Potential fix for code scanning alert no. 8: Insecure local authentication

[maxschwinghammer]

Potential fix for https://github.com/FreshKeeper/AndroidApp/security/code-scanning/8

The best way to fix the problem is to leverage an Android KeyStore-backed cryptographic key for guarding changes to the biometric-enabled state, ensuring that only an authenticated user (with a registered biometric) can enable this setting by successfully unlocking a secret key. Specifically:

• Generate a symmetric encryption key using Android's KeyStore, requiring biometric user authentication for use.
• On biometric prompt, supply a Cipher (as a CryptoObject) that can only be used after successful biometric authentication.
• In onAuthenticationSucceeded, require and use the Cipher (via result.getCryptoObject().getCipher()), ensuring that biometric authentication was needed to access the key/cipher operation. Update the setting state only in this context.
• This change should be implemented within the onBiometricSwitchChanged method in ProfileSettingsViewModel.

The following edits are required:

1. Add functions for generating and retrieving the KeyStore-backed key, similar to the provided secure example.
2. Prepare a Cipher before launching the biometric prompt and use it as a CryptoObject.
3. Change the call to authenticate() to pass a CryptoObject.
4. In onAuthenticationSucceeded, verify and use the Cipher from the CryptoObject before updating state.

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