Study Of The Verifiability Of Attribute-Based Encryption With Outsourcing

As we all know, cloud servers have the powerful capacities of computing and storage. Users can not only store the data in the cloud servers, but also outsource large and complex calculation to cloud servers. Data owners encrypt data using the attribute-based encryption scheme, the ciphertext is stored in the cloud servers, which largely reduce the local storage's cost and realize data sharing effectively and securely.Due to the decryption algorithm efficiency of attribute-based encryption scheme is low,how to outsource some decryption operations to the cloud servers; Due to the cloud is not fully trusted, how to verify the validity of cloud server outsourcing decryption operation. Hence, it has the important theoretical significance and application value to research and establish the verifiable technology of Attribute-Based Encryption with outsourced decryption in cloud environment. The main work shows as follows:This thesis firstly analyzes LW's scheme and points out its defects such as the cost of decryption growing with the complexity of the access structures and the number of policies. Pointing at these deficiencies, according to the thought of Lai et al.'s scheme,an attribute-based encryption with efficient verifiable outsourced decryption scheme is put forward, and proved to be selective choosen-plaintext security under the RCCA model. The user's private key is divided into two parts: transformation key and retrieve key with the blind key technology in the proposed scheme. After receiving the transformation key and the request of outsourcing decryption, cloud servers perform some decryption operations, which can realize the function of outsourcing decryption.We add the ciphertext of an extra random message and a checksum valum to the original ciphertext, which can be used to check if the transformation is done by cloud correctly in our KP-ABE Scheme with verifiable outsourced decryption. Compared with the LW's KP-ABE scheme,the proposed scheme can not only reduce the time of decryption of local users from linear growth to be constant, but also verify the validity of cloud server outsourcing decryption operation.Although the first scheme can implement outsourcing decryption and verifiable function, the length of the ciphertext is doubled. An improved attribute-based encryption with efficient verifiable outsourced decryption scheme is proposed in this thesis based on the Qin et al.'s Key Encapsulation Mechanism. Therefore,the improved scheme adopt the Key Encapsulation Mechanism, making the output of encryption algorithm include a verification key which is used to check if the transformation is doneby cloud correctly. Compared with the existing schemes, the improved scheme can not only reduce the cost of the local calculation and verify the validity of cloud server outsourcing decryption operation, but also keep the size of ciphertext unchanged.