###### Abstract  
Population aging is currently a tough problem of many countries. How to utilize modern technologies (including both information and medical technologies) to improve the service quality of health information is an important issue. Personal Health Record (PHR) could be regarded as a kind of health information records of individuals. A ciphertext policy attribute-based encryption (CP-ABE) is a cryptographic primitive for fine-grained access control of outsourced data in clouds. In order to enable patients to effectively store his medical records and PHR data in medical clouds, we propose an improved multi-user CP-ABE scheme with the functionality of keyword search which enables data users to seek for specific ciphertext in the cloud server by using a specific keyword. Additionally, we adopt an independent proxy server in the proposed system architecture to isolate the communication between clients and the cloud server, so as to prevent cloud servers from suffering direct attacks and also reduce the computational loading of cloud servers. Compared with the previous approach, the proposed encryption algorithm takes less running time and the ciphertext length is also relatively short. Moreover, the procedures of re-encryption and pre-decryption only require one exponentiation computation, respectively.  
attribute-based encryption; ciphertext policy; medical cloud; keyword search; proxy re-encryption +
Footnote †: 2021: 11, 63. [[https://dx.doi.org/10.3390/app11010063](https://dx.doi.org/10.3390/app11010063)]([https://dx.doi.org/10.3390/app11010063](https://dx.doi.org/10.3390/app11010063))
Electronic health record (EHR)This record contains medical history, medication records, prescription, medication allergies, etc. It can be accessed by specific hospitals with the the patient's consent, so that medical personnel can easily search the relevant information about the patient. In addition, the record can be updated by patients and medical institutions at any time. With various information, medical staff are able to make more accurate and efficient diagnoses.  
Electronic medical record (EMR)Nowadays, the electronic medical records used by medical institutions, the process of patient diagnosis and treatment are all kept in the medical cloud system by the physicians. The access right of the electronic medical records is owned by the physician, meaning that these data could be accessed by hospitals through the medical clouds.  
In medical cloud environments, there will be three main roles: a medical data owner, a cloud server and a medical user. The medical data owner will store his/her own health data on the cloud server and then authorize specific users such as patients and medical staffs to access the data. However, since cloud servers are not completely trustworthy, there will be many security misgivings such as data leakage and data loss. When the data are stored in the clouds with the form of ciphertext, it would be difficult to perform the functionality of keyword search. Consequently, a suitable cryptographic mechanism which could deal with the above scenarios is quite important. In this work, we elaborate on the merits of attribute-based encryption mechanisms and come up with a multi-user ciphertext policy attribute-based encryption scheme with the functionality of keyword search for medical cloud systems.