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Key-aggregate cryptosystems (KAC) have attracted significant attention from the research community because of their elegance and efficiency in enforcing predefined access control policies for outsourced data. The data owner computes a constant-size aggregate key that is capable of decrypting a subset of outsourced data items. Based on the access control policy, the data owner securely transmits the aggregate key to a user authorized for the corresponding subset of data items. For practical access control scenarios, a KAC needs to satisfy additional flexibility requirements. We propose a practically motivated, novel cryptographic primitive called key-aggregate proxy re-encryption that allows temporary delegation of decryption capabilities of an aggregate key to one or more other aggregate keys without carrying out any secure transmissions. The existing key-aggregate cryptosystems face two important issues in …