Refresh Tokens & Token Rotation – Keeping Auth Secure Long-Term

When using JWTs or OAuth tokens, one challenge is:

“What happens when the access token expires?”

Answer: Use a Refresh Token.

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🧭 Why Refresh Tokens Exist

• Access tokens should be short-lived (e.g., 15 mins).

• But logging the user in every 15 mins? 🤯 No thanks.

• So we issue a long-lived refresh token to get new access tokens silently.

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🔄 How Refresh Tokens Work (Simplified Flow)

1. User logs in

   • Server returns:

     • access_token (expires soon)

     • refresh_token (expires later or never)

2. Client uses access_token for API calls.

3. When it expires, client sends:

    	 POST /token/refresh Authorization: Bearer <refresh_token>
    	```
    

4. Server verifies it, and returns a new access_token.

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🛡️ Secure Refresh Token Strategy

🔐 Best Practice	Why
Store refresh tokens securely (e.g., EncryptedSharedPreferences on Android)	Prevent token theft
Rotate refresh tokens on every use	Prevent reuse attacks
Invalidate old refresh tokens immediately after rotation	Avoid replay
Bind token to device / IP / fingerprint	Add extra layer of safety
Don’t send refresh tokens to 3rd-party frontends	They’re long-lived and sensitive

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🔄 Token Rotation (Modern Best Practice)

Each time a refresh token is used, issue a new one and invalidate the old.

• Prevents replay attacks (stealing and reusing a refresh token).

• Some services (e.g., Auth0, Okta) do this by default.

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User → refreshes → gets new access + new refresh Old refresh → marked invalid

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🔥 Android-Specific Notes

• Store access/refresh tokens in:

  • EncryptedSharedPreferences (recommended)

  • Jetpack Security Crypto Library

• NEVER store in plaintext.

• Always prefer using AppAuth or FirebaseAuth over rolling your own flow.

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🧠 Gotchas

• Don’t use the same refresh token forever. Rotate.

• Handle refresh token expiration (e.g., user logs in again).

• Monitor refresh usage for abuse detection.