Quantitative Toolchain Assurance

The software bill of materials (SBOM) concept aims to include more information about a software build such as copyrights, dependencies and security references. But SBOM lacks visibility into the process for building a package. Efforts such as Supply-chain Levels for Software Artifacts (SLSA) try to remedy this by focusing on the quality of the build process. But they lack quantitative assessment of that quality. They are purely qualitative. A new form of assurance case and new technique for structuring it, called process reduction, are presented. An assurance case for a toolchain is quantitative and when structured as a process reduction can measure the strength of the toolchain via the strength of the reduction. An example is given for a simple toolchain.