# Towards Secure Searchable Electronic Health Records Using Consortium Blockchain
###### Abstract
Searchable encryption is a technology that allows users to save ciphertext documents while ensuring the functionality to search their documents with keywords. In recent years, two different types of Searchable Encryption (SE) have been widely openly discussed, namely Symmetrical Searchable Encryption (SSE) and Public Key Encryption with Keyword Search (PEKS) or Asymmetrical Searchable Encryption (ASE). The downside of the SSE method is that there are several keys used for encryption and decryption and multi-keyword search cannot be supported [6]. PEKS was initially suggested by [PERSON] et al. [7], which is primitive encryption that solves the above addition issues efficiently. However, the vulnerability against Keyword Guessing Attacks (KGA) is an issue that reduces the security of the PEKS scheme. Recently, efforts have been spent on developing the PEKS scheme to solve the KGA problem, for example, in [8], in a Public-key Authenticated Encryption with Keyword Search (PAEKS) scheme a keyword is not only encrypted but also authenticated by the data owner. Another study [9] uses the method of renewing the keywords in the key server periodically to prevent the key server from being compromised. In [10] they used the smart contract as an effective solution against KGA. Besides this, Access Control (AC) in systems is typically carried out in three steps: Identification, Authentication and Authorization [11]. To implement AC mechanisms with SE methods, the proxy re-encryption (PRE) scheme can provide efficient Access Control for the patients with regard to their data. PRE is an encoding scheme allowing a third party (proxy) to modify a single party-encoded ciphertext to decode it by another party approved to do so [3]. The motivation of this paper is to develop an EHRs sharing protocol-based blockchain. It can be used to store, manage, and to exchange EHRs. The protocol should preserve security and privacy in the method used to share medical data, in addition to taking into consideration the minimization of communication and computational cost. In dealing with previous challenges, we proposed a secrecy and efficient blockchain protocol for EHRs sharing based on Public-key Encryption with Keyword Search (PEKS) using Elliptic-curve Diffie-Hellman (ECDH) with a Proxy Re-Encryption (PRE) scheme to preserve the data security and patient privacy. In summary, the contributions of the developed scheme are as follows. 1. To address the problem of poor AC of patients over their EHR, we suggest using a PRE-based blockchain to preserve the privacy of EHR and patients' fine-grained AC by re-encrypting the data by the patient's public key after each access by a third party (data researchers);
2. To address the problem of patient identity disclosure, we suggest storing the real identity in a private blockchain and using a unique blockchain address as a pointer to their identity;
3. To address the problem of preserving confidentiality in an EHR-sharing blockchain, we suggest using PEKS with a conjunctive keyword searchable scheme to store the encrypted index in the smart contract on a consortium Ethereum blockchain then storing the EHR in a private blockchain and periodically using a renewal key server technique to prevent compromising the key server and to solve the KGA problem. The remainder of the paper is arranged as follows. Section 2 outlines existing studies linked to our study, and describes the background of this work. Section 3 presents the developed system architecture and performance analysis of the EHR consortium. In Section 4, we conduct an experimental study, and we compute the computational and communication overheads of our proposed protocol implementation in Ethereum. An overall discussion regarding the proposed protocol is given in Section 5. Finally, Section 6 is the last section of our paper.