Implement BankID Supabase identity linking and session creation
epic-bankid-vipps-login-services-task-009 — Complete the BankID Authentication Service post-validation flow: link the verified BankID identity to an existing Supabase user account or create a new one, store the verified identity level in the User Identity Repository, and establish a high-trust Supabase session via the Auth Session Manager with appropriate claims indicating BankID verification.
Acceptance Criteria
Technical Requirements
Execution Context
Tier 5 - 253 tasks
Can start after Tier 4 completes
Implementation Notes
Implement identity linking via a Supabase Edge Function (TypeScript/Deno) to keep JWT claim minting server-side and prevent client-side privilege escalation. The Flutter service calls the edge function with the validated BankID assertion token; the edge function performs the upsert and returns a signed Supabase JWT. Use `supabase.rpc('link_bankid_identity', { pid_hash, identity_level })` wrapped in a Dart try/catch with typed failure classes. Define a sealed class hierarchy for BankIdLinkResult: Success(session), UserNotInOrg, DuplicateIdentity, LinkingFailure(error).
Store refresh token in flutter_secure_storage under key 'supabase_refresh_token_{userId}'. The BLoC should transition: BankIdValidated → IdentityLinking → Authenticated | LinkingError. Ensure atomicity: the Supabase edge function must use a single `BEGIN/COMMIT` block for user creation + identity row insertion.
Testing Requirements
Write unit tests for the identity-link service covering: (1) happy path — existing user matched by personnummer hash, session created, claims verified; (2) new user creation path — no match found, user created, identity row inserted, session issued; (3) duplicate identity rejection — same personnummer linked twice returns error without creating duplicate; (4) RPC failure — Supabase call throws, BLoC emits BankIdLinkingFailure, no partial state remains; (5) invalid assertion input — null or malformed BankID assertion rejected before any Supabase call. Mock Supabase client using a test double. Verify JWT custom claims structure using a JWT decode utility in tests. Verify Secure Storage write is called with correct token keys.
Coverage target: 90% for the identity-link service class.
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.