Read SP 800-63B-4 Section 4.2 (AAL2 requirements): required authenticator types include phishing-resistant single-factor hardware cryptographic devices, or multi-factor combinations such as memorized secret plus OTP authenticator.
Confirm passkey implementation: syncable passkeys (FIDO2/WebAuthn credentials stored in a cloud keychain) are explicitly permitted at AAL2 in 800-63B-4; device-bound passkeys are also permitted; verify your WebAuthn relying party implementation uses the correct rpId and enforces user verification (UV=required).
For TOTP authenticators combined with passwords: confirm the OTP seed is stored securely, codes are time-limited (TOTP uses 30-second windows), and replay protection is implemented to reject reused valid codes.
Implement reauthentication at the intervals required by 800-63B-4: subscribers must be reauthenticated at least once per 12 hours during an extended session, and immediately when a risk-based trigger is detected.
Test resistance to phishing: phishing-resistant authenticators (hardware security keys, passkeys) must bind the authentication to the origin; confirm your WebAuthn implementation does not accept cross-origin credentials.
Document the authenticator type, AAL achieved, and any compensating controls in the Digital Identity Acceptance Statement or equivalent artifact for the system authorization package.
Known gotchas
800-63B-4 permits syncable passkeys at AAL2 as of the final July 2025 release; prior draft versions of 800-63-4 restricted syncable passkeys — integrations built against draft text may reflect outdated restrictions.
User verification (UV) flag in WebAuthn must be required (not preferred) to achieve AAL2 with a passkey; setting UV to preferred allows authentication without biometric or PIN confirmation and degrades the assurance level.
Federal agencies are additionally bound by OMB M-22-09 and the FICAM Roadmap which may impose requirements above the NIST 800-63B-4 baseline; confirm applicable federal policy overlays before finalizing the authenticator assurance level claims.
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