Data sharing is an important functionality in cloud storage. In this paper, the authors show how to securely, efficiently and flexibly share data with others in cloud storage. They describe new public-key cryptosystems that produce constant-size cipher texts such that efficient delegation of decryption rights for any set of cipher texts is possible. The novelty is that one can aggregate any set of secret keys and make them as compact as a single key, but encompassing the power of all the keys being aggregated.