Implementing Secure Offline Login for Android Apps
Many Android applications require users to login, but what happens when internet connectivity is unavailable? In this article, we will explore the security best practices to implement a secure offline login feature for Android apps.
Understanding the Current Implementation
Currently, the offline login feature is implemented by setting a pin code during the user’s first sign in. This pin is stored locally, and during subsequent offline login attempts, the pin is checked to grant access. Although this approach provides basic offline login functionality, is it secure enough?
Enhancing Security with Android System API
Instead of using custom user sign-in, a more secure approach is to leverage the Android system API for in-app authentication. This API supports both biometric and device credential authentication and is backed by the Trusted Execution Environment (TEE), ensuring the security of the authentication process.
Cryptographically Binding Sensitive Data
To protect the app’s sensitive data, it should be cryptographically bound to the in-app authentication. This can be achieved by using the Android keystore system to generate a new secret key, which is stored inside the TEE. This key acts as a primary (master) key that encrypts and decrypts the app’s sensitive data and auth token. The actual encryption keys are kept outside of the TEE and are only unlocked when the in-app user authentication is successful. This ensures that even if the OS is compromised, the app data remains encrypted and secure.
Validating Auth Token and Data Access
Before granting data access, it is essential to decrypt and verify the auth token provided by the web service. This ensures that the token is not expired and that the user is allowed offline access. By implementing this validation step, you can further enhance the security of the offline login feature.
Implementing these security best practices will ensure that your offline login feature is robust and protects the app’s sensitive data, even in the absence of internet connectivity.
If you’re looking for an easier way to implement these security features, consider using the Android Jetpack Library. This library provides various tools and components to simplify the implementation of secure offline login in your Android app.