Abstract: Data sharing is an important functionality in cloud storage. This work, show how to efficiently, securely, and flexibly share data with others in cloud storage. It describes new public-key cryptosystems that produce constant-size ciphertexts such that efficient delegation of decryption rights for any set of ciphertexts is possible. More importantly one can aggregate any set of secret keys and make them as compact as a single key, but having the power of all the keys being aggregated. This compact aggregate key can be conveniently sent to others or be stored in a smart card with very limited secure storage.

Keywords: Cloud storage, data sharing, encryption, key-aggregation.