How do I authenticate users in a mobile app and backend API?

To authenticate users in a mobile app + backend API, the usual modern pattern is:

1. The user signs in on the mobile app
2. Your backend verifies their identity
3. Your backend issues tokens
4. The app sends those tokens with API requests
5. The backend validates the tokens on every request

Recommended approach: OAuth 2.0 / OpenID Connect with JWTs

For most apps, this is the standard and safest approach.

Typical flow

• User enters email/password, or uses Google/Apple sign-in, or another identity provider.
• The mobile app sends credentials to your auth server over HTTPS.
• The auth server verifies them and returns:
  • an access token (short-lived)
  • a refresh token (longer-lived)
• The app uses the access token in API calls:
  • Authorization: Bearer <access_token>
• When the access token expires, the app uses the refresh token to get a new one.

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Option 1: Use a third-party identity provider

Examples:

• Auth0
• Firebase Authentication
• AWS Cognito
• Azure AD B2C
• Okta

Pros

• Faster to implement
• Handles password storage, social login, MFA, token refresh
• Better security defaults

Cons

• Vendor dependency
• Cost at scale
• Less control

Best for

Most teams unless you have special requirements.

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Option 2: Build your own auth system

If you need full control, implement:

Backend responsibilities

• Store passwords securely with bcrypt, scrypt, or Argon2
• Verify credentials
• Issue JWT access tokens
• Issue refresh tokens
• Support logout / token revocation
• Support password reset and email verification

Mobile app responsibilities

• Sign in and store tokens securely
• Send access token on requests
• Refresh access token when needed
• Clear tokens on logout

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Secure token storage on mobile

Do not store tokens in plain text or insecure local storage.

Use:

• iOS Keychain
• Android Keystore / Encrypted SharedPreferences
• Cross-platform wrappers if using React Native / Flutter

Avoid:

• plaintext files
• insecure AsyncStorage for sensitive tokens unless encrypted

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Access token vs refresh token

Access token

• Short-lived, e.g. 5–15 minutes
• Sent with every API request
• If stolen, limited damage due to short lifespan

Refresh token

• Longer-lived, e.g. days or weeks
• Used only to obtain a new access token
• Should be stored more securely than the access token
• Ideally rotate refresh tokens on every use

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Backend validation

On each request, your API should:

1. Read Authorization: Bearer <token>
2. Verify signature
3. Check expiration
4. Confirm issuer/audience if applicable
5. Load the user identity/claims
6. Authorize the action

If the token is invalid or expired, return 401 Unauthorized.

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Example request

GET /api/profile
Authorization: Bearer eyJhbGciOi...

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Example authentication flow

Login

POST /auth/login
Content-Type: application/json

{
  "email": "user@example.com",
  "password": "secret"
}

Response

{
  "access_token": "eyJ...",
  "refresh_token": "def...",
  "expires_in": 900
}

Use token

GET /api/orders
Authorization: Bearer eyJ...

Refresh

POST /auth/refresh
Content-Type: application/json

{
  "refresh_token": "def..."
}

New response

{
  "access_token": "new_access_token",
  "refresh_token": "new_refresh_token"
}

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Important security practices

• Always use HTTPS
• Hash passwords with Argon2/bcrypt/scrypt
• Use short-lived access tokens
• Rotate refresh tokens
• Store tokens securely on-device
• Add rate limiting to login endpoints
• Support MFA if possible
• Use secure logout and token revocation
• Validate all authorization on the backend; never trust the mobile app

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If you want session-based auth instead

You can also use cookies and server sessions, but for mobile apps token-based auth is usually simpler and more common. Cookies can work, but you need to manage CSRF and cookie handling carefully.

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Practical recommendation

If you’re starting a new app:

• Use OAuth/OpenID Connect
• Use JWT access tokens
• Use refresh tokens
• Store secrets in Keychain/Keystore
• Consider a managed provider like Auth0/Firebase/Cognito

If you want, I can also give you:

• a diagram of the full flow
• a Node.js/Express backend example
• a React Native or Flutter client example
• a secure JWT + refresh token implementation pattern