# Blockchain-Based Multiple Authorities Attribute-Based Encryption for EHR Access Control Scheme
###### Abstract  
The Internet of Medical Things (IOMT) is critical in improving electronic device precision, dependability, and productivity. Researchers are driving the development of digital healthcare systems by connecting available medical resources and healthcare services. However, there are concerns about the security of sharing patients' electronic health records. In response to the prevailing problems such as difficulties in sharing medical records between different hospitals and patients' inability to grasp the usage of their medical records, we propose a patient-controlled and cloud-chain collaborative multi-authority attribute-based encryption for EHR sharing with verifiable outsourcing decryption and hiding access policies (VO-PH-MAABE). This scheme uses blockchain to store the validation parameters by utilizing its immutable, which data users use to verify the correctness of third-party outsourcing decryption results. In addition, we use policy-hiding technology to protect data privacy so that data security is guaranteed. Moreover, we use blockchain technology to establish trust among multiple authorities and utilize Shamir secret sharing and smart contracts to compute keys or tokens for attributes managed across multiple administrative domains, which avoids a single point of failure and reduces communication and computation overhead on the data user side. Finally, the ciphertext indistinguishability security under the chosen plaintext attack is demonstrated under the stochastic prediction model and compared with other schemes in terms of functionality, communication overhead, and computation overhead. The experimental results show the effectiveness of this scheme. I 2022 2022 2022 20222222222222222222222222222222222222222222222  
decrypt, CP-ABE is considered an ideal solution for securely sharing outsourced data in public clouds. It is more suitable for access control scenarios for healthcare data. However, in most existing CP-ABE programs [1; 2; 3], all participants trust an authority. Due to the centralized authority problem, the mischievous attribute authority (AA) even causes the misuse of private keys. It distributes the attribute keys to illegal users, leading to unauthorized data access. However, some multi-AA extension schemes did emerge later, dividing the attribute Universe into multiple administrative domains. Each AA manages a disjoint set of attributes, which gives rise to trust issues between attribute domains and does not fundamentally solve the single point of failure, as well as the data users bearing high computation and communication overheads. Moreover, in conventional CP-ABE, the access policy embedded in the ciphertext is publicly accessible, so an attacker can indirectly deduce sensitive identity information about the data owner and the data user [2]. In addition, CP-ABE schemes contain a large amount of bilinear computation during encryption and decryption, which consumes many computational resources and limits the application of CP-ABE [4]. The emerging blockchain technology brings a glimmer of light to these challenges. Blockchain is a distributed ledger technology with tamper-proof features, which enables anyone to host a distributed ledger and keep a permanent record of transactions. Moreover, blockchain technology can establish multi-party trust. Smart contracts deployed on the blockchain enable a collaborative computing process with multiple authorities from different attribute domains and generate attribute keys for users to decrypt data. Since there are performance bottlenecks in blockchain at this stage, and EHR usually includes large-scale, cross-media health data, such as CT, X-ray, and other medical image data, it is inefficient to store and share EHRs independently using blockchain. Thus, it is necessary to combine cloud storage and blockchain to complement each other for secure and efficient EHR sharing.