Authentication is the process of verifying the user's identity. In apps, it's the gateway to the experience. Poorly implemented authentication compromises security and frustrates users. This guide presents methods, protocols, and best practices for implementing secure, user-friendly authentication.
What is Authentication
Authentication answers the question, “Who are you?” It's different from authorization, which answers, "What can you do?" The user proves their identity, usually with something they know (password), something they have (phone) or something they are (biometrics).
Importance of Authentication
Without proper authentication, anyone can access private data. Leaks, fraud and abuse are possible. Product reputation and user trust are at stake.
Authentication Methods
Username and Password
Traditional method. Simple to implement, but vulnerable to attack if passwords are weak or stored incorrectly.
Social Login
Login via Google, Apple, Facebook. Fast for the user, delegates security to trusted providers. Reduces friction in registration.
Magic Link
Link sent by email that authenticates when clicked. Eliminates password, but depends on email access.
OTP (One-Time Password)
Code sent by SMS or email. Valid for a short period. Common as second factor.
Biometrics
Fingerprint, Face ID. Maximum convenience on devices that support it. Local, not shared storage.
Passkeys
New standard that eliminates passwords. Uses public-key cryptography. Apple, Google and Microsoft support it.
Protocols and Standards
OAuth 2.0
Default for delegated authorization. Allows apps to access resources without receiving a password. Social login base.
OpenID Connect
Identity layer over OAuth 2.0. Adds authentication to the authorization flow.
SAML
Enterprise standard for SSO. XML-based. Common in corporate environments.
JWT (JSON Web Token)
Token that carries signed claims. Stateless, verifiable by the server without consulting the database.
Typical Authentication Flow
- User provides credentials.
- Server validates and generates token.
- Token is stored on the device.
- Requests include token in the header.
- Server validates token and authorizes access.
Tokens: Access and Refresh
Access Token
Short-lived token for accessing APIs. Expires quickly (minutes to hours).
Refresh Token
Long-lived token to obtain new access tokens. Safely stored. Allows long sessions without reauthentication.
Token Rotation
Refresh token generates a new refresh token with each use. If one is compromised, it is invalidated on the next rotation.
Secure Storage
iOS Keychain
Native API for storing sensitive data. Encrypted by the system. Use for tokens and credentials.
Android Keystore
Similar to Keychain. Stores cryptographic keys with hardware protection when available.
Encrypted SharedPreferences
For Android, simpler alternative. Encrypts data in SharedPreferences.
Never in Plain Text
Do not store tokens in local files, logs, or code. Attackers can easily extract it.
Multi-Factor Authentication (MFA)
What is it
Requires multiple factors to authenticate. Something you know + something you have, for example.
Why Use
Even if the password is compromised, the attacker needs the second factor. Significantly increases security.
Common Implementations
- SMS OTP (less secure, vulnerable to SIM swap).
- TOTP via authenticator app (Google Authenticator, Authy).
- Push notification to approve login.
- Biometrics as a second factor.
Login Security
Rate Limiting
Limit login attempts. Prevents brute force.
Account Lockout
Blocks account after multiple failures. Combines with rate limiting.
CAPTCHA
Differentiates humans from bots. Adds friction, use sparingly.
Anomaly Detection
Identify suspicious logins: new location, new device, unusual time. Require additional verification.
User Experience
Minimize Friction
The more steps, the more abandonment. Social login and biometrics reduce friction.
Remind the User
Long sessions prevent frequent reauthentication. Use refresh tokens.
Clear Feedback
Specific error messages (within security reason). "Incorrect email or password" is better than "Authentication error".
Account Recovery
Clear flow for password reset. Email, SMS or security questions.
Social Login: Pros and Cons
Advantages
- Quick registration, fewer fields.
- User no longer creates a password.
- Providers take care of security.
Disadvantages
- Dependence on third parties.
- User may have privacy concerns.
- If provider goes down, login becomes unavailable.
Good Practices
Offer options: social + traditional. Allow linking multiple methods. Do not force social login.
Sign in with Apple
Mandatory
If the app offers social login, Apple requires it to offer Sign in with Apple. App Store Rule.
Hide My Email
User can hide real email. App receives relay address from Apple.
Implementation
Use AuthenticationServices framework. Similar flow to OAuth.
Passkeys: The Future
How They Work
Public/private key pair. Private key stays on the device, never transmitted. Authentication by cryptographic challenge.
Advantages
- Phishing resistant.
- There is no password to leak.
- Synchronize between user devices.
Adoption
Apple, Google and Microsoft support it. FIDO2/WebAuthn standard. Growing adoption.
Session and Logout
Session Management
Track active sessions. Allow the user to view and revoke.
Secure Logout
Invalidate tokens on the server. Wipe sensitive data from the device.
Inactivity Timeout
Log out after period of non-use. Balance security and convenience.
Common Errors
Plain Text Passwords
Never store passwords without hashing. Use bcrypt, scrypt or Argon2.
No Expiration Tokens
Eternal tokens are risk. Always set expiration.
Client Only Validation
All validation must happen on the server. Customer can be manipulated.
Telltale Error Messages
"User does not exist" vs "Email not registered" reveal information. Be generic.
Conclusion
Secure authentication balances protection and user experience. Use established protocols, implement MFA, store tokens correctly, and minimize friction. Passkeys represent the future, but passwords still rule. Build for today's reality while preparing for evolution.
##FAQs
1) Is JWT safe? Yes, if implemented correctly. Use HTTPS, validate subscription, set short expiration.
2) Should I offer SMS login? As a second factor, it is better than nothing. But TOTP authenticator is more secure than SMS.
3) Is Sign in with Apple mandatory? If you offer any social login, yes. It's the App Store rule.
4) How long should an access token last? It depends on the context. 15-60 minutes is common. Refresh tokens renew the session.
5) Is biometrics safe for authentication? Yes. Biometric data stays on the device and is never transmitted. It's convenience with security.
Also read
- Authentication in Applications - Best Practices with Checklist
- Social Login in Applications: Implementation and Best Practices
- Social login in practice: the decision roadmap before implementing it in your app
- Application Security: Mobile Protection Guide
- Authentication in Applications - Best Practices with Examples
- Android Native Development: Complete Guide with Kotlin