authentication

SKILL.md

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

Credential Handling

Credential State Checking

Token Validation

Existing Account Setup Flows

Passkeys

ASWebAuthenticationSession (OAuth)

Password AutoFill Credentials

Biometric Authentication

Security Boundaries

SwiftUI SignInWithAppleButton

Common Mistakes

Review Checklist

References

Sign in with Apple

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

UIKit: ASAuthorizationController Setup

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

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
    ) {
        switch (error as? ASAuthorizationError)?.code {
        case .canceled, .notInteractive:
            break
        case .failed:
            showError("Authorization failed")
        default:
            showError("Authorization failed: \(error.localizedDescription)")
        }
    }
}

Credential Handling

Credential data
Required handling

user
Persist this stable, per-team identifier for credential-state checks.

email, fullName
These optional values arrive only on first authorization; cache them immediately.

identityToken, authorizationCode
Send them to the server for validation or exchange; never trust them as client-side proof.

Treat realUserStatus only as a fraud-prevention signal, not authentication
proof.

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.

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

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.

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, and exp has not
passed. Exchange the short-lived authorization code on the server and store
the resulting app session token in Keychain.

Existing Account Setup Flows

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

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

    let controller = ASAuthorizationController(
        authorizationRequests: [appleIDRequest, passwordRequest]
    )
    controller.delegate = self
    controller.presentationContextProvider = self
    controller.performRequests(
        options: .preferImmediatelyAvailableCredentials
    )
}

Call this in viewDidAppear or on app launch. If no existing credentials
are found, the delegate receives a .notInteractive error -- handle it
silently and show your normal login UI.

Passkeys

Use passkeys only for a relying-party domain configured with a webcredentials:
Associated Domain and AASA entry. Registration and assertion each require a
fresh server challenge, an ASAuthorizationPlatformPublicKeyCredentialProvider,
an authorization controller with an active presentation anchor, and server-side
verification before issuing a session.

Load references/passkeys.md for the canonical
registration, assertion, result handling, AutoFill-assisted, and physical
security-key flows.

ASWebAuthenticationSession (OAuth)

Use ASWebAuthenticationSession for OAuth and third-party authentication
(Google, GitHub, etc.). Never use WKWebView for auth flows.

import AuthenticationServices

final class OAuthController: NSObject, ASWebAuthenticationPresentationContextProviding {
    private weak var presentationAnchor: ASPresentationAnchor?

    init(presentationAnchor: ASPresentationAnchor) {
        self.presentationAnchor = presentationAnchor
    }

    func startOAuthFlow() {
        let authURL = URL(string:
            "https://provider.com/oauth/authorize?client_id=YOUR_ID&redirect_uri=myapp://callback&response_type=code"
        )!
        let session = ASWebAuthenticationSession(
            url: authURL, callback: .customScheme("myapp")
        ) { callbackURL, error in
            guard let callbackURL, error == nil,
                  let code = URLComponents(url: callbackURL, resolvingAgainstBaseURL: false)?
                      .queryItems?.first(where: { $0.name == "code" })?.value else { return }
            Task { await self.exchangeCodeForTokens(code) }
        }
        session.presentationContextProvider = self
        session.prefersEphemeralWebBrowserSession = true  // No shared cookies
        session.start()
    }

    func presentationAnchor(for session: ASWebAuthenticationSession) -> ASPresentationAnchor {
        guard let presentationAnchor else {
            fatalError("ASWebAuthenticationSession needs the active window")
        }
        return presentationAnchor
    }
}

In SwiftUI, use @Environment(\.webAuthenticationSession) and call
authenticate(using:callback:preferredBrowserSession:additionalHeaderFields:)
with .customScheme("myapp") or .https(host:path:); prefer .ephemeral
only when the provider flow should avoid shared browser cookies.

Password AutoFill Credentials

Offer ASAuthorizationPasswordProvider alongside Sign in with Apple using the
single controller in Existing Account Setup Flows.
Handle ASPasswordCredential in that controller's delegate.

Set textContentType on text fields for AutoFill to work:

usernameField.textContentType = .username
passwordField.textContentType = .password

Biometric Authentication

Use LAContext from LocalAuthentication for local re-authentication before
showing account settings or starting sensitive actions. Do not treat a returned
Bool as proof to unlock a stored secret; protect secrets with Keychain access
control instead. See references/keychain-biometric.md
for the canonical LAContext, fallback, SecAccessControl, and
.biometryCurrentSet patterns.

Required: Add NSFaceIDUsageDescription to Info.plist. Missing this
key crashes on Face ID devices.

Security Boundaries

This skill owns user-facing account authentication: Sign in with Apple,
passkeys, Password AutoFill, ASAuthorizationController, OAuth session
presentation, credential state, and local biometric re-authentication. Route
deep security work to swift-security: Keychain architecture/migration,
CryptoKit, Secure Enclave, certificate pinning/trust, keychain sharing, storage
hardening, and OWASP MASVS/MASTG. Keep only the storage minimum here: tokens and
secrets belong in Keychain; LAContext.evaluatePolicy alone must not release
protected secrets.

SwiftUI SignInWithAppleButton

Use SignInWithAppleButton in SwiftUI views when the login surface is SwiftUI.
Request .fullName and .email, downcast a successful result to
ASAuthorizationAppleIDCredential, and pass it to the shared
Token Validation flow. Style with
.signInWithAppleButtonStyle(...).

Common Mistakes

Assuming a saved local session means the Apple ID credential is still valid.
Check credential state at launch and handle revocation notifications.

Showing a full login screen before trying existing account setup flows.
Treat .notInteractive as the normal "no local credential" path.

Force-unwrapping email or fullName. Cache them on first authorization and
handle nil later.

Creating an ASAuthorizationController without a presentation context
provider. Authorization UI needs the active presentation anchor.

Storing identity tokens, authorization codes, access tokens, passwords, or
passkey server state in UserDefaults, files, or Core Data. Store secrets in
Keychain and keep relying-party passkey verification server-side.

Adding passkey requests without webcredentials: Associated Domains for the
relying-party domain, or trying to use app-native passkeys for unrelated
websites.

Expanding authentication work into CryptoKit, Secure Enclave, certificate
pinning, or OWASP MASVS. Route those to swift-security.

Review Checklist

 "Sign in with Apple" capability added in Xcode project

 ASAuthorizationControllerPresentationContextProviding implemented

 Credential state checked on every app launch (getCredentialState(forUserID:completion:))

 credentialRevokedNotification observer registered; sign-out handled

 email and fullName cached on first authorization (not assumed available later)

 identityToken sent to server for validation, not trusted client-side only

 Tokens stored in Keychain, not UserDefaults or files

 performExistingAccountSetupFlows called before showing login UI

 Error cases handled: .canceled, .failed, .notInteractive

 NSFaceIDUsageDescription in Info.plist for biometric auth

 ASWebAuthenticationSession used for OAuth (not WKWebView)

 prefersEphemeralWebBrowserSession set for OAuth when appropriate

 textContentType set on username/password fields for AutoFill

 Passkey relying party has webcredentials: Associated Domains configured

 Passkey registration/assertion challenges come from the server and are verified server-side

 Deep Keychain, CryptoKit, Secure Enclave, certificate pinning, and MASVS work routed to swift-security

References

Keychain & biometric patterns: references/keychain-biometric.md

Passkey patterns: references/passkeys.md

AuthenticationServices

ASAuthorizationAppleIDProvider

ASAuthorizationAppleIDCredential

ASAuthorizationController

ASWebAuthenticationSession

Supporting passkeys

ASAuthorizationPlatformPublicKeyCredentialProvider

ASAuthorizationPasswordProvider

SignInWithAppleButton

Implementing User Authentication with Sign in with Apple