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To keep data confidential against unauthorized cloud servers and users, cryptographic access control mechanisms must be adopted. However, user revocation is a challenging issue since it would inevitably require data re-encryption, and may need user secret key updates. Considering the complexity of fine-grained access control policy and the large number of users in cloud, this issue would become extremely difficult to resolve. In this paper, we focus on this challenging open issue and present a secure and efficient revocation scheme. We propose a modified CP-ABE algorithm to set up a fine-grained access control method, in which user revocation is achieved based on the theory of Shamir's Secret Sharing. Compared with existing schemes, our scheme introduces a minimal overhead not only to the data owner but also to cloud servers. Collusions between cloud servers and revoked users can be avoided as long as the key-update protocol is honestly executed. Meanwhile, the data owner can delegate key updates to the cloud servers without disclosing data contents, user attributes, and the access policy information. Moreover, our scheme maintains the important feature that the revocation won't affect the users whose attribute set is a superset of the revoked user's.