Keeping secrets in incomplete databases.

Controlled query evaluation (CQE) preserves confidentiality in information systems at runtime. A confidentiality policy specifies the information a certain user is not allowed to know. At each query, a censor checks whether the answer would enable the user to learn any classified information. In that case, the answer is distorted, either by lying or by refusal. We introduce a framework in which CQE can be analyzed wrt. possibly incomplete logic databases. For each distortion method, lying and refusal, a class of confidentiality-preserving mechanisms is presented. Furthermore, we specify a third approach that combines lying and refusal and compensates the disadvantages of the respective uniform methods. The enforcement methods are compared to the existing methods for complete databases. [ABSTRACT FROM AUTHOR]