Creating Your First Security DFD

Start with a context diagram showing your system as a single process interacting with external entities. This high-level view establishes system boundaries and identifies all external interactions. For a web application, external entities might include end users, administrators, payment processors, email services, and potential attackers. Draw trust boundaries around your system to clearly delineate what you control versus external dependencies.

Decompose the main process into major components, creating a Level 1 DFD. Focus on architectural components rather than detailed functionality. A typical web application might decompose into web server, application server, database, authentication service, and background job processors. Show data flows between components, marking trust boundaries where components have different privilege levels or security contexts.

Continue decomposition where security analysis requires more detail. Areas handling sensitive data, implementing security controls, or interfacing with external systems often benefit from Level 2 diagrams. However, avoid over-decomposition that obscures rather than clarifies security relationships. The goal is sufficient detail for threat identification, not complete functional documentation.

Add security annotations systematically. For each element, consider authentication requirements, authorization levels, data sensitivity, encryption status, and validation performed. Use consistent notation—perhaps colors for trust levels, line styles for encryption status, or symbols for authentication types. Include a clear legend explaining your notation choices.