The cloud storage is an online storage which can be accessible from different types of connected users. The cloud storage providing easy accessibility and also providing reliability. The cloud storage also providing strong protection to the users' data, recovery management and these all are provided at low cost. The characteristics of cloud making it as famous i.e., securely, efficiency and flexibility over data. In this paper, the authors are proposing a novel public key encryption which was also known as Key Aggregate Cryptosystem (KAC). Key aggregate cryptosystem provides a constant size cipher texts so that the efficient delegation of decryption credentials will be given to any set of cipher texts.