# MedChain: Efficient Healthcare Data Sharing via Blockchain
## 2 Related Works  
In the medical and healthcare sector, legacy systems generally only exchange medical resources internally [13] and are not interoperable with external systems [14]. Yet, evidences [15; 16] show numerous benefits from connecting these systems for integrated and improved healthcare, calling health informatics researchers for an interconnection solution among different organizations. One of the most important challenges is inter-organizational data sharing [17], demanding the medical data collected by one healthcare provider to be securely accessible to other entities, such as a doctor or a research organization. Cloud computing is considered to be an immediate solution for medical data storage and sharing [6; 18; 19] because it is scalable, highly available, and flexible in pricing. Yet, due to the privacy and confidentiality requirements in healthcare data sharing, additional security means must be applied to mitigate the risks of cloud sourcing in healthcare and public health industry [7]. Many studies [4; 20] have been conducted to address the security and privacy issues. For example, Reference [21] analyzes the security and privacy issues in the access and management of EHRs. References [22; 23] explores the solution for search over encrypted health records in a public cloud. Reference [24] studies the identity exposure problem in cloud-based healthcare applications and proposes an anonymous authentication approach. However, cloud services are not fully trusted by users, due to the infrastructure security, data ownership, and vendor lock-in issues in cloud sourcing [7; 25; 26]. Our experience [18] shows the importance of inter-organizational healthcare data sharing. In Macau, for example, the hemodialysis center does not provide a platform for examination result sharing. Patients have to carry the paper/CD-based results to hospitals for diagnosis. Our previous work [18] provides a hybrid cloud-based medical resource sharing solution for electronic healthcare record (EHR) sharing, which relies on the trust of an external cloud service provider. This work replaces the cloud with two decentralized networks to remove the needs of the external party. To win user trust, blockchain-enabled medical record sharing has been extensively discussed within the last two years [8]. [PERSON] et al. reported their early adoption in Reference [27]. They treat blockchain as a database for storing health records. Likewise, References [28; 29] use permissioned blockchain as a repository of health data to attain access accountability and data integrity. These approaches need to transfer the actual data to blockchain servers, bringing extra overhead in data transmission. Reference [30] uses blockchain to store only the address of shared data. However, data sharing is not sufficiently discussed. Recently, MeDShare [31] enables the sharing of patient medical data to third-party research institutes in cloud repositories. It uses a smart contract for data access auditing and access control. However, MeDShare also shares the user trust issue in cloud computing. In another design for medical image sharing [32], the source files are still stored at the end of healthcare providers and only the URLs for file access are stored on the blockchain. Unfortunately, these solutions do not provide an efficient approach for data search over the blockchain. Since blockchain is not search-friendly, looking up a specific record would be very slow with the increase of data. MedRec [33] and MedBlock [34] provide a breadcrumb mechanism for a record search. Breadcrumbs maintain the address of blocks containing the records of a patient, grouped by a healthcare provider or department. Reference [35] implements the keyword-based search over encrypted data for record matching. However, they still share three limitations. First, a blockchain only stores immutable records. Yet, the actual locations of data are likely to be changed.