Leveraging Timestamps to Create Secure and Feeless Evidence Management

In decentralized trustless data utilization, evidence (log) management and verification play a crucial role in assessing the reliability of other users (nodes). For secure evidence management, permissionless blockchains do not assume trust in a third-party community, which makes them highly verifiable. However, they require computational resources and usage fees. In this paper, we propose a new protocol, Decentralized Timestamp with Directed Acyclic Graph (DT-DAG), that enables data to be verified through graph traversal of joint-signatures by introducing lightweight local chains between users. DT-DAG allows users to maintain and use an evidence management system by managing only lightweight local chains using their resources (e.g., smartphones), thereby eliminating the need for fees for using external ones. Users can also verify the existence of the evidence through permissionless joint-signature verification. Our implementation of the proposed protocol using IPFS as an example framework acts as a proof-of-concept, demonstrating the feasibility of the DT-DAG in a local environment.