## 2 Related Works  
Several approaches have been proposed to ensure the integrity and authenticity of long-term time-stamping (LTTS) systems, which leverage cryptographic techniques, trusted third parties, and decentralized architectures [24]. While some focus on formal security models, others explore blockchain-based solutions to eliminate central authorities. [PERSON] et al. [25] introduced a formal security model for LTTS schemes based on Message Authentication Codes (MACs), archives, and transient keys, addressing the lack of cryptographic analysis in alternative approaches. Despite enhancing robustness against cryptographic advancements, their solution depends on trusted third parties (e.g., archives and TSAs), incurs storage overhead, and requires frequent cryptographic updates. Similarly, [PERSON] et al. [26] explored decentralized timestamping for long-term digital archives but still relied on timestamping servers, making their approach vulnerable to centralization risks. Unlike these methods, our approach leverages blockchain to eliminate the need for trusted third parties while ensuring verifiable timestamps and built-in dispute resolution. Blockchain-based timestamping has gained traction as an alternative to centralized solutions. [PERSON] and [PERSON] [27] proposed a scheme that records hash digests on a private blockchain, ensuring long-term verifiability without relying on external authorities. However, a private blockchain cannot provide guarantees as a public one, and the transaction delays introduced by the consensus mechanism and the risk of loss of keys affecting verification pose significant challenges. [PERSON] [28] extended this concept by implementing a permissioned blockchain for secure document timestamping, integrating off-chain storage and a quarantine mechanism. Although document management was enhanced, centralized control over the blockchain raises concerns regarding governance. Similarly, [PERSON] et al. [29] leveraged Ethereum blockchain to leverage transactions for timestamping, benefitting from its immutability but introducing privacy risks and high operational costs. To address these limitations, our approach adopts a blockchain model that stores only data hashes, thus eliminating privacy risks and significantly reducing operational costs. Scalability and computational efficiency remain key concerns in long-term trusted timestamping (LTTS). [PERSON] et al. [30] introduced IoETTS, a decentralized scheme for the Internet of Energy (IoE), leveraging public blockchain to improve data integrity. However, the computational and energy overhead associated with blockchain operations, particularly those relying on Proof of Work consensus, conflicts with the efficiency goals of IoE. Similarly, [PERSON] et al. [31] proposed a scalable LTTS scheme using commitment signatures and bilinear pairing accumulators to reduce transaction costs, but their approach lacks analysis of real-world deployment. In addition to system implementations, theoretical analyses of LTTS security and efficiency have emerged. [PERSON] [32] formalized timestamping mechanisms using transitive prefix authentication graphs, providing a structured comparison of efficiency and security trade-offs. However, maintaining authenticated prefix structures introduces storage and computational overhead, limiting large-scale applicability. [PERSON] et al. [33] examined client-side security risks, identifying vulnerabilities in key management and cryptographic resilience, but did not offer concrete mitigation strategies. Existing solutions often involve trade-offs between security, decentralization, scalability, and efficiency. In contrast, our approach leverages a public blockchain (Ethereum) to ensure decentralization and scalability, while maintaining the integrity of the stored hash and reducing operational costs. Blockchain technology has been used in several contexts where it is fundamental that data remain unchanged, tamper-proof, and provable [34; 35; 36].