Writing a Security Requirements Doc for Hardware
Security that is not written down does not get built. A threat model that lives in someone’s head, or in a slide deck nobody revisits, does not change what engineers implement. A security requirements document is the bridge: it turns the threats you identified into specific, testable statements that become part of what the product must do, and what testing must verify. Here is how to write one for hardware.
Why Requirements, Not Just a Threat Model
A threat model identifies what could go wrong. It does not, by itself, change the product. Engineers build to requirements, and if security is not among them, it competes for attention against features that are written down, and loses. The requirements document is what gives security the same standing as anything else the product must do.
Writing requirements also forces precision. A threat model can say the debug port is a risk; a requirement must say the debug interface shall be disabled via fuse in production units, verified by a failed read attempt. That precision is what an engineer can implement and a tester can check, and it is what turns a concern into a built-and-verified property.
Start from the Threat Model
The requirements flow from the threat model, so build that first, from attack trees, trust boundaries, and the asset-and-attacker analysis. Each significant threat becomes one or more requirements that mitigate it, and the traceability from threat to requirement is what ensures the document addresses real risks rather than a generic wish list.
This linkage matters in both directions. Every requirement should trace back to a threat it counters, so you can justify it; every serious threat should trace forward to a requirement that addresses it, so you can see nothing was dropped. A requirements document with no threat model behind it tends to be either incomplete or full of items nobody can justify.
Make Each Requirement Testable
A requirement you cannot test is a wish. Each one should state a condition that can be objectively verified, ideally with the test method implied. Avoid vague verbs like ensure secure and handle properly; state the specific behavior and the bar it must meet.
The strong version names the mechanism, the algorithm, the failure behavior, and the test, so an engineer knows exactly what to build and a tester knows exactly how to confirm it. The weak version can be argued met or unmet by anyone, which means it will be neither built reliably nor tested meaningfully.
What the Document Must Cover
A hardware security requirements document spans three areas, each tied back to a threat the model named:
- Hardware decisions. The permanent choices, whether there is a secure element, whether debug can be fused off, how keys are stored, which interfaces are exposed, stated so they drive the BOM and the layout before fabrication. For example, the device shall store the signing key in a secure element from which it cannot be exported.
- Firmware and boot. Secure boot and what it verifies, a signed update mechanism with rollback protection, key handling, input validation at trust boundaries, and no debug hooks in production builds. A downgrade threat becomes the device shall reject firmware older than the current version via an anti-rollback counter.
- Provisioning and keys. How keys are generated, injected, and managed at manufacturing, per-device, generated on the device where possible, with no central store of private keys, because provisioning is where per-device security is won or lost and the requirement is what stops the line defaulting to a shared key.
Anchor It in a Standard
Rather than inventing every requirement, anchor the document in an established standard like the OWASP IoT Security Verification Standard, then add product-specific items. ISVS provides a vetted, leveled set of requirements, so you start from a known-good baseline and spend your effort on what is unique to your device.
Borrowing from a standard also makes the document defensible and complete. You can state the assurance level the product targets, show the requirements derived from it, and add the product-specific ones on top, far stronger than a from-scratch list that inevitably misses categories a maintained standard has already thought through.
Prioritize, Then Trace End to End
Not every requirement is equally critical, and the document should say so, marking which are mandatory for ship and which are desirable, with the rating tied to the severity of the threat each addresses. That distinction is what survives contact with a deadline: when something has to give, a document that separates must from should lets the team cut the desirable hardening rather than the essential controls.
Underneath the priorities runs traceability: threat to requirement to implementation to test. Each requirement carries an identifier, links to the threat it mitigates, and links to the test that verifies it. That chain lets anyone confirm a threat was addressed, built, and proven, makes the document auditable, and shows immediately where a link is missing.
Keep It Alive through Development
A requirements document written once and filed is nearly useless, because products change: new features add threats and therefore requirements, and design changes affect existing ones. Kept current, reviewed when the design evolves and consulted during implementation and testing, it becomes the security backbone of the project, engineers building to it, testers verifying against it, reviewers checking changes against it. That ongoing use is what separates a document that improves a product from one that satisfied a checkbox.
From Document to Verified Product
The payoff is a product where security was specified, built, and tested as a first-class concern, with a paper trail from the threats to the verification. A good requirements document, traced from a real threat model and verified by a real test plan, is the difference between hoping a product is secure and knowing what it does and why, a concrete, checkable set of behaviors that someone built and someone confirmed.
Where This Fits
Turning a threat model into a testable hardware security requirements document, and tracing it through to verification, is part of the secure-development support I provide. If you want help writing requirements your engineers can build and your testers can verify, that is the kind of work we do at Berkner Tech.
