Data exchange is an important functionality in cloud storage. In this article, we show you how to securely, efficiently and flexibly share data with others in cloud storage. The new public key cryptosystems that produce encrypted texts of constant size in such a way that it is possible the efficient delegation of decryption rights for any set of encrypted texts. The novelty is that you can add any set of secret keys and make them as compact as a single key, but encompassing the power of all the keys that are added. In other words, the holder of the secret key may release a constant-sized aggregate key for flexible encryption set options in the cloud storage, but the other files encrypted outside the set remain confidential. This compact aggregate key can conveniently be sent to others or stored on a smart card with very limited secure storage.