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Data sharing is playing vital role in the cloud storage. Using cloud storage user can store and share their data very securely and efficiently. So data access security becomes the critical section to be focused. In this article, we describe a technique which works on how a user can share their data partially over cloud storages. This technique introduces a special type of encryption called as keyaggregate cryptosystem which produces constant size ciphertext. In this technique user can provide a constant-size aggregate key for different choices of ciphertext classes in cloud storage, but the other encrypted files outside the class remain confidential. We implemented this cryptosystem for public-key patient-controlled encryption for flexible hierarchy.