authentication

What this skill does

# Authentication

Implement authentication flows on iOS using the AuthenticationServices
framework, including Sign in with Apple, passkeys, OAuth/third-party web
auth, Password AutoFill, and biometric re-authentication.

## Contents

- [Sign in with Apple](#sign-in-with-apple)
- [Credential Handling](#credential-handling)
- [Credential State Checking](#credential-state-checking)
- [Token Validation](#token-validation)
- [Existing Account Setup Flows](#existing-account-setup-flows)
- [Passkeys](#passkeys)
- [ASWebAuthenticationSession (OAuth)](#aswebauthenticationsession-oauth)
- [Password AutoFill Credentials](#password-autofill-credentials)
- [Biometric Authentication](#biometric-authentication)
- [Security Boundaries](#security-boundaries)
- [SwiftUI SignInWithAppleButton](#swiftui-signinwithapplebutton)
- [Common Mistakes](#common-mistakes)
- [Review Checklist](#review-checklist)
- [References](#references)

## Sign in with Apple

Add the "Sign in with Apple" capability in Xcode before using these APIs.

### UIKit: ASAuthorizationController Setup

```swift
import AuthenticationServices

final class LoginViewController: UIViewController {
    func startSignInWithApple() {
        let provider = ASAuthorizationAppleIDProvider()
        let request = provider.createRequest()
        request.requestedScopes = [.fullName, .email]

        let controller = ASAuthorizationController(authorizationRequests: [request])
        controller.delegate = self
        controller.presentationContextProvider = self
        controller.performRequests()
    }
}

extension LoginViewController: ASAuthorizationControllerPresentationContextProviding {
    func presentationAnchor(for controller: ASAuthorizationController) -> ASPresentationAnchor {
        view.window!
    }
}
```

### Delegate: Handling Success and Failure

```swift
extension LoginViewController: ASAuthorizationControllerDelegate {
    func authorizationController(
        controller: ASAuthorizationController,
        didCompleteWithAuthorization authorization: ASAuthorization
    ) {
        guard let credential = authorization.credential
            as? ASAuthorizationAppleIDCredential else { return }

        let userID = credential.user  // Stable, unique, per-team identifier
        let email = credential.email  // nil after first authorization
        let fullName = credential.fullName  // nil after first authorization
        let identityToken = credential.identityToken  // JWT for server validation
        let authCode = credential.authorizationCode  // Short-lived code for server exchange

        // Save userID to Keychain for credential state checks
        // See references/keychain-biometric.md for Keychain patterns
        saveUserID(userID)

        // Send identityToken and authCode to your server
        authenticateWithServer(identityToken: identityToken, authCode: authCode)
    }

    func authorizationController(
        controller: ASAuthorizationController,
        didCompleteWithError error: any Error
    ) {
        let authError = error as? ASAuthorizationError
        switch authError?.code {
        case .canceled:
            break  // User dismissed
        case .failed:
            showError("Authorization failed")
        case .invalidResponse:
            showError("Invalid response")
        case .notHandled:
            showError("Not handled")
        case .notInteractive:
            break  // Non-interactive request failed -- expected for silent checks
        default:
            showError("Unknown error")
        }
    }
}
```

## Credential Handling

`ASAuthorizationAppleIDCredential` properties and their behavior:

| Property | Type | First Auth | Subsequent Auth |
|---|---|---|---|
| `user` | `String` | Always | Always |
| `email` | `String?` | Provided if requested | `nil` |
| `fullName` | `PersonNameComponents?` | Provided if requested | `nil` |
| `identityToken` | `Data?` | JWT encoded as UTF-8 data | JWT encoded as UTF-8 data |
| `authorizationCode` | `Data?` | Short-lived code | Short-lived code |
| `realUserStatus` | `ASUserDetectionStatus` | Fraud-prevention signal | Do not rely on later attempts |

**Critical:** `email` and `fullName` are provided ONLY on the first
authorization. Cache them immediately during the initial sign-up flow. If the
user later deletes and re-adds the app, these values will not be returned.

```swift
func handleCredential(_ credential: ASAuthorizationAppleIDCredential) {
    // Always persist the user identifier
    let userID = credential.user

    // Cache name and email IMMEDIATELY -- only available on first auth
    if let fullName = credential.fullName {
        let name = PersonNameComponentsFormatter().string(from: fullName)
        UserProfile.saveName(name)  // Persist to your backend
    }
    if let email = credential.email {
        UserProfile.saveEmail(email)  // Persist to your backend
    }
}
```

## Credential State Checking

Check credential state on every app launch. The user may revoke access at
any time via Settings > Apple Account > Sign-In & Security.

```swift
func checkCredentialState() {
    let provider = ASAuthorizationAppleIDProvider()
    guard let userID = loadSavedUserID() else {
        showLoginScreen()
        return
    }

    provider.getCredentialState(forUserID: userID) { state, _ in
        DispatchQueue.main.async {
            switch state {
            case .authorized:
                proceedToMainApp()
            case .revoked:
                // User revoked -- sign out and clear local data
                signOut()
                showLoginScreen()
            case .notFound:
                showLoginScreen()
            case .transferred:
                // App transferred to new team -- migrate user identifier
                migrateUser()
            @unknown default:
                showLoginScreen()
            }
        }
    }
}
```

### Credential Revocation Notification

```swift
NotificationCenter.default.addObserver(
    forName: ASAuthorizationAppleIDProvider.credentialRevokedNotification,
    object: nil,
    queue: .main
) { _ in
    // Sign out immediately
    AuthManager.shared.signOut()
}
```

## Token Validation

The `identityToken` is a JWT. Send it to your server for validation --
never trust it client-side alone.

```swift
func sendTokenToServer(credential: ASAuthorizationAppleIDCredential) async throws {
    guard let tokenData = credential.identityToken,
          let token = String(data: tokenData, encoding: .utf8),
          let authCodeData = credential.authorizationCode,
          let authCode = String(data: authCodeData, encoding: .utf8) else {
        throw AuthError.missingToken
    }

    var request = URLRequest(url: URL(string: "https://api.example.com/auth/apple")!)
    request.httpMethod = "POST"
    request.setValue("application/json", forHTTPHeaderField: "Content-Type")
    request.httpBody = try JSONEncoder().encode(
        ["identityToken": token, "authorizationCode": authCode]
    )

    let (data, response) = try await URLSession.shared.data(for: request)
    guard (response as? HTTPURLResponse)?.statusCode == 200 else {
        throw AuthError.serverValidationFailed
    }
    let session = try JSONDecoder().decode(SessionResponse.self, from: data)
    // Store session token in Keychain -- see references/keychain-biometric.md
    try KeychainHelper.save(session.accessToken, forKey: "accessToken")
}
```

Server-side, validate the JWT against Apple's public keys at
`https://appleid.apple.com/auth/keys` (JWKS). Verify: `iss` is
`https://appleid.apple.com`, `aud` matches your bundle ID, `exp` not passed.

## Existing Account Setup Flows

On launch, silently check for existing Sign in with Apple and password
credentials before showing a login screen:

```swift
func performExistingAccountSetupFlows() {
    let appleIDRequest = ASAuthorizationAppleIDProvider().createRequest()
    let passwordRequest = ASAuthorizationPasswordProvider().createReques