arXiv:2610.34279v1 Announce Type: new Abstract: Recently, many blockchain-based medical data sharing systems using off-chain storage have been proposed. However, these systems have some drawbacks such as being incompletely decentralized, low adaptability to cloud storages, high cost, and low security. It is non-trivial to address the drawbacks due to the transparency, immutability, and decentralization of a blockchain platform. Our goal is to design a decentralized medical data sharing system with off-chain storage that addresses all the above drawbacks while satisfying data protection and privacy laws. We propose the VASCO (View-based AccesS COntrol) system that does not rely on any central server including a key generation one, not rely on any special cloud storage including IPFS, not use a secure channel, encrypts both data and metadata, and supports end-to-end encryption. We achieve it by designing eight chaincodes rigorously based on view-based access control. For a medical data ecosystem that consists of many stakeholders who employ different data standards, storage formats, and transmission protocols, we present how typical flows of medical data in the ecosystem can be achieved by using the proposed chaincodes with interoperability. We showed that VASCO is superior to the existing systems in terms of both functionality and regulatory compliance. We also tested the scalability of VASCO using a blockchain of 21 peer nodes with different specifications and verified interoperability through 30 combinations of standards, formats, and protocols. VASCO is a promising medical data sharing system that can provide secure and efficient sharing of medical data while preserving patient privacy and satisfying regulatory compliance with interoperability. The result of this study provides a potential to improve patient outcomes and advance medical research for the medical data ecosystem.