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Cyber engineering is getting more significant in not only manufacturing but also business applications. Information management technologies play a key role in cyber engineering. In order to realize the cyber engineering, objects in information systems are required to be not only secure but also consistent in the presence of security threats and multiple conflicting transactions. There are so far many discussions on each of the security management and concurrence control. However, there is no discussion on synchronization mechanisms to realize both of them. In this paper, we discuss a novel synchronization protocol to make an information system not only secure but also consistent. In the role-based access control model, authorized access requests are specified in roles which are collections of access rights. A transaction issued by a subject is assigned with a subfamily of the roles granted to the subjects, which are named purpose. We first define legal information flow relations among purposes. Based on the legal information flow relations, we discuss a purpose-marking (PM) protocol to prevent illegal information flow to occur by performing conflicting transactions in a serializable way. We evaluate the PM protocol in terms of the number of transactions to be aborted.