Cloud computing is a novel model for computing and storing. It enables elasticity, on-demand and low-cost usage of computing resources. Electronic health record (EHR) is an emerging patient-oriented paradigm for sharing of medical data. With the arrival of cloud computing, health care industries outsource their EHR to the cloud servers but, at the same time there is increased demand and concern for outsourced EHR’s security also. The major concerns in data outsourcing are the implementation of access policies and policies modification. To address these issues, the optimal solution is Ciphertext Policy Attribute Based Encryption (CP-ABE). CP-ABE allows the patients to describe their own access policies and implement those policies on their data before outsourcing into the cloud servers. But there are major limitations like key escrow and user revocation problems. In this paper, we proposed a modified CP-ABE scheme with user revocation to strengthen data outsourcing system in cloud architecture. The proposed system addresses the key-escrow and revocation problems. 1) The key-escrow problem is solved by using two-authority computation between the key generator authority and cloud server and 2) An immediate attribute modification method is used to achieve fine-grained user revocation. Security analysis and performance evaluation demonstrates that the proposed system is efficient to achieve security in outsourced EHRs in cloud servers.
