Who is touching my cloud

Advanced access controls have been proposed to secure sensitive data maintained by a third party. A subtle issue in such systems is that some access credentials may be leaked due to various reasons, which could severely damage data security. In this paper, we investigate leakage tracing enabled access control over outsourced data, so that one can revoke the suspected leaked credentials or prepare judicial evidences for legal procedure if necessary. Specifically, we propose a leaked access credential tracing (LACT) framework to secure data outsourced to clouds and formalize its security model. Following the framework, we construct a concrete LACT scheme that is provably secure. The proposed scheme offers fine-grained access control over outsourced data, by which the data owner can specify an access policy to ensure that the data is only accessible to the users meeting the policy. In case of suspectable illegal access to outsourced data with leaked credentials, a tracing procedure can be invoked to tracing in a black-box manner at least one of the users who leaked their access credentials. The tracing procedure can run without the cloud service provider being disturbed. Analysis shows that the introduction of tracing access credential leakage incurs little additional cost to either data outsourcing or access procedure.