Cloud storage services enable users to remotely store their data and eliminate excessive local installation of software and hardware. One critical issue is how to enable a secure data collaboration service including data access and update in cloud computing. A data collaboration service is to support the availability and consistency of the shared data among multi-users. In this paper, we propose a secure and efficient data collaboration scheme SECO. In SECO, we employ a twolevel hierarchical

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... vel hierarchical identity based encryption (HIBE) to guarantee data confidentiality against untrusted cloud. This paper is the first attempt to explore secure cloud data collaboration service that precludes information leakage and enables a one-to-many encryption paradigm, data writing operation and fine-grained access control simultaneously. Security analysis indicates that the SECO enforces fine-grained access control and collusion resistant. Extensive performance analysis and experiment results demonstrate that SECO is highly efficient and low overhead on computation and communication.