如何使用 Android KeyStore 安全地存储任意字符串？

Question

我希望能够在 Android KeyStore 中安全地存储一些敏感字符串。 我从服务器获取字符串，但我有一个用例需要我保留它们。 KeyStore 将只允许从与分配给我的应用程序的 UID 相同的 UID 进行访问，并且它将使用设备主密码对数据进行加密，因此我的理解是我不必进行任何额外的加密来保护我的数据。 我的问题是，我缺少有关如何写入数据的信息。 只要省略对 KeyStore.store(null) 的调用，我下面的代码就可以完美运行。 那个代码失败了，只要我把数据放到 KeyStore 之后我不能存储它，那么我就不能持久化它。

我想我遗漏了一些关于 KeyStore API 的东西，但我不知道是什么。 任何帮助表示赞赏！

String metaKey = "ourSecretKey";
String encodedKey = "this is supposed to be a secret";
byte[] encodedKeyBytes = new byte[(int)encodedKey.length()];
encodedKeyBytes = encodedKey.getBytes("UTF-8");
KeyStoreParameter ksp = null;

//String algorithm = "DES";
String algorithm = "DESede";
SecretKeyFactory secretKeyFactory = SecretKeyFactory.getInstance(algorithm);
SecretKeySpec secretKeySpec = new SecretKeySpec(encodedKeyBytes, algorithm);
SecretKey secretKey = secretKeyFactory.generateSecret(secretKeySpec);

KeyStore keyStore = KeyStore.getInstance(KeyStore.getDefaultType());

keyStore.load(null);

KeyStore.SecretKeyEntry secretKeyEntry = new KeyStore.SecretKeyEntry(secretKey);
keyStore.setEntry(metaKey, secretKeyEntry, ksp);

keyStore.store(null);

String recoveredSecret = "";
if (keyStore.containsAlias(metaKey)) {
    KeyStore.SecretKeyEntry recoveredEntry = (KeyStore.SecretKeyEntry)keyStore.getEntry(metaKey, ksp);
    byte[] bytes = recoveredEntry.getSecretKey().getEncoded();
    for (byte b : bytes) {
        recoveredSecret += (char)b;
     }
}
Log.v(TAG, "recovered " + recoveredSecret);

Answer 1

我的前提是我可以使用 AndroidKeyStore 来保护任意数据块，并将它们称为“密钥”。 然而，我越深入研究，就越清楚地发现 KeyStore API 与安全相关的对象深深地纠缠在一起：证书、KeySpecs、Providers 等。将其弯曲到该目的的路径。

但是，AndroidKeyStore 可用于帮助我保护敏感数据。 我可以用它来管理我将用来加密应用程序本地数据的加密密钥。 通过结合使用 AndroidKeyStore、CipherOutputStream 和 CipherInputStream，我们可以：

• 在设备上生成、安全存储和检索加密密钥
• 加密任意数据并将其保存在设备上（在应用程序的目录中，它将受到文件系统权限的进一步保护）
• 访问和解密数据以供后续使用。

下面是一些示例代码，演示了如何实现这一点。

try {
    KeyStore keyStore = KeyStore.getInstance("AndroidKeyStore");
    keyStore.load(null);

    String alias = "key3";

    int nBefore = keyStore.size();

    // Create the keys if necessary
    if (!keyStore.containsAlias(alias)) {

        Calendar notBefore = Calendar.getInstance();
        Calendar notAfter = Calendar.getInstance();
        notAfter.add(Calendar.YEAR, 1);
        KeyPairGeneratorSpec spec = new KeyPairGeneratorSpec.Builder(this)
            .setAlias(alias)
            .setKeyType("RSA")
            .setKeySize(2048)
            .setSubject(new X500Principal("CN=test"))
            .setSerialNumber(BigInteger.ONE)
            .setStartDate(notBefore.getTime())
            .setEndDate(notAfter.getTime())
            .build();
        KeyPairGenerator generator = KeyPairGenerator.getInstance("RSA", "AndroidKeyStore");
        generator.initialize(spec);

        KeyPair keyPair = generator.generateKeyPair();
    }
    int nAfter = keyStore.size();
    Log.v(TAG, "Before = " + nBefore + " After = " + nAfter);

    // Retrieve the keys
    KeyStore.PrivateKeyEntry privateKeyEntry = (KeyStore.PrivateKeyEntry)keyStore.getEntry(alias, null);
    RSAPrivateKey privateKey = (RSAPrivateKey) privateKeyEntry.getPrivateKey();
    RSAPublicKey publicKey = (RSAPublicKey) privateKeyEntry.getCertificate().getPublicKey();

    Log.v(TAG, "private key = " + privateKey.toString());
    Log.v(TAG, "public key = " + publicKey.toString());

    // Encrypt the text
    String plainText = "This text is supposed to be a secret!";
    String dataDirectory = getApplicationInfo().dataDir;
    String filesDirectory = getFilesDir().getAbsolutePath();
    String encryptedDataFilePath = filesDirectory + File.separator + "keep_yer_secrets_here";

    Log.v(TAG, "plainText = " + plainText);
    Log.v(TAG, "dataDirectory = " + dataDirectory);
    Log.v(TAG, "filesDirectory = " + filesDirectory);
    Log.v(TAG, "encryptedDataFilePath = " + encryptedDataFilePath);

    Cipher inCipher = Cipher.getInstance("RSA/ECB/PKCS1Padding", "AndroidOpenSSL");
    inCipher.init(Cipher.ENCRYPT_MODE, publicKey);

    Cipher outCipher = Cipher.getInstance("RSA/ECB/PKCS1Padding", "AndroidOpenSSL");
    outCipher.init(Cipher.DECRYPT_MODE, privateKey);

    CipherOutputStream cipherOutputStream = 
        new CipherOutputStream(
            new FileOutputStream(encryptedDataFilePath), inCipher);
    cipherOutputStream.write(plainText.getBytes("UTF-8"));
    cipherOutputStream.close();

    CipherInputStream cipherInputStream = 
        new CipherInputStream(new FileInputStream(encryptedDataFilePath),
            outCipher);
    byte [] roundTrippedBytes = new byte[1000]; // TODO: dynamically resize as we get more data

    int index = 0;
    int nextByte;
    while ((nextByte = cipherInputStream.read()) != -1) {
        roundTrippedBytes[index] = (byte)nextByte;
        index++;
    }
    String roundTrippedString = new String(roundTrippedBytes, 0, index, "UTF-8");
    Log.v(TAG, "round tripped string = " + roundTrippedString);

} catch (NoSuchAlgorithmException e) {
    Log.e(TAG, Log.getStackTraceString(e));
} catch (NoSuchProviderException e) {
    Log.e(TAG, Log.getStackTraceString(e));
} catch (InvalidAlgorithmParameterException e) {
    Log.e(TAG, Log.getStackTraceString(e));
} catch (KeyStoreException e) {
    Log.e(TAG, Log.getStackTraceString(e));
} catch (CertificateException e) {
    Log.e(TAG, Log.getStackTraceString(e));
} catch (IOException e) {
    Log.e(TAG, Log.getStackTraceString(e));
} catch (UnrecoverableEntryException e) {
    Log.e(TAG, Log.getStackTraceString(e));
} catch (NoSuchPaddingException e) {
    Log.e(TAG, Log.getStackTraceString(e));
} catch (InvalidKeyException e) {
    Log.e(TAG, Log.getStackTraceString(e));
} catch (BadPaddingException e) {
    Log.e(TAG, Log.getStackTraceString(e));
} catch (IllegalBlockSizeException e) {
    Log.e(TAG, Log.getStackTraceString(e));
} catch (UnsupportedOperationException e) {
    Log.e(TAG, Log.getStackTraceString(e));
}

Answer 2

您可能已经注意到，使用 Android Keystore 处理不同的 API 级别存在问题。

Scytale是一个开源库，它为 Android 密钥库提供了一个方便的包装器，这样您就无需编写样板，可以直接进入加密/解密。

示例代码：

// Create and save key
Store store = new Store(getApplicationContext());
if (!store.hasKey("test")) {
   SecretKey key = store.generateSymmetricKey("test", null);
}
...

// Get key
SecretKey key = store.getSymmetricKey("test", null);

// Encrypt/Decrypt data
Crypto crypto = new Crypto(Options.TRANSFORMATION_SYMMETRIC);
String text = "Sample text";

String encryptedData = crypto.encrypt(text, key);
Log.i("Scytale", "Encrypted data: " + encryptedData);

String decryptedData = crypto.decrypt(encryptedData, key);
Log.i("Scytale", "Decrypted data: " + decryptedData);

Answer 3

我已经重新设计了 Patrick Brennan 接受的答案。 在 Android 9 上，它产生了 NoSuchAlgorithmException。 已弃用的 KeyPairGeneratorSpec 已替换为 KeyPairGenerator。 还有一些工作需要解决有关填充的异常。

代码注释了所做的更改：“***”

@RequiresApi(api = Build.VERSION_CODES.M)
public static void storeExistingKey(Context context) {

    final String TAG = "KEY-UTIL";

    try {
        KeyStore keyStore = KeyStore.getInstance("AndroidKeyStore");
        keyStore.load(null);

        String alias = "key11";
        int nBefore = keyStore.size();

        // Create the keys if necessary
        if (!keyStore.containsAlias(alias)) {

            Calendar notBefore = Calendar.getInstance();
            Calendar notAfter = Calendar.getInstance();
            notAfter.add(Calendar.YEAR, 1);


            // *** Replaced deprecated KeyPairGeneratorSpec with KeyPairGenerator
            KeyPairGenerator spec = KeyPairGenerator.getInstance(
                    // *** Specified algorithm here
                    // *** Specified: Purpose of key here
                    KeyProperties.KEY_ALGORITHM_RSA, "AndroidKeyStore");
            spec.initialize(new KeyGenParameterSpec.Builder(
                    alias, KeyProperties.PURPOSE_DECRYPT | KeyProperties.PURPOSE_ENCRYPT) 
                    .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_RSA_PKCS1) //  RSA/ECB/PKCS1Padding
                    .setKeySize(2048)
                    // *** Replaced: setStartDate
                    .setKeyValidityStart(notBefore.getTime())
                    // *** Replaced: setEndDate
                    .setKeyValidityEnd(notAfter.getTime())
                    // *** Replaced: setSubject
                    .setCertificateSubject(new X500Principal("CN=test"))
                    // *** Replaced: setSerialNumber
                    .setCertificateSerialNumber(BigInteger.ONE)
                    .build());
            KeyPair keyPair = spec.generateKeyPair();
            Log.i(TAG, keyPair.toString());
        }

        int nAfter = keyStore.size();
        Log.v(TAG, "Before = " + nBefore + " After = " + nAfter);

        // Retrieve the keys
        KeyStore.PrivateKeyEntry privateKeyEntry = (KeyStore.PrivateKeyEntry) keyStore.getEntry(alias, null);
        PrivateKey privateKey = privateKeyEntry.getPrivateKey();
        PublicKey publicKey = privateKeyEntry.getCertificate().getPublicKey();


        Log.v(TAG, "private key = " + privateKey.toString());
        Log.v(TAG, "public key = " + publicKey.toString());

        // Encrypt the text
        String plainText = "This text is supposed to be a secret!";
        String dataDirectory = context.getApplicationInfo().dataDir;
        String filesDirectory = context.getFilesDir().getAbsolutePath();
        String encryptedDataFilePath = filesDirectory + File.separator + "keep_yer_secrets_here";

        Log.v(TAG, "plainText = " + plainText);
        Log.v(TAG, "dataDirectory = " + dataDirectory);
        Log.v(TAG, "filesDirectory = " + filesDirectory);
        Log.v(TAG, "encryptedDataFilePath = " + encryptedDataFilePath);

        // *** Changed the padding type here and changed to AndroidKeyStoreBCWorkaround
        Cipher inCipher = Cipher.getInstance("RSA/ECB/PKCS1Padding", "AndroidKeyStoreBCWorkaround");
        inCipher.init(Cipher.ENCRYPT_MODE, publicKey);

        // *** Changed the padding type here and changed to AndroidKeyStoreBCWorkaround
        Cipher outCipher = Cipher.getInstance("RSA/ECB/PKCS1Padding", "AndroidKeyStoreBCWorkaround");
        outCipher.init(Cipher.DECRYPT_MODE, privateKey);

        CipherOutputStream cipherOutputStream =
                new CipherOutputStream(
                        new FileOutputStream(encryptedDataFilePath), inCipher);
        // *** Replaced string literal with StandardCharsets.UTF_8
        cipherOutputStream.write(plainText.getBytes(StandardCharsets.UTF_8));
        cipherOutputStream.close();

        CipherInputStream cipherInputStream =
                new CipherInputStream(new FileInputStream(encryptedDataFilePath),
                        outCipher);
        byte[] roundTrippedBytes = new byte[1000]; // TODO: dynamically resize as we get more data

        int index = 0;
        int nextByte;
        while ((nextByte = cipherInputStream.read()) != -1) {
            roundTrippedBytes[index] = (byte) nextByte;
            index++;
        }

        // *** Replaced string literal with StandardCharsets.UTF_8
        String roundTrippedString = new String(roundTrippedBytes, 0, index, StandardCharsets.UTF_8);
        Log.v(TAG, "round tripped string = " + roundTrippedString);

    } catch (NoSuchAlgorithmException | UnsupportedOperationException | InvalidKeyException | NoSuchPaddingException | UnrecoverableEntryException | NoSuchProviderException | KeyStoreException | CertificateException | IOException e | InvalidAlgorithmParameterException e) {
        e.printStackTrace();
}

注意：“AndroidKeyStoreBCWorkaround”允许代码跨不同的 API 工作。

如果有人可以评论此更新解决方案中的任何缺点，我将不胜感激。 否则，如果有更多加密知识的人有信心更新帕特里克的答案，那么我将删除这个答案。