Privacy Enhanced Access Control by Means of Policy Blinding

Traditional techniques of enforcing an access control policy rely on an honest reference monitor to enforce the policy. However, for applications where the resources are sensitive, the access control policy might also be sensitive. As a result, an honest-but-curious reference monitor would glean some interesting information from the requests that it processes. For example if a requestor in a role psychiatrist is granted access to a document, the patient associated with that document probably has a psychiatric problem. The patient would consider this sensitive information, and she might prefer the honest-but-curious reference monitor to remain oblivious of her mental problem. We present a high level framework for querying and enforcing a role based access control policy that identifies where sensitive information might be disclosed. We then propose a construction which enforces a role based access control policy cryptographically, in such a way that the reference monitor learns as little as possible about the policy. (The reference monitor only learns something from repeated queries). We prove the security of our scheme showing that it works in theory, but that it has a practical drawback. However, the practical drawback is common to all cryptographically enforced access policy schemes. We identify several approaches to mitigate the drawback and conclude by arguing that there is an underlying fundamental problem that cannot be solved. We also show why attribute based encryption techniques do not not solve the problem of enforcing policy by an honest but curious reference monitor.