The important functionality of cloud is data storage. In this paper, the authors are going to discuss about how to flexibly, efficiently and securely share the data with another client in cloud storage. New public-key cryptosystems is described here that produces constant-size cipher texts that has efficient delegation of decryption rights for any set of cipher-texts are possible. The new thing is that one can aggregate any set of secret keys and make them as concise as a single key, but including power of all keys being aggregated. The secret key holder can free a constant-size aggregate key for flexible choices of cipher text set in cloud storage, but the other encrypted files which are outside the set remain secretly.