# Secure Outsourced Blockchain-Based Medical Data Sharing System Using Proxy Re-Encryption +
###### Abstract  
The security and privacy of electronic health records (EHRs) have received considerable attention from healthcare workers and researchers. To ensure security, various encryption and decryption schemes as well as key management protocols have been developed. However, owing to sharing and scalability issues, additional security technologies have been proposed. Nonetheless, these technologies cause other problems, such as efficiency issues. Blockchain-based EHR management systems have been proposed to overcome computational overhead. However, because most blockchain systems are installed by outsourcing companies, EHRs may be leaked to the company. Hence, we herein propose a blockchain-based EHR management scheme with proxy re-encryption. In this scheme, we set a proxy server that re-encrypts the ciphertext between file servers, thereby solving EHR sharing issues. Furthermore, because the server is separated from the blockchain system, the outsourcing company cannot manipulate the server or access the records. In addition, the blockchain assists in access control by using smart contracts, thereby enabling secure and efficient EHR sharing. By performing security analysis, we prove that our proposed scheme solves the aforementioned security problems. In addition, we experimentally demonstrate the efficient operation of the proposed system. EHR; blockchain; proxy re-encryption +
Footnote †: 29 August 2021  
rency, e.g., Bitcoin [11] and Ethereum [12]. Blockchain is used to store not only the data of currency transactions, but also various types of historical data such as the distribution process, status of shared items, security protocols [13], and electronic healthcare data [14, 15]. In particular, many blockchain-based medical data management systems have been proposed, such as MedRec [16] and MedShare [17]. However, the proposed systems exhibit several vulnerabilities. MedRec [16] and its extended version [18] employ a public blockchain; therefore, efficiency may be a concern. In addition, MedShare operates access control only with the blockchain; hence, encrypted medical data can be delivered to unauthorized users, and efficiency problems due to decryption and encryption may occur [17, 19]. Other solutions for overcoming both privacy and sharing issues have been proposed. [PERSON] et al. provided a framework that preserves personal health records based on attribute-based encryption (ABE) [20], and [PERSON] et al. proposed an extended version that employs attribute-based signcryption (ABSC) [21]. Furthermore, [PERSON] et al. developed a homomorphic encryption-based electronic health record (EHR) management system [22]. However, advanced encryption schemes such as ABE and homomorphic encryption involve computational overhead; therefore, they are not suitable for managing large volumes of medical data. The solutions presented above for secure medical data management systems cannot overcome the computational overhead problem owing to the outsourcing of blockchain [23, 24]. Most healthcare institutes are unable to develop systems that include blockchain and cryptography schemes owing to insufficient IT expertise [25]. However, when an outsourcing company is employed to implement a file server and a blockchain server, the healthcare data stored in the file server may be accessed by the company. In particular, this occurs when a decryption key is lost, or the stored data are deleted because of security attacks, natural disasters, etc. In this case, the data stored by the outsourcing company may be leaked. Therefore, a security solution that prevents the company from accessing privacy-sensitive healthcare data is required. Proxy re-encryption (PRE) is a promising solution to this issue. It transfers a ciphertext into another ciphertext, enabling the receiver to decrypt the encrypted data with its own secret key [26]. This method does not reveal any sensitive data, including plaintext or secret keys of the sender or receiver, during the entire process.