Mobile App Authentication: Login, Signup, OTP, Social Login and Security

Mobile app authentication is how an app confirms who a user is and keeps them signed in safely. A sound setup picks sign-in methods that fit the app's risk and audience (passkeys, social login, email and password, or one-time codes), adds multi-factor authentication where the stakes justify it, issues short-lived access tokens with rotating refresh tokens, stores them in the Keychain or Keystore, and enforces authorization on the server for every request. Recovery, logout and session management complete the picture.

Authentication answers "who is this?" Authorization answers "what may they do?" Mobile apps often get the first right and the second wrong: the app hides a button, but the API still returns another user's data if asked. Every API endpoint must check that the authenticated user is allowed to perform the action. The broader security picture is covered in how to build secure mobile apps.

The app sends the user's credentials or proof of identity to an authentication server. The server verifies it and returns an access token (short-lived, attached to API calls) and a refresh token (longer-lived, used only to get new access tokens). The app keeps both in secure storage, refreshes the access token before it expires, and discards both on logout.

Still the most widely understood method. Support password managers and autofill, allow long passphrases, check new passwords against known breached passwords, rate-limit attempts, and hash passwords on the server with a modern algorithm. Never send or store passwords in plain text, and never log them.

One-time codes by SMS or email remove the need to remember a password. They're convenient, but SMS codes can be intercepted or redirected through SIM swapping, so treat them as moderate-assurance. Make codes short-lived and single-use, limit attempts, and support platform autofill for one-time codes to reduce friction.

Passkeys use public-key cryptography: the device holds a private key unlocked by biometrics or device PIN, and the server stores only a public key. They resist phishing and there's no shared secret to leak. iOS supports them through the AuthenticationServices framework and Android through Credential Manager, with syncing across a user's devices. Design recovery carefully for users who lose access to all their devices.

Signing in with Google, Apple or similar providers shortens signup considerably. Use the providers' official SDKs or OAuth 2.0 with PKCE through the system browser, following the native-app best practices in RFC 8252, rather than an embedded web view. Apple's App Review Guidelines require apps offering certain third-party login options to also offer an equivalent privacy-focused option such as Sign in with Apple, with listed exceptions.

Face ID, Touch ID and Android BiometricPrompt confirm that the person holding the device is its owner. In practice they unlock a credential or key stored in the Keychain or Keystore, which then authenticates with your server. Always provide a fallback such as device passcode, and don't treat a biometric check performed on the device as proof the server can trust on its own.

Add a second factor where the impact of account takeover is high: financial actions, health data, admin access. Authenticator apps and passkeys are stronger than SMS. Use step-up authentication, asking for an extra factor only when the user attempts a sensitive action, to keep everyday use smooth.

Keep access tokens short-lived and scoped. Use refresh tokens to obtain new ones, rotate refresh tokens on use, and detect reuse of an old refresh token as a sign of theft. Handle refresh in one shared networking layer, as described in mobile app API integration, so screens never deal with expired tokens directly.

Store tokens in the iOS Keychain and in Android Keystore-backed encrypted storage. Don't keep them in UserDefaults, SharedPreferences, plain files, JavaScript storage without encryption, or crash logs. Clear them fully on logout and when the app detects a compromised session.

Recovery is where many accounts are actually taken over, so protect it as carefully as login. Logout should revoke the refresh token on the server, not just delete it locally. For sensitive apps, show users their active sessions and devices with the ability to sign them out remotely. Both Apple and Google also require in-app account deletion for apps that let users create accounts.

Let users explore before forcing signup where possible, offer the fastest options first, support autofill and password managers, show clear error messages that don't reveal whether an account exists, and keep users signed in for a sensible period. The onboarding guide covers when to ask for an account.

For most consumer apps: passkeys plus social login, with email as a fallback and biometrics for returning users. For fintech and health apps: add MFA and step-up checks for sensitive actions. For B2B and internal apps: enterprise SSO. Whatever you choose, the token handling and server-side authorization described above apply. For the wider build context, see the mobile app development guide.

Good authentication balances friction and risk: fast sign-in options for everyday use, stronger factors where the stakes are high, and careful handling of tokens, sessions, recovery and logout behind the scenes. Pair it with server-side authorization and the broader practices in mobile app security.