Abstract: An important functionality of cloud storage is data sharing. Secure and efficient data sharing is the main aspect. Cloud storage can provide strong protection, good reliability and accessibility, disaster recovery, and lowest cost. On cloud storage anyone can share data as much they want to i.e. only selected content can be shared. Cryptography helps the data owner to share the data in a safer way. So user encrypts data and uploads on server. Aggregate key cryptosystem produces a constant size key such that efficient delegation of decryption rights for any file is possible in a secure manner. Here a public-key cryptosystem is used, which generates a public key which is used to decrypt the file. But the public is aggregated with one more key. The difference is one can collect a set of secret keys and make them as small size as a single key with holding the same ability to decrypt the file securely. This compact aggregate key can be efficiently sent to the clients or to be stored in a smart card with little secure storage.
Keywords: Aggregate key, data sharing, Encryption, Decryption, secret key.