Correctness Criteria for Multilevel Secure Transactions

The benefits of distributed systems and shared database resources are widely recognized, but they often cannot be exploited by users who must protect their data by using label-based access controls. In particular, users of label-based data need to read and write data at different security levels within a single database transaction, which is not currently possible without violating multilevel security constraints. This paper presents a formal model of multilevel transactions which provide this capability. We define four ACIS (atomicity, consistency, isolation, and security) correctness properties of multilevel transactions. While atomicity, consistency and isolation are mutually achievable in standard single-site and distributed transactions, we show that the security requirements of multilevel transactions conflict with some of these goals. This forces trade-offs to be made among the ACIS correctness properties, and we define appropriate partial correctness properties. Due to such trade-offs, an important problem is to design multilevel transaction execution protocols which achieve the greatest possible degree of correctness. These protocols must provide a variety of approaches to making trade-offs according to the differing priorities of various users. We present three transaction execution protocols which achieve a high degree of correctness. These protocols exemplify the correctness trade-offs proven in the paper, and offer realistic implementation options.