auth-factor-design

Decision aid for authentication factor architecture. Use when designing or reviewing any system whose unlock, login, or operation gating depends on more than one factor — and especially when biometrics, hardware tokens, or "something you know" enter the design.

The skill forces three explicit exercises: a factor inventory, a coercion-resistance matrix, and a lockout/recovery policy. Without all three, the design isn't complete.

Triggers

• "factor design" / "MFA" / "multi-factor"
• "FIDO2 PIN" / "WebAuthn PIN"
• "biometric policy" / "uv=true"
• "coercion resistance" / "rubber-hose"
• "PRF hot path" / "hmac-secret"
• "what you know vs what you have"
• "duress code" / "duress factor"

When NOT to use this skill

• Pure password-based authentication for low-stakes systems (use OWASP ASVS L1 guidance)
• OAuth/OIDC client design (different layer; use sdlc-complete security-architect for protocol selection)
• Network-layer mutual TLS (cert authentication, not user authentication)

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Section 1: The three factor classes

Class	Examples	What "knowing it" means
Have	Hardware token, USB drive, smart card, phone with attestation	Possession demonstrates auth
Know	Password, PIN, passphrase, recovery code, pattern, security questions	Recall demonstrates auth
Are	Fingerprint, face, voice, gait, behavioral biometrics	Body demonstrates auth

A "true" multi-factor system requires factors from at least two distinct classes. Three of the same class is not multi-factor — two passwords is one factor twice.

Common misclassifications

• SMS code: not a "have" factor on its own — SIM-swappable, network-interceptable. Treat as a weak channel-bound code, not a possession proof.
• TOTP from phone: weak "have" if phone is uncontrolled (any app can read TOTP seed if leaked). Stronger if seed is in a hardware-backed enclave (Apple Secure Enclave, Android Strongbox).
• Email confirmation link: not a factor — it's a recovery channel that demonstrates control of the email account, which itself depends on factors.
• "Security question" answers: nominally "know" but socially harvestable; treat as a fallback recovery hint, not a factor.

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Section 2: Factor inventory exercise (mandatory)

Before declaring an auth design complete, fill out:

Factor	Class	Specific instance	Required for which operations?	Lost-recovery procedure
YubiKey 5 OTP slot	have	hardware HMAC-SHA1 PRF	unlock, secret derivation	revoke at HQ, reissue
YubiKey Bio (FIDO2 PRF)	are+have	fingerprint + hmac-secret w/ uv=true	unlock, signing	revoke + reissue (≤4 weeks per fingerprint enrollment)
FIDO2 PIN	know	PIN on Bio key	required when uv=true	reset via reset-protocol; bricks key after N failures
Recovery passphrase	know	6-word BIP39 phrase	emergency rotation only	offline backup at HQ

Required: at least one row from each of two classes for any operation that protects high-value assets. Document operations that bypass this rule with explicit risk acceptance.

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Section 3: Coercion-resistance matrix (mandatory)

For each factor, document whether it can be coerced and what mitigates the coercion.

Factor	Coercible?	How	Mitigation
Password / PIN	Yes — operator under duress reveals it	Mitigation: duress code that triggers fail-closed silent lockout
Hardware token	Yes — physical seizure	Mitigation: time-lock, witness requirement, second-factor at HQ
Biometric	Always coerced. Operator's body is present; "extracting" the biometric is trivial under duress	Mitigation: pair with a "know" factor (FIDO2 PIN) so coerced biometric alone fails
Geolocation	Coercible (move operator)	Mitigation: not a real factor; use as risk signal only

Anti-pattern: only have + are (review finding H2)

A system with hardware token + biometric and no "know" factor is not coercion-resistant. The attacker compels the operator to come to the device with the token in hand and the finger ready. There is nothing the operator can decline to provide.

A FIDO2 PIN as the third factor — REQUIRED for PRF assertion via uv=true — adds:

• A factor only the operator's mind contains (truly something they "know")
• A lockout-after-N-tries property (FIDO2 PIN bricks the key after retries; default ~8)
• Optional duress codes (some authenticators support a separate "wipe key on this PIN" entry)

Cost: zero hardware. Recommended for any production deployment of FIDO2/WebAuthn PRF.

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Section 4: FIDO2 / WebAuthn PRF design

Suggested default

For hardware-backed authenticated key derivation:

• FIDO2 hmac-secret extension (PRF in WebAuthn-2 terminology) on a token with both PIN and biometric
• uv: required in the assertion — forces user verification
• PIN required — forces "know" alongside "have+are"
• Per-RP credential isolation — different RP IDs derive different PRF outputs

Tooling

Use case	Suggested tool	Vetted alternatives
Production C/C++	libfido2 (Yubico's reference C lib)	python-fido2 only behind a sandbox; PyHanko/cose for high-level signing
FIDO2 CLI	fido2-token, fido2-assert (libfido2 binaries)	OpenSC for smart-card-style use
Browser-side WebAuthn	Web Crypto API + navigator.credentials.create/get	Server libs: webauthn4j (JVM), simplewebauthn (Node), python-fido2 (server)
Testing	Yubico's fido2-token -L, virtual authenticators (Chrome devtools, soft-fido2)	Hardware test rigs only for pre-production validation

Anti-pattern: Python in the PRF hot path (review finding H3)

python-fido2 in the biometric PRF assertion path introduces a wide attack surface for the most sensitive operation in the system:

• python-fido2, cryptography, cffi, pyusb — any of these getting a typosquat or supply-chain compromise = PRF interception
• The C path through libfido2 is ~50KB statically linkable, audited, fewer transitive deps

Remediation options:

1. Use libfido2 C binaries (fido2-token, fido2-assert) directly — they support hmac-secret. Drop Python from the hot path entirely.
2. If Python is mandatory: vendor and pin python-fido2 + transitive deps with hash-locked wheels in your bootstrap; verify per-wheel SHA-256 against an offline manifest; reproducibly build the bootstrap.
3. Either way: run the PRF call inside a sandboxed namespace (bubblewrap, no network, no /proc, read-only mounts).

RP ID guidance

rp.id in WebAuthn / FIDO2 is canonically a domain name. Some authenticators behave oddly with non-domain RP IDs (e.g., "agent-box"). For non-web contexts:

• Prefer a domain you control even if not actually web-served (agent-box.example.com)
• Test with multiple authenticator firmware versions before locking RP ID
• Document if you proceed with non-domain (review L1 finding)

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Section 5: Lockout / retry policy

Factor	Failure response
FIDO2 PIN	Hard lockout after N failures (default ~8); brick key on M (default 3 lockout cycles); requires reset protocol with PIN+attestation
Biometric on hardware token	Fall back to PIN (if PIN set); on N failures of bio, force PIN; never "skip bio"
Application password	Rate-limit + account lock with side-channel notification; CAPTCHA after 3 failures (anti-bot, not anti-human)
Recovery code (single-use)	Mark used immediately; rotate active set; alert on use

Required exercise: document the exact behavior

For each authenticator in your system, document:

Factor: FIDO2 PIN on Bio key
Wrong-PIN behavior:
  After 1: warning + 1-second delay
  After 3: 5-second delay
  After 5: 30-second delay
  After 8: full lockout, requires reset
  Lockout cycles before key brick: 3 (default)
Reset procedure:
  fido2-token -R (factory reset, wipes credentials)
  Re-enroll fingerprint, re-set PIN
  Re-register with all RPs (operationally expensive — design for this)

If your design doesn't have this written down, it's incomplete.

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Section 6: Duress / coercion countermeasures

When the threat model includes coercion (high-value targets, journalists, security operators in hostile environments), build in:

Countermeasure	How
Duress PIN	Separate PIN that triggers visible-success but secret wipe. Hardware support varies; some FIDO2 firmware supports it, most don't. Document if your hardware does.
Time-lock	Operations require N-minute delay; operator can secretly trigger a longer delay during which silent alarm fires
Witness requirement	High-value operations require a second operator present (cryptographically verified — e.g., second YubiKey assertion)
Geographic / device fence	Operations only succeed from authorized locations or hardware; revocable centrally
Silent canary / heartbeat	Operator periodically demonstrates non-coercion; missed heartbeat triggers alert (review M7: heartbeat is killable by root, mitigate via signed live image)

Important: every coercion countermeasure has operational cost. A duress PIN that the operator forgets locks them out as effectively as the attacker does. Test the failure modes.

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Section 7: Worked examples

Review finding H2 — "what you have + what you are" only

Original design: 2-of-2 with one biometric (YubiKey 5 + YubiKey Bio with fingerprint).

What this skill flags:

• Section 3 anti-pattern: no "know" factor → coercion gives the attacker everything
• Section 6: no duress countermeasure documented

Remediation:

• Set FIDO2 PIN on Bio key; require uv=true in PRF assertion
• Document PIN lockout policy
• Optionally add duress procedure (geographic alert, second-operator witness)

Review finding H3 — python-fido2 in PRF hot path

Original design: Python program calls python-fido2 to do the biometric PRF assertion.

What this skill flags:

• Section 4 anti-pattern: Python deps in hot path → wide attack surface

Remediation:

• Replace with libfido2 C tools (fido2-token, fido2-assert -h hmac-secret)
• If Python is mandatory: vendor + hash-lock all transitive deps; sandbox the call

Review finding M4 — biometric retry behavior unclear

Original design: uv: REQUIRED documented, but lockout/fallback behavior not specified.

What this skill flags:

• Section 5 mandatory exercise: exact lockout behavior must be documented

Remediation:

• Run fido2-token -L against a test key, document observed behavior under N failures
• Verify Bio key was not factory-defaulted to "no PIN" (if so, set one)
• Add behavior to factor-design-rationale.md per-factor

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Section 8: Output format

When this skill is invoked as part of a review, produce a factor-design-rationale.md (template in templates/factor-design-rationale.md) with:

• Factor inventory table (Section 2)
• Coercion-resistance matrix (Section 3)
• Lockout/retry policy per factor (Section 5)
• Hardware/firmware constraints (FIDO2 version, PRF support, UV behavior)

If issues are found, produce findings in the standard format:

### Finding: <SHORT-NAME>

**Severity**: BLOCK | HIGH | MEDIUM | LOW
**Section**: auth-factor-design <section reference>
**Issue**: <one paragraph>
**Remediation**: <concrete fix>
**References**: <NIST SP 800-63B, FIDO2 spec, etc.>

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Related

• Companion skill: physical-threat-modeling (Tier 2) — coercion is in its threat library
• Companion skill: degraded-mode-design (Tier 2) — what happens when factors fail
• Companion skill: chain-of-trust-design — the factors only matter if the verifier is trusted
• Template: factor-design-rationale.md

Standards referenced

• NIST SP 800-63B — Digital Identity Guidelines: Authentication
• FIDO2 / CTAP2 specification
• WebAuthn Level 2 / Level 3 specifications
• OWASP ASVS 4.0 §2 (Authentication)
• libfido2 documentation