Abstract

Cloud storage is nowadays very popular storage system. Cloud storage is a storage of data online in cloud which is accessible from multiple and connected resources. Cloud storage can provide good accessibility, reliability, strong protection, disaster recovery, and lowest cost. here describes how securely ,efficiently and flexibly sharing data with others in cloud storage. In modern cryptography, a primary problem often studied is that leveraging the secrecy of a small piece of knowledge into the ability to perform the cryptographic functions (e.g., encryption, authentication) multiple times making a decryption key more powerful in the sense that it allows decryption of multiple cipher texts, without increasing its size. This problem is solved by introducing a special type of public-key encryption which is called as key-aggregate cryptosystem (KAC).KAC produces constant-size ciphertexts such that efficient delegation of decryption rights for any set of ciphertexts are possible. Any set of secret keys can be aggregated and make them as single key, which encompasses power of all the keys being aggregated. This aggregate key can be sent to the others for decryption of ciphertext set. The remaining encrypted files outside the set are remains confidential and secure. This compact aggregate key can be conveniently sent to others or to be stored in a smartcard with very limited storage. This scheme give the first public-key patient-controlled encryption for flexible hierarchy, which was yet to be known.

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