Data sharing is an important function in cloud storage. Secure, efficient, and flexible sharing of data with others in cloud storage is studied. The public-key cryptosystems describes that any number of fixed-size cipher texts can be produced that helps in efficient assignment of decryption rights for any set of cipher text. The innovative idea is that one can aggregate any set of secret keys and make them as compact as a single key, but covering the power of all the keys being aggregated. In other words, the secret key holder can release a constant-size aggregate key for flexible choices of cipher text set in cloud storage, but the other encrypted files outside the set remain confidential. This compact aggregate key can be conveniently sent to others or be stored in a smart card with very limited secure storage.