Local Authentication on Android

During local authentication, an app authenticates the user against credentials stored locally on the device. In other words, the user "unlocks" the app or some inner layer of functionality by providing a valid PIN, password, or fingerprint, verified by referencing local data. Generally, this process is invoked for reasons such providing a user convenience for resuming an existing session with the remote service or as a means of step-up authentication to protect some critical function.

Testing Biometric Authentication

Overview

Android Marshmallow (6.0) introduced public APIs for authenticating users via fingerprint. Access to the fingerprint hardware is provided through the FingerprintManager class. An app can request fingerprint authentication by instantiating a FingerprintManager object and calling its authenticate() method. The caller registers callback methods to handle possible outcomes of the authentication process (i.e. success, failure, or error). Note that this method doesn't constitute strong proof that fingerprint authentication has actually been performed - for example, the authentication step could be patched out by an attacker, or the "success" callback could be called using instrumentation.

Better security is achieved by using the fingerprint API in conjunction with the Android KeyGenerator class. With this method, a symmetric key is stored in the Keystore and "unlocked" with the user's fingerprint. For example, to enable user access to a remote service, an AES key is created which encrypts the user PIN or authentication token. By calling setUserAuthenticationRequired(true) when creating the key, it is ensured that the user must re-authenticate to retrieve it. The encrypted authentication credentials can then be saved directly to regular storage on the the device (e.g. SharedPreferences). This design is a relatively safe way to ensure the user actually entered an authorized fingerprint. Note however that this setup requires the app to hold the symmetric key in memory during cryptographic operations, potentially exposing it to attackers that manage to access the app's memory during runtime.

An even more secure option is using asymmetric cryptography. Here, the mobile app creates an asymmetric key pair in the Keystore and enrolls the public key on the server backend. Later transactions are then signed with the private key and verified by the server using the public key. The advantage of this is that transactions can be signed using Keystore APIs without ever extracting the private key from the Keystore. Consequently, it is impossible for attackers to obtain the key from memory dumps or by using instrumentation.

Static Analysis

Begin by searching for FingerprintManager.authenticate() calls. The first parameter passed to this method should be a CryptoObject instance which is a wrapper class for crypto objects supported by FingerprintManager. Should the parameter be set to null, this means the fingerprint authorization is purely event-bound, likely creating a security issue.

The creation of the key used to initialize the cipher wrapper can be traced back to the CryptoObject. Verify the key was both created using the KeyGenerator class in addition to setUserAuthenticationRequired(true) being called during creation of the KeyGenParameterSpec object (see code samples below).

Make sure to verify the authentication logic. For the authentication to be successful, the remote endpoint must require the client to present the secret retrieved from the Keystore, a value derived from the secret, or a value signed with the client private key (see above).

Safely implementing fingerprint authentication requires following a few simple principles, starting by first checking if that type of authentication is even available. On the most basic front, the device must run Android 6.0 or higher (API 23+). Four other prerequisites must also be verified:

Fingerprint hardware must be available:

     FingerprintManager fingerprintManager = (FingerprintManager)
                    context.getSystemService(Context.FINGERPRINT_SERVICE);
    fingerprintManager.isHardwareDetected();

The user must have a protected lockscreen:

     KeyguardManager keyguardManager = (KeyguardManager) context.getSystemService(Context.KEYGUARD_SERVICE);
     keyguardManager.isKeyguardSecure();

At least one finger should be registered:

    fingerprintManager.hasEnrolledFingerprints();

The application should have permission to ask for a user fingerprint:

    context.checkSelfPermission(Manifest.permission.USE_FINGERPRINT) == PermissionResult.PERMISSION_GRANTED;

If any of the above checks fail, the option for fingerprint authentication should not be offered.

It is important to remember that not every Android device offers hardware-backed key storage. The KeyInfo class can be used to find out whether the key resides inside secure hardware such as a Trusted Execution Environment (TEE) or Secure Element (SE).

SecretKeyFactory factory = SecretKeyFactory.getInstance(getEncryptionKey().getAlgorithm(), ANDROID_KEYSTORE);
                KeyInfo secetkeyInfo = (KeyInfo) factory.getKeySpec(yourencryptionkeyhere, KeyInfo.class);
secetkeyInfo.isInsideSecureHardware()

On certain systems, it is possible to enforce the policy for biometric authentication through hardware as well. This is checked by:

    keyInfo.isUserAuthenticationRequirementEnforcedBySecureHardware();

Fingerprint Authentication using a Symmetric Key

Fingerprint authentication may be implemented by creating a new AES key using the KeyGenerator class by adding setUserAuthenticationRequired(true) in KeyGenParameterSpec.Builder.

    generator = KeyGenerator.getInstance(KeyProperties.KEY_ALGORITHM_AES, KEYSTORE);

    generator.init(new KeyGenParameterSpec.Builder (KEY_ALIAS,
          KeyProperties.PURPOSE_ENCRYPT | KeyProperties.PURPOSE_DECRYPT)
          .setBlockModes(KeyProperties.BLOCK_MODE_CBC)
          .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_PKCS7)
          .setUserAuthenticationRequired(true)
          .build()
    );

    generator.generateKey();

To perform encryption or decryption with the protected key, create a Cipher object and initialize it with the key alias.

    SecretKey keyspec = (SecretKey)keyStore.getKey(KEY_ALIAS, null);

    if (mode == Cipher.ENCRYPT_MODE) {
        cipher.init(mode, keyspec);

Keep in mind, a new key cannot be used immediately - it has to be authenticated through the FingerprintManager first. This involves wrapping the Cipher object into FingerprintManager.CryptoObject which is passed to FingerprintManager.authenticate() before it will be recognized.

    cryptoObject = new FingerprintManager.CryptoObject(cipher);
    fingerprintManager.authenticate(cryptoObject, new CancellationSignal(), 0, this, null);

When the authentication succeeds, the callback method onAuthenticationSucceeded(FingerprintManager.AuthenticationResult result) is called at which point, the authenticated CryptoObject can be retrieved from the result.

public void authenticationSucceeded(FingerprintManager.AuthenticationResult result) {
    cipher = result.getCryptoObject().getCipher();

    (... do something with the authenticated cipher object ...)
}

Fingerprint Authentication using an Asymmetric Key Pair

To implement fingerprint authentication using asymmetric cryptography, first create a signing key using the KeyPairGenerator class, and enroll the public key with the server. You can then authenticate pieces of data by signing them on the client and verifying the signature on the server. A detailed example for authenticating to remote servers using the fingerprint API can be found in the Android Developers Blog.

A key pair is generated as follows:

KeyPairGenerator.getInstance(KeyProperties.KEY_ALGORITHM_EC, "AndroidKeyStore");
keyPairGenerator.initialize(
        new KeyGenParameterSpec.Builder(MY_KEY,
                KeyProperties.PURPOSE_SIGN)
                .setDigests(KeyProperties.DIGEST_SHA256)
                .setAlgorithmParameterSpec(new ECGenParameterSpec("secp256r1"))
                .setUserAuthenticationRequired(true)
                .build());
keyPairGenerator.generateKeyPair();

To use the key for signing, you need to instantiate a CryptoObject and authenticate it through FingerprintManager.

Signature.getInstance("SHA256withECDSA");
KeyStore keyStore = KeyStore.getInstance("AndroidKeyStore");
keyStore.load(null);
PrivateKey key = (PrivateKey) keyStore.getKey(MY_KEY, null);
signature.initSign(key);
CryptoObject cryptObject = new FingerprintManager.CryptoObject(signature);

CancellationSignal cancellationSignal = new CancellationSignal();
FingerprintManager fingerprintManager =
        context.getSystemService(FingerprintManager.class);
fingerprintManager.authenticate(cryptoObject, cancellationSignal, 0, this, null);

You can now sign the contents of a byte array inputBytes as follows.

Signature signature = cryptoObject.getSignature();
signature.update(inputBytes);
byte[] signed = signature.sign();

Note that in cases where transactions are signed, a random nonce should be generated and added to the signed data. Otherwise, an attacker could replay the transaction.
To implement authentication using symmetric fingerprint authentication, use a challenge-response protocol.

Additional Security Features

Android Nougat (API 24) adds the setInvalidatedByBiometricEnrollment(boolean invalidateKey) method to KeyGenParameterSpec.Builder. When invalidateKey value is set to "true" (the default), keys that are valid for fingerprint authentication are irreversibly invalidated when a new fingerprint is enrolled. This prevents an attacker from retrieving they key even if they are able to enroll an additional fingerprint.

Dynamic Analysis

Patch the app or use runtime instrumentation to bypass fingerprint authentication on the client. For example, you could use Frida to call the onAuthenticationSucceeded callback method directly. Refer to the chapter "Tampering and Reverse Engineering on Android" for more information.

References

OWASP Mobile Top 10 2016

M4 - Insecure Authentication - https://www.owasp.org/index.php/Mobile_Top_10_2016-M4-Insecure_Authentication

OWASP MASVS

V4.7: "Biometric authentication, if any, is not event-bound (i.e. using an API that simply returns "true" or "false"). Instead, it is based on unlocking the keychain/keystore."

CWE

CWE-287 - Improper Authentication
CWE-604 - Use of Client-Side Authentication

Local Authentication on Android