<P> A number of security threats are associated with cloud data services: not only traditional security threats, such as network eavesdropping, illegal invasion, and denial of service attacks, but also specific cloud computing threats, such as side channel attacks, virtualization vulnerabilities, and abuse of cloud services . The following security requirements limit the threats . </P> <P> Data confidentiality is the property that data contents are not made available or disclosed to illegal users . Outsourced data is stored in a cloud and out of the owners' direct control . Only authorized users can access the sensitive data while others, including CSPs, should not gain any information of the data . Meanwhile, data owners expect to fully utilize cloud data services, e.g., data search, data computation, and data sharing, without the leakage of the data contents to CSPs or other adversaries . </P> <P> Access controllability means that a data owner can perform the selective restriction of access to her or his data outsourced to cloud . Legal users can be authorized by the owner to access the data, while others cannot access it without permissions . Further, it is desirable to enforce fine - grained access control to the outsourced data, i.e., different users should be granted different access privileges with regard to different data pieces . The access authorization must be controlled only by the owner in untrusted cloud environments . </P> <P> Data integrity demands maintaining and assuring the accuracy and completeness of data . A data owner always expects that her or his data in a cloud can be stored correctly and trustworthily . It means that the data should not be illegally tampered, improperly modified, deliberately deleted, or maliciously fabricated . If any undesirable operations corrupt or delete the data, the owner should be able to detect the corruption or loss . Further, when a portion of the outsourced data is corrupted or lost, it can still be retrieved by the data users . </P>

What technique csp does for cloud storage security