Table 6. Summary of STPA-DFSec and STPA-Sec steps.

Summary and comparison of STPA-DFSec and STPA-Sec approaches with differences marked.

Basic four steps	STPA-DFSec details	STPA-Sec details
Step 1: Define the purpose of the analysis	Identify system-level losses, vulnerabilities, and constraints. Link vulnerabilities with corresponding losses and security attributes+. A general losses list is provided+	Identify system-level losses, vulnerabilities, and constraints
Step 2: Model the system structure	Model the system by functional interaction structure based on data flows*. A common function set for FIS is provided+	Model the system by functional control structure based on the control loop
Step 3: Identify insecure items	Use adapted guide words* (“not being executed”, “being executed” and “being executed but exceeding the time limits”) to identify insecure function behaviors	Use guide words (“not providing”, “providing”, “too early, too late, out of order”, “stopped too soon, applied too long”) to identify insecure control actions
Step 4: Identify loss scenarios	Use adapted guide words* (“function itself”, “execution environment (incl. function inputs, calling behaviors, computing resources, and links)”) to identify loss scenarios	Use guide words (“unsafe controller behavior”, “inadequate feedback and information”, “involving the control path”, “related to the controlled process”) to identify loss scenarios

Notes:

⁺Added features of the STPA-DFSec.

^*Modified steps in comparison with the original STPA-Sec.