MASTG-KNOW-0002: FingerprintManager

Deprecated

Reason: The FingerprintManager class is deprecated in Android 9 (API level 28) and should not be used for new applications. Instead, use the BiometricPrompt API or the Biometric library for Android.

Use instead:

Biometric Authentication

Android 6.0 (API level 23) introduced public APIs for authenticating users via fingerprint, but is deprecated in Android 9 (API level 28). 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 overloaded using dynamic instrumentation.

You can achieve better security by using the fingerprint API in conjunction with the Android KeyGenerator class. With this approach, a symmetric key is stored in the Android 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 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 token can then be saved directly on the device (e.g. via Shared Preferences). This design is a relatively safe way to ensure the user actually entered an authorized fingerprint.

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.

Implementation¶

This section describes how to implement biometric authentication by using the FingerprintManager class. Please keep in mind that this class is deprecated and the Biometric library should be used instead as a best practice. This section is just for reference, in case you come across such an implementation and need to analyze it.

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:

The permission must be requested in the Android Manifest:

<uses-permission
    android:name="android.permission.USE_FINGERPRINT" />

Fingerprint hardware must be available:

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

The user must have a protected lock screen:

KeyguardManager keyguardManager = (KeyguardManager) context.getSystemService(Context.KEYGUARD_SERVICE);
keyguardManager.isKeyguardSecure();  //note if this is not the case: ask the user to setup a protected lock screen

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();

The following describes how to do fingerprint authentication using a symmetric key pair.

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);

The callback method onAuthenticationSucceeded(FingerprintManager.AuthenticationResult result) is called when the authentication succeeds. The authenticated CryptoObject can then be retrieved from the result.

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

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

The following describes how to do 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 cryptoObject = 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.