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Cloud computing as a promising technology and paradigm can provide various data services, such as data sharing and distribution, which allows users to derive benefits without the need for deep knowledge about them. However, the popular cloud data services also bring forth many new data security and privacy challenges. Cloud service provider untrusted, outsourced data security, hence collusion attacks from cloud service providers and data users become extremely challenging issues. To resolve these issues, we design the basic parts of secure re-encryption scheme for data services in a cloud computing environment, and further propose an efficient and secure re-encryption algorithm based on the EIGamal algorithm, to satisfy basic security requirements. The proposed scheme not only makes full use of the powerful processing ability of cloud computing but also can effectively ensure cloud data security. Extensive analysis shows that our proposed scheme is highly efficient and provably secure under existing security model. Our contributionTo resolve these issues, an efficient and secure protection method for protecting the sensitive data confidentiality is desired in the cloud. In this paper, we first design a secure re-encryption model for cloud outsourced data and propose an efficient and secure re-encryption algorithm based on EIGamal.(1) Secret key query. The challenger runs sk 1 ← Keygen(par) to generate a secret key sk 1 .(2) Re-encryption key query. The adversary inputs (pk 1 , pk 2 ), and the challenger runs rekey oi ← ReKeygen(par, sk 1 , pk 2 ) to generate a re-encryption key rekey oi . (3) Re-encryption query. The adversary inputs (pk 1 , pk 2 , CT), and the challenger runs CT′ ← ReEnc (par, rekey oi , CT) to generate the re-encrypted ciphertext. 4580

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