React Native Keychain: Biometric Access Control Issues

Understanding Biometric Prompts with React Native Keychain

When developing mobile applications, securing sensitive data is paramount. React Native Keychain offers a robust solution for storing credentials and other private information securely. A key feature that many developers want to leverage is the ability to enforce biometric authentication (like Face ID or fingerprint scans) before accessing stored data. However, as some developers have discovered, getting this biometric access control to work consistently across different devices and platforms can be a bit tricky. This article dives into the common issues encountered and provides insights into how React Native Keychain's accessControl option is intended to function, along with potential reasons for unexpected behavior.

The Promise and Practice of Biometric Security

The core idea behind using accessControl: Keychain.ACCESS_CONTROL.BIOMETRY_ANY is straightforward: you tell the keychain that any form of biometric authentication should be required when interacting with a specific piece of data. This means that whether a user has Face ID, a fingerprint scanner, or another biometric method set up, the system should prompt them to use it. For example, when you attempt to retrieve a password using Keychain.getGenericPassword(), you might expect a biometric prompt to appear, ensuring that only the authenticated user can access that sensitive information. Similarly, when storing data with Keychain.setGenericPassword(), you might want to tie that initial storage to a biometric verification.

This is particularly useful for high-security applications where even a brief window of unprotected data access is unacceptable. By requiring a biometric prompt at critical junctures, you significantly enhance the security posture of your application, providing users with a seamless yet highly effective layer of protection. The convenience of biometrics, combined with the underlying security of the device's keychain, creates a powerful combination for safeguarding user data.

Common Scenarios and Observed Behavior

Let's look at how this feature plays out in practice. A typical scenario involves setting a password and then retrieving it, both with the accessControl option specified. However, the results can be surprisingly varied.

• Retrieving Without Storing Biometrics: If you only set the accessControl when retrieving a password (i.e., Keychain.getGenericPassword({ service: 'myPasswordService', accessControl: Keychain.ACCESS_CONTROL.BIOMETRY_ANY })), you might find that the prompt never appears. The password is often returned without any user verification, regardless of the device type or biometric setup. This can be quite alarming, as it implies that the security measure you thought you implemented is not actually being enforced during the retrieval process.

• Storing and Retrieving Biometrics: The behavior becomes more nuanced when you set the accessControl during both the storage (Keychain.setGenericPassword) and retrieval (Keychain.getGenericPassword) operations. The observed outcomes can differ significantly:

  • iOS Simulators & Android Emulators (No Biometrics): On an iPhone Simulator (whether Face ID is enrolled or not) or an Android Emulator that hasn't been configured with biometrics, you might notice that both storing and retrieving the password happen without any prompt. This suggests that in environments lacking a native biometric setup, the accessControl might be silently bypassed.
  • Android Devices (including emulators with biometrics): On actual Android devices, including emulators with biometrics configured, the experience is often different. You might be prompted for biometrics on both the set and get operations, especially if there's a noticeable delay between them. This indicates that Android's implementation might be more stringent or that the system's handling of keychain operations is more sensitive to sequential requests.
  • iPhone Devices: On a physical iPhone device, the behavior might be further refined. You might find that the biometric prompt only appears when you attempt to retrieve the password, not necessarily when storing it initially. This behavior aligns more closely with the expectation that sensitive data should require verification upon access, rather than just upon creation.

These inconsistencies highlight that the accessControl flag isn't a universal switch that guarantees a prompt everywhere. Its effectiveness and behavior are deeply intertwined with the underlying operating system's security framework, the specific device hardware, and the device's current biometric configuration.

Troubleshooting the Inconsistent Prompts

If you're facing these issues, here are some steps to consider and common pitfalls to avoid:

1. Always set accessControl on both set and get: While it might not always guarantee a prompt on set, including it in both operations often leads to more predictable behavior, especially on certain platforms like Android. The system might interpret the accessControl more strictly when it's defined for the entire lifecycle of the data in the keychain.

2. Understand Platform Differences: iOS and Android handle keychain data and biometric prompts differently. iOS's Keychain Services are known for their granular control, while Android's Keystore system has its own mechanisms. The react-native-keychain library abstracts these, but the underlying OS behaviors will still surface.

3. Device and Emulator Variations: As observed, simulators and emulators might not perfectly replicate the security behaviors of real devices. Always test on physical hardware whenever possible, especially for security-sensitive features.

4. Biometric Setup: Ensure that the device you are testing on actually has biometrics (Face ID, fingerprint) set up and enabled. If no biometrics are configured on the device, the system has no biometric method to prompt for, and thus the BIOMETRY_ANY flag might be ignored or handled differently.

5. Timing and Delays: In some cases, especially on Android, there might be a very small window between storing and retrieving data. If the get operation happens almost instantaneously after the set operation, the system might consider the data