In section 4.3 we looked at an overview of the typical informal system development process and saw how that process is supplemented by formal techniques. The formal specification and formal verification phases of the formal system development paths (fig. 12-1) are used to increase the level of assurance that a system will meet its security requirements. Figure 12-1. System Development Paths. The formal specification satisfies the phase of system development corresponding to the informal functional specification. In chapter 9 we discussed mathematical concepts for defining a security model of a system, and in section 9.1 we summarized several paths—formal and informal—for demonstrating correspondence of the system to the model (illustrated in figure 9-2). Section 9.7 discussed specific techniques for following the informal paths without having to use a formal specification. The formal techniques for showing correspondence covered in this chapter closely follow the philosophy of the informal techniques, and reviewing section 9.7 will help you put these formal techniques into perspective. Despite the similarity in some of their formal methods, you should distinguish between the process of writing a formal model and the process of writing a formal specification. Formal specification
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