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A Key Approach To Secure Cloud Computing Is For The Data Owner To Store Encrypted Data In The Cloud, And Issue Decryption Keys To Authorized Users. Then, When A User Is Revoked, The Data Owner Will Issue Re-encryption Commands To The Cloud To Re-encrypt The Data, To Prevent The Revoked User From Decrypting The Data, And To Generate New Decryption Keys To Valid Users, So That They Can Continue To Access The Data. However, Since A Cloud Computing Environment Is Comprised Of Many Cloud Servers, Such Commands May Not Be Received And Executed By All Of The Cloud Servers Due To Unreliable Network Communications. In This Paper, We Solve This Problem By Proposing A Time-based Re-encryption Scheme, Which Enables The Cloud Servers To Automatically Re-encrypt Data Based On Their Internal Clocks. Our Solution Is Built On Top Of A New Encryption Scheme, Attribute-based Encryption, To Allow Fine-grain Access Control, And Does Not Require Perfect Clock Synchronization For Correctness.