Assurance Notes: Threat Modeling for Cryptographic APIs

Cryptographic APIs are deceptively small surfaces that hide large, high-impact threat models. Most incidents we see start with a missing assumption in the API contract rather than a broken primitive. This note captures the minimum threat model we expect before a modeling and verification sprint begins.

Define the trust boundary

• Who provides inputs, and which inputs are attacker-controlled?
• Which keys are long-lived, and which are derived per session?
• What is the failure mode when entropy is low or unavailable?

Enumerate abuse cases

We explicitly list the abusive uses of the API. If the API can be called in the wrong order, with partial state, or with repeated nonces, write those paths down and decide whether to hard-fail or to harden.

Side-channel budget

Constant-time behavior, memory access patterns, and timing variance are first-class requirements. If you cannot bound the side-channel surface, shrink the API or isolate the sensitive path.

Make assumptions executable

The most valuable output of threat modeling is a short list of assumptions you can formalize. Capture them in a Lean model, turn them into invariants, and use them as oracles for property tests and differential fuzzing.

Operational guardrails

Keys should be rotateable, audit logs should be tamper-evident, and any failure should be noisy. If you cannot tell when things go wrong, you are betting the business on silence.

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