Biometric Authentication Without Cryptographic Binding

暗号化バインディングのない生体認証

Risk: high

説明

暗号化バインディングのない生体認証は、アプリケーションが生体認証（指紋、顔認識など）を実装する際、それを暗号化キーや安全な資格情報に適切にバインドしていない場合に発生します。この脆弱性により、攻撃者は保護されたリソースに直接アクセスしたり、認証トークンを傍受してリプレイしたりすることで、生体認証をバイパスできるようになります。生体データが保護されたデータやセッションと暗号化してリンクされていないため、生体認証チェックは単なるUI層の検証にすぎず、APIまたはストレージレベルで回避される可能性があります。

例

Dart

import 'package:flutter/material.dart';
import 'package:local_auth/local_auth.dart';
import 'dart:convert';
import 'package:shared_preferences/shared_preferences.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Biometric Auth Demo',
      theme: ThemeData(primarySwatch: Colors.blue),
      home: LoginScreen(),
    );
  }
}

class LoginScreen extends StatefulWidget {
  @override
  _LoginScreenState createState() => _LoginScreenState();
}

class _LoginScreenState extends State<LoginScreen> {
  final LocalAuthentication auth = LocalAuthentication();
  bool _isAuthenticated = false;
  final String apiKey = "secret_api_key_12345";

  Future<void> _authenticate() async {
    bool authenticated = false;
    try {
      authenticated = await auth.authenticate(
        localizedReason: 'Authenticate to access the app',
        options: const AuthenticationOptions(
          stickyAuth: true,
          biometricOnly: true,
        ),
      );
    } catch (e) {
      print(e);
    }

    if (authenticated) {
      setState(() {
        _isAuthenticated = true;
      });

      // VULNERABLE: Storing sensitive data without cryptographic binding
      final prefs = await SharedPreferences.getInstance();
      prefs.setString('auth_status', 'authenticated');
      prefs.setString('api_key', apiKey);
    }
  }

  void _accessSecureData() async {
    final prefs = await SharedPreferences.getInstance();
    // VULNERABLE: Only checking local flag without cryptographic verification
    if (prefs.getString('auth_status') == 'authenticated') {
      final retrievedApiKey = prefs.getString('api_key');
      print('Accessed secure data: $retrievedApiKey');
    } else {
      print('Authentication required');
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Biometric Auth Demo')),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            if (!_isAuthenticated)
              ElevatedButton(
                onPressed: _authenticate,
                child: Text('Authenticate with Biometrics'),
              )
            else
              Column(
                children: [
                  Text('Authentication Successful!'),
                  SizedBox(height: 20),
                  ElevatedButton(
                    onPressed: _accessSecureData,
                    child: Text('Access Secure Data'),
                  ),
                ],
              ),
          ],
        ),
      ),
    );
  }
}

Kotlin

import android.hardware.biometrics.BiometricPrompt
import android.os.Build
import androidx.annotation.RequiresApi

@RequiresApi(Build.VERSION_CODES.P)
class BiometricAuthVulnerable {

    private var cryptoObject: BiometricPrompt.CryptoObject? = null

    init {
        // In a vulnerable implementation, the CryptoObject might be null or not properly bound
        cryptoObject = null
    }

    fun authenticate(biometricPrompt: BiometricPrompt, callback: BiometricPrompt.AuthenticationCallback) {
        val promptInfo = BiometricPrompt.PromptInfo.Builder()
            .setTitle("Biometric Authentication")
            .setSubtitle("Authenticate using your fingerprint")
            .setNegativeButtonText("Cancel")
            .build()

        // The cryptoObject is passed but is null or not properly initialized,
        // meaning the biometric data isn't cryptographically bound.
        biometricPrompt.authenticate(promptInfo, cryptoObject, callback)
        println("Authentication initiated without proper cryptographic binding.")
    }

    // In a real scenario, you would have a callback to handle success/failure
    class AuthenticationCallback : BiometricPrompt.AuthenticationCallback() {
        override fun onAuthenticationSucceeded(result: BiometricPrompt.AuthenticationResult) {
            super.onAuthenticationSucceeded(result)
            println("Authentication succeeded, but without cryptographic binding, the result is vulnerable.")
            // Access granted without proper security measures.
        }

        override fun onAuthenticationFailed() {
            super.onAuthenticationFailed()
            println("Authentication failed.")
        }

        override fun onAuthenticationError(errorCode: Int, errString: CharSequence) {
            super.onAuthenticationError(errorCode, errString)
            System.err.println("Authentication error: $errString")
        }
    }
}

fun main() {
    // This is a simplified example and would require an Activity context
    // and proper BiometricPrompt initialization in a real Android application.
    println("This is a demonstration of vulnerable biometric authentication.")
    println("In a real app, the CryptoObject would be missing or improperly used.")
}

Java

import android.hardware.biometrics.BiometricPrompt;
import android.os.Build;
import androidx.annotation.RequiresApi;

import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;
import java.security.KeyStore;

@RequiresApi(api = Build.VERSION_CODES.P)
public class SecureBiometricAuthCrypto {

    private static final String KEY_NAME = "biometric_key";
    private KeyStore keyStore;

    public SecureBiometricAuthCrypto() {
        try {
            keyStore = KeyStore.getInstance("AndroidKeyStore");
            keyStore.load(null);
            if (!keyStore.containsAlias(KEY_NAME)) {
                generateKey();
            }
        } catch (Exception e) {
            throw new RuntimeException("Failed to initialize Keystore", e);
        }
    }

    private void generateKey() {
        try {
            KeyGenerator keyGenerator = KeyGenerator.getInstance("AES", "AndroidKeyStore");
            keyGenerator.init(null); // No specific parameters needed for AndroidKeyStore
            keyGenerator.generateKey();
        } catch (Exception e) {
            throw new RuntimeException("Failed to generate key", e);
        }
    }

    private BiometricPrompt.CryptoObject getCryptoObject() {
        try {
            SecretKey key = (SecretKey) keyStore.getKey(KEY_NAME, null);
            Cipher cipher = Cipher.getInstance("AES/CBC/PKCS7Padding");
            cipher.init(Cipher.ENCRYPT_MODE, key);
            return new BiometricPrompt.CryptoObject(cipher);
        } catch (Exception e) {
            throw new RuntimeException("Failed to get CryptoObject", e);
        }
    }

    public void authenticate(BiometricPrompt biometricPrompt, BiometricPrompt.AuthenticationCallback callback) {
        BiometricPrompt.PromptInfo promptInfo = new BiometricPrompt.PromptInfo.Builder()
                .setTitle("Biometric Authentication")
                .setSubtitle("Authenticate using your fingerprint")
                .setNegativeButtonText("Cancel")
                .build();

        // Properly obtain and pass the CryptoObject to bind the biometric data
        BiometricPrompt.CryptoObject cryptoObject = getCryptoObject();
        if (cryptoObject != null) {
            biometricPrompt.authenticate(promptInfo, cryptoObject, callback);
            System.out.println("Authentication initiated with cryptographic binding.");
        } else {
            System.err.println("Failed to obtain CryptoObject. Authentication cannot be securely performed.");
        }
    }

    public static class AuthenticationCallback extends BiometricPrompt.AuthenticationCallback {
        @Override
        public void onAuthenticationSucceeded(BiometricPrompt.AuthenticationResult result) {
            super.onAuthenticationSucceeded(result);
            if (result.getCryptoObject() != null) {
                // The getCryptoObject() will return the CryptoObject used during authentication.
                // You can now use the Cipher object within it to perform cryptographic operations
                // on the data that the user intended to protect with their biometric.
                System.out.println("Authentication succeeded with cryptographic binding. Cipher: " + result.getCryptoObject().getCipher().getAlgorithm());
                // Proceed with secure operations using the cipher.
            } else {
                System.err.println("Authentication succeeded, but CryptoObject is null. This should not happen in a secure implementation.");
                // Handle this error appropriately.
            }
        }

        @Override
        public void onAuthenticationFailed() {
            super.onAuthenticationFailed();
            System.out.println("Authentication failed.");
        }

        @Override
        public void onAuthenticationError(int errorCode, CharSequence errString) {
            super.onAuthenticationError(errorCode, errString);
            System.err.println("Authentication error: " + errString);
        }
    }

    public static void main(String[] args) {
        // This is a simplified example and would require an Activity context
        // and proper BiometricPrompt initialization in a real Android application.
        System.out.println("This demonstrates secure biometric authentication using CryptoObject.");
    }
}

推奨事項

生体認証を安全に実装するには、生体認証による検証を機密データまたは認証プロセスに暗号化してバインドします。デバイスの安全なハードウェア（Keystore/Keychainなど）を使用して、生体認証が成功した後にのみアクセスできる暗号化キーを生成および保存します。これにより、攻撃者がUI層をバイパスしたり、保存された設定にアクセスしたりしても、復号化キーはハードウェアで保護され生体認証でゲートされているため、生体認証チェックを通過せずに保護されたデータにアクセスすることはできません。

コード例:

Dart

import 'package:flutter/material.dart';
import 'package:local_auth/local_auth.dart';
import 'package:flutter_secure_storage/flutter_secure_storage.dart';
import 'package:encrypt/encrypt.dart' as encrypt;

class SecureBiometricAuth {
  final LocalAuthentication auth = LocalAuthentication();
  final FlutterSecureStorage secureStorage = FlutterSecureStorage();

  // For securely storing encryption key
  final String keyId = 'biometric_protected_key';

  Future<bool> authenticateUser() async {
    return await auth.authenticate(
      localizedReason: 'Authenticate to access the app',
      options: const AuthenticationOptions(
        stickyAuth: true,
        biometricOnly: true,
      ),
    );
  }

  Future<void> secureData(String sensitiveData) async {
    if (await authenticateUser()) {
      // Generate cryptographic key after biometric authentication
      final key = encrypt.Key.fromSecureRandom(32);
      final iv = encrypt.IV.fromSecureRandom(16);

      // Store encryption key in secure storage
      await secureStorage.write(key: keyId, value: key.base64);
      await secureStorage.write(key: '${keyId}_iv', value: iv.base64);

      // Encrypt sensitive data
      final encrypter = encrypt.Encrypter(encrypt.AES(key));
      final encrypted = encrypter.encrypt(sensitiveData, iv: iv);

      // Store encrypted data - only useful with the key
      final prefs = await SharedPreferences.getInstance();
      prefs.setString('encrypted_sensitive_data', encrypted.base64);

      return true;
    }
    return false;
  }

  Future<String?> retrieveSecureData() async {
    if (await authenticateUser()) {
      // Get encryption key (requires biometric auth to access secure storage)
      final keyString = await secureStorage.read(key: keyId);
      final ivString = await secureStorage.read(key: '${keyId}_iv');

      if (keyString == null || ivString == null) return null;

      // Get encrypted data
      final prefs = await SharedPreferences.getInstance();
      final encryptedData = prefs.getString('encrypted_sensitive_data');
      if (encryptedData == null) return null;

      // Decrypt data using key from secure storage
      final key = encrypt.Key.fromBase64(keyString);
      final iv = encrypt.IV.fromBase64(ivString);
      final encrypter = encrypt.Encrypter(encrypt.AES(key));

      return encrypter.decrypt64(encryptedData, iv: iv);
    }
    return null;
  }
}

Kotlin

// Recommended Kotlin code for Recommendation (Secure Data Access with CryptoObject)
import android.hardware.biometrics.BiometricPrompt
import android.os.Build
import androidx.annotation.RequiresApi
import java.nio.charset.StandardCharsets
import java.security.KeyStore
import javax.crypto.Cipher
import javax.crypto.KeyGenerator
import javax.crypto.NoSuchPaddingException
import javax.crypto.SecretKey
import javax.crypto.spec.IvParameterSpec
import java.security.InvalidAlgorithmParameterException
import java.security.InvalidKeyException
import java.security.NoSuchAlgorithmException
import java.util.Base64

@RequiresApi(Build.VERSION_CODES.P)
class SecureBiometricDataAccess {

    private val KEY_NAME = "biometric_data_key"
    private val TRANSFORMATION = "AES/CBC/PKCS7Padding"
    private val keyStore: KeyStore

    init {
        try {
            keyStore = KeyStore.getInstance("AndroidKeyStore")
            keyStore.load(null)
            if (!keyStore.containsAlias(KEY_NAME)) {
                generateKey()
            }
        } catch (e: Exception) {
            throw RuntimeException("Failed to initialize Keystore", e)
        }
    }

    private fun generateKey() {
        try {
            val keyGenerator = KeyGenerator.getInstance("AES", "AndroidKeyStore")
            keyGenerator.init(null)
            keyGenerator.generateKey()
        } catch (e: Exception) {
            throw RuntimeException("Failed to generate key", e)
        }
    }

    private fun getCryptoObject(): BiometricPrompt.CryptoObject? {
        return try {
            val key = keyStore.getKey(KEY_NAME, null) as SecretKey
            val cipher = Cipher.getInstance(TRANSFORMATION)
            cipher.init(Cipher.DECRYPT_MODE, key, IvParameterSpec(ByteArray(cipher.blockSize)))
            BiometricPrompt.CryptoObject(cipher)
        } catch (e: NoSuchAlgorithmException | NoSuchPaddingException | InvalidKeyException | InvalidAlgorithmParameterException) {
            RuntimeException("Failed to get CryptoObject for decryption", e)
            null
        }
    }

    // Simulate encrypted data storage
    private val encryptedData = "some_encrypted_data" // In a real app, this would be fetched

    fun authenticateAndDecrypt(biometricPrompt: BiometricPrompt, callback: BiometricPrompt.AuthenticationCallback) {
        val promptInfo = BiometricPrompt.PromptInfo.Builder()
            .setTitle("Biometric Authentication for Data Access")
            .setSubtitle("Authenticate to decrypt sensitive data")
            .setNegativeButtonText("Cancel")
            .build()

        val cryptoObject = getCryptoObject()
        if (cryptoObject != null) {
            biometricPrompt.authenticate(promptInfo, cryptoObject, callback)
            println("Authentication initiated for data decryption.")
        } else {
            System.err.println("Failed to obtain CryptoObject for decryption.")
        }
    }

    class AuthenticationCallback : BiometricPrompt.AuthenticationCallback() {
        override fun onAuthenticationSucceeded(result: BiometricPrompt.AuthenticationResult) {
            super.onAuthenticationSucceeded(result)
            val cryptoObject = result.cryptoObject
            if (cryptoObject != null) {
                val cipher = cryptoObject.cipher
                try {
                    // Simulate fetching encrypted data
                    val encryptedBytes = Base64.getDecoder().decode("YWFhYWFhYWFhYWFhYWFhYQ==") // Replace with actual encrypted data
                    val decryptedBytes = cipher?.doFinal(encryptedBytes)
                    val decryptedData = decryptedBytes?.toString(StandardCharsets.UTF_8)
                    println("Biometric authentication successful. Decrypted data: $decryptedData")
                    // Now you can securely use the decrypted data.
                } catch (e: Exception) {
                    System.err.println("Error during decryption: ${e.message}")
                }
            } else {
                System.err.println("Authentication succeeded, but CryptoObject is null.")
            }
        }

        override fun onAuthenticationFailed() {
            super.onAuthenticationFailed()
            println("Biometric authentication failed.")
        }

        override fun onAuthenticationError(errorCode: Int, errString: CharSequence) {
            super.onAuthenticationError(errorCode, errString)
            System.err.println("Authentication error: $errString")
        }
    }
}

fun main() {
    // Requires an Android environment for BiometricPrompt
    println("This demonstrates secure data access after biometric authentication.")
}

Java

// Recommended Java code for Recommendation (Secure Data Access with CryptoObject)
import android.hardware.biometrics.BiometricPrompt;
import android.os.Build;
import androidx.annotation.RequiresApi;

import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.NoSuchPaddingException;
import javax.crypto.SecretKey;
import javax.crypto.spec.IvParameterSpec;
import java.nio.charset.StandardCharsets;
import java.security.InvalidAlgorithmParameterException;
import java.security.InvalidKeyException;
import java.security.KeyStore;
import java.security.NoSuchAlgorithmException;
import java.util.Base64;

@RequiresApi(api = Build.VERSION_CODES.P)
public class SecureBiometricDataAccess {

    private static final String KEY_NAME = "biometric_data_key";
    private static final String TRANSFORMATION = "AES/CBC/PKCS7Padding";
    private KeyStore keyStore;

    public SecureBiometricDataAccess() {
        try {
            keyStore = KeyStore.getInstance("AndroidKeyStore");
            keyStore.load(null);
            if (!keyStore.containsAlias(KEY_NAME)) {
                generateKey();
            }
        } catch (Exception e) {
            throw new RuntimeException("Failed to initialize Keystore", e);
        }
    }

    private void generateKey() {
        try {
            KeyGenerator keyGenerator = KeyGenerator.getInstance("AES", "AndroidKeyStore");
            keyGenerator.init(null);
            keyGenerator.generateKey();
        } catch (Exception e) {
            throw new RuntimeException("Failed to generate key", e);
        }
    }

    private BiometricPrompt.CryptoObject getCryptoObject() {
        try {
            SecretKey key = (SecretKey) keyStore.getKey(KEY_NAME, null);
            Cipher cipher = Cipher.getInstance(TRANSFORMATION);
            cipher.init(Cipher.DECRYPT_MODE, key, new IvParameterSpec(new byte[cipher.getBlockSize()]));
            return new BiometricPrompt.CryptoObject(cipher);
        } catch (NoSuchAlgorithmException | NoSuchPaddingException | InvalidKeyException | InvalidAlgorithmParameterException e) {
            throw new RuntimeException("Failed to get CryptoObject for decryption", e);
        }
    }

    // Simulate encrypted data storage
    private String encryptedData = "some_encrypted_data"; // In a real app, this would be fetched

    public void authenticateAndDecrypt(BiometricPrompt biometricPrompt, BiometricPrompt.AuthenticationCallback callback) {
        BiometricPrompt.PromptInfo promptInfo = new BiometricPrompt.PromptInfo.Builder()
                .setTitle("Biometric Authentication for Data Access")
                .setSubtitle("Authenticate to decrypt sensitive data")
                .setNegativeButtonText("Cancel")
                .build();

        BiometricPrompt.CryptoObject cryptoObject = getCryptoObject();
        if (cryptoObject != null) {
            biometricPrompt.authenticate(promptInfo, cryptoObject, callback);
            System.out.println("Authentication initiated for data decryption.");
        } else {
            System.err.println("Failed to obtain CryptoObject for decryption.");
        }
    }

    public static class AuthenticationCallback extends BiometricPrompt.AuthenticationCallback {
        @Override
        public void onAuthenticationSucceeded(BiometricPrompt.AuthenticationResult result) {
            super.onAuthenticationSucceeded(result);
            if (result.getCryptoObject() != null) {
                Cipher cipher = result.getCryptoObject().getCipher();
                try {
                    // Simulate fetching encrypted data
                    byte[] encryptedBytes = Base64.getDecoder().decode("YWFhYWFhYWFhYWFhYWFhYQ=="); // Replace with actual encrypted data
                    byte[] decryptedBytes = cipher.doFinal(encryptedBytes);
                    String decryptedData = new String(decryptedBytes, StandardCharsets.UTF_8);
                    System.out.println("Biometric authentication successful. Decrypted data: " + decryptedData);
                    // Now you can securely use the decrypted data.
                } catch (Exception e) {
                    System.err.println("Error during decryption: " + e.getMessage());
                }
            } else {
                System.err.println("Authentication succeeded, but CryptoObject is null.");
            }
        }

        @Override
        public void onAuthenticationFailed() {
            super.onAuthenticationFailed();
            System.out.println("Biometric authentication failed.");
        }

        @Override
        public void onAuthenticationError(int errorCode, CharSequence errString) {
            super.onAuthenticationError(errorCode, errString);
            System.err.println("Authentication error: " + errString);
        }
    }

    public static void main(String[] args) {
        // Requires an Android environment for BiometricPrompt
        System.out.println("This demonstrates secure data access after biometric authentication.");
    }
}

リンク

• OWASP Mobile Top 10
• Android Keystore System
• iOS Secure Enclave

標準

• OWASP_MASVS_L1:
  • MSTG_STORAGE_1
  • MSTG_CRYPTO_1
• OWASP_MASVS_L2:
  • MSTG_AUTH_8
  • MSTG_AUTH_10
• HIPAA_CONTROLS:
  • SECURITY221
  • SECURITY212
  • SECURITY213
  • SECURITY251