###### Abstract  
With continuous advancements in Internet technology and the increased use of cryptographic techniques, the cloud has become the obvious choice for data sharing. Generally, the data are outsourced to cloud storage servers in encrypted form. Access control methods can be used on encrypted outsourced data to facilitate and regulate access. Multi-authority attribute-based encryption is a propitious technique to control who can access encrypted data in inter-domain applications such as sharing data between organizations, sharing data in healthcare, etc. The data owner may require the flexibility to share the data with known and unknown users. The known or closed-domain users may be internal employees of the organization, and unknown or open-domain users may be outside agencies, third-party users, etc. In the case of closed-domain users, the data owner becomes the key issuing authority, and in the case of open-domain users, various established attribute authorities perform the task of key issuance. Privacy preservation is also a crucial requirement in cloud-based data-sharing systems. This work proposes the SP-MAACS scheme, a secure and privacy-preserving multi-authority access control system for cloud-based healthcare data sharing. Both open and closed domain users are considered, and policy privacy is ensured by only disclosing the names of policy attributes. The values of the attributes are kept hidden. Characteristic comparison with similar existing schemes shows that our scheme simultaneously provides features such as multi-authority setting, expressive and flexible access policy structure, privacy preservation, and scalability. The performance analysis carried out by us shows that the decryption cost is reasonable enough. Furthermore, the scheme is demonstrated to be adaptively secure under the standard model. electronic health records; access control; cloud storage; attribute-based encryption; multiple authorities; privacy preservation +
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health records (EHRs). For example, let us assume that a patient is suffering from chronic heart disease and they use body sensors to record their blood pressure. As patient monitoring services make their EHR available online, their doctors can treat them efficiently. In addition, in some unfavorable conditions when they may need medical attention, any treating doctor can access their data to make the right diagnosis. The availability of their EHRs can save their life. Patients can maintain their EHR data on cloud storage servers and receive various advantages such as 24/7 access, easy management, and tracking and sharing of the EHR data for treatment as well as beneficiary purposes. However, cloud-based data outsourcing is not trusted because the data owners lose physical control of their data. Performing data encryption before uploading to the cloud is one solution to the above problem. This limits accessibility for legitimate users, as they will need decryption keys for accessing the data. Data owners can only issue the keys to known users. Other legitimate users cannot access the encrypted data in this case. Therefore, there is a need for a secure data-sharing system allowing the data owner to provide access control to data in a fine-grained manner. The system should have proper attributes and key management facilities to serve all legitimate users. To accomplish this outsourcing paradigm, the cloud-based EHR approach is motivated by the concepts of \"Availability, Scalability, Cost efficiency, and Convenience\" [2]. Dissemination of patient data through cloud-based EHRs is advantageous, but it must be performed with such care that the privacy of patients is protected. Various cryptographic access control models have been defined to provide data security and control unauthorized data access. For this need, attribute-based encryption, also known as ABE [3], seems to be a suitable option. It allows for fine-grained and flexible access control.