and resilience across diverse industries, including healthcare, finance, and IoT, paving the way for robust identity management in decentralized ecosystems [1; 147; 150; 147].