Key-aggregate authentication cryptosystem for data sharing in dynamic cloud storage

Sharing encrypted data with different users via public cloud storage is an important functionality. Therefore, we propose a key-aggregate authentication cryptosystem that can generate a constant-size key that supports flexible delegation of decryption rights for any set of ciphertexts. The size of the key is independent of the number of maximum ciphertexts, meaning that the expense of our scheme is stable no matter how frequently users upload files to the cloud server. In addition, the authentication process in our scheme solves the key-leakage problem of data sharing. The data owner can extract an aggregated key that includes indices of the ciphertexts, the identity of the delegate, and the expiration date of the key. The key with the public parameters is used by the cloud server to identity the person or entity requesting a download, allowing the cloud server to control the right to download. Remarkably, we proved that the authentication key cannot be forged, and the message in this key cannot be denied. The method that is used to achieve efficient and secure data sharing in dynamic cloud storage must be stable in expense and leakage-resilient. Our scheme simultaneously satisfies both of these requirements.