Implement biometric enrollment after first-time login
epic-bankid-vipps-login-services-task-011 — Implement the post-authentication biometric enrollment flow in the Biometric Authentication Service: after successful BankID or Vipps first-time login, prompt the user to enroll biometrics, securely store the enrollment confirmation flag in Secure Storage, and associate the biometric enrollment with the current Supabase user identity so subsequent logins can use biometric shortcut.
Acceptance Criteria
Technical Requirements
Execution Context
Tier 6 - 158 tasks
Can start after Tier 5 completes
Implementation Notes
Implement enrollment as a dedicated `enrollBiometrics(String userId)` method on `BiometricAuthServiceImpl`. Before showing the prompt, call `isBiometricEnrolled(userId)` — if already enrolled, return `BiometricEnrollmentState.enrolled` immediately. The BLoC should handle `EnrollBiometricsEvent` after the post-login navigation guard triggers. Use a consistent key scheme: `'biometric_enrolled_
Do not use `userId` directly as part of a plaintext key if it contains sensitive info — hash the userId portion of the key with a stable hash (MD5 or SHA-1 is sufficient here since this is not a security secret, just a namespacing mechanism). The enrollment prompt UI (bottom sheet) should be triggered from the router's post-login redirect logic, not from within the service itself — keep the service pure and side-effect-free with respect to UI.
Testing Requirements
Unit tests for BiometricAuthService enrollment: (1) first-time login with biometrics available → prompt shown, user confirms → Secure Storage write called with correct key/value; (2) first-time login with biometrics available → user declines → declined flag written, no enrollment flag; (3) biometrics unavailable → enrollment silently skipped, no Secure Storage write; (4) Secure Storage write throws → error caught, state set to enrollmentError, no crash; (5) second login with enrolled flag present → enrollment prompt not shown; (6) isBiometricEnrolled returns true when flag is present and false when absent. Mock flutter_secure_storage using an in-memory map. Widget tests for the enrollment bottom sheet: confirm and decline paths trigger correct BLoC events.
The PKCE OAuth flow requires the code verifier to survive an app backgrounding during the Vipps redirect, which can trigger OS memory pressure and clear in-memory state. If the verifier is lost between authorization request and callback, the token exchange fails and the user is stranded with a confusing error.
Mitigation & Contingency
Mitigation: Store the PKCE code verifier in AuthTokenStore (Flutter Secure Storage) immediately after generation, before launching the Vipps redirect. Clear it only after a successful or explicitly failed token exchange.
Contingency: If state loss occurs in production, implement a retry flow that generates a new PKCE pair and restarts the authorization URL request, with a user-visible 'Try again' prompt rather than a generic error.
Resuming a Supabase session after biometric verification requires the session token to still be valid. If the session has expired in the background (e.g., after a long device offline period), biometric success will not produce a valid session, and the user will see a confusing 'Face ID worked but still logged out' experience.
Mitigation & Contingency
Mitigation: Before presenting the biometric prompt, check session token expiry. If expired, skip biometrics and route directly to full BankID/Vipps re-authentication. Only offer biometric re-auth if the stored refresh token is still within its validity window.
Contingency: If session expiry during biometric flow occurs in production, implement a graceful transition message ('Your session has expired — please log in again') that preserves the user's last-used authentication method preference.
BankID and Vipps may return different user identifiers (personnummer, phone number, sub claim) that must be correctly linked to an existing Supabase auth user. If the linking logic has edge cases (e.g., user previously registered via email/password), duplicate Supabase accounts may be created.
Mitigation & Contingency
Mitigation: Design the identity linking logic with explicit disambiguation: check for existing users by personnummer before creating a new Supabase identity. Implement the linking via Supabase Edge Function to keep the logic server-side and auditable.
Contingency: Implement an admin-facing account merge tool in the admin portal to resolve duplicate accounts if they occur. Add a Supabase unique constraint on the personnummer field to make duplicates fail loudly rather than silently.
The Vipps nin (personnummer) scope requires explicit approval from Vipps as part of the merchant agreement. If this scope approval is not in place before the production release, the Vipps flow will succeed but return no personnummer, making the primary business value (membership data gap fill) non-functional without user-visible error.
Mitigation & Contingency
Mitigation: Apply for Vipps nin scope approval as part of the merchant onboarding process, well before Phase 2 launch. Implement the service to gracefully handle absent nin claims and show users a clear message if personnummer could not be retrieved.
Contingency: If nin scope is delayed, ship the Vipps login flow without personnummer write-back first (delivering login value immediately) and add personnummer sync as a post-approval update with no UI changes required.