Sharing of Electronic Health Records (EHRs) is of significant importance in health care. Lately, a cloudbased electronic health record sharing scheme has been used extensively to share patient records among various healthcare organizations. However, cloud centralization may compromise patients' privacy and security. Due to the special features of blockchain, it is important to see this technology as a promising solution to resolve these issues. This article proposes a privacy-preserving, secure EHR sharing and access control framework based on blockchain technology. The proposal aims to implement EHR blockchain technology and ensure that electronic records are stored safely by specifying user access permissions. We emulate the cryptographic primitives and use smart contracts to describe the relationships between the EHR owner and EHR user through the proposed system on the Ethereum blockchain. We assess the proposal results based on encryption and decryption time and the costs of the smart contract. The encryption and decryption times are proportional to the size of the EHR, which varies from 128 KB to 128 MB. When it comes to encryption, the smallest EHR takes 0.0012 s to encrypt, while the largest EHR, which is 128 MB, takes 1.4149 s. On the other hand, a 128 KB EHR takes 0.0013 s to decrypt, whereas a 128 MB EHR requires 1.6284 s. As a result, performance evaluation and security analysis confirm that the proposal is secure for practical application.

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In recent years, cloud computing has been widely used in various fields and is gaining importance in healthcare systems. Patients' health data are outsourced to cloud storage, enabling healthcare professionals to easily access health information from anywhere and at any time to improve health services. Once patient data are stored in the cloud, they are vulnerable to attacks such as data loss, denial of service (DoS), distributed denial of service (DDoS) and other sorts of cyberattacks. Data confidentiality and patient privacy are more of a problem in the cloud computing context due to their public availability. If a patient's personal information is stolen, he or she may face a range of problems. These are concerns that necessitate more security. The transmission of this sensitive information over the internet is always susceptible to hacking. Therefore, the privacy of patients' data is considered one of healthcare organizations' main issues. To overcome this problem, encryption mechanisms that place a significant emphasis on securing data within the cloud environment are used to preserve sensitive health data. A hybrid cryptography approach is employed in this paper to ensure the secure sharing of health data over the cloud. To maintain data privacy and secrecy, a hybrid cryptography mechanism for storing and transporting data to and from the cloud is used. To protect data from malevolent insiders, the encryption key is separated into two halves, controlling access to patient records via a specific technique. This paper shows the implementation and performance evaluation of the proposal as a functional system prototype. The evaluation is based on the key generation time, the record encryption time, the record decryption time, the record upload time and the record download time for different user numbers and different file sizes varying from 0.1 MB to 500 MB. The findings show that the proposal performs better than other state-of-the-art systems and can practically share secure health data in cloud environments.