尝试在 GCM 模式下解密消息时出现 AEADBadTagException

Question

我正在编写一个有很多安全限制的应用程序：它需要安全地存储加密文件，并且必须能够解密它们。此外，操作员需要能够在没有应用程序的情况下解密文件。

为了存档，我在我的电脑上生成了一个 KeyPair，将 public 部分放在我的应用程序中，在应用程序中生成了一个 AES SecretKey 密钥，使用我的 public 密钥加密并保存它（供操作员使用），并将未加密的密钥放入 AndroidKeyStore.

为了加密消息，我从 KeyStore 收到 SecretKey，加密我的消息，获取我使用的 IV 以及加密的 SecretKey，并将它们写入定义的顺序为字节数组 (iv->encryptedSecretKey->encryptedMessage)。

为了解密，我反向尝试相同的操作：获取字节数组，读取 iv 和 encryptedSecretKey，然后将其余的 (encryptedMessage) 传递给我的密码进行解密。问题是，cipher.doFinal(encryptedMessage) 正在抛出一个 javax.crypto.AEADBadTagException这是由 android.security.KeyStoreException: Signature/MAC verification failed.

引起的

我已经检查过加密的消息和我要解密的消息完全一样。我不知道我做错了什么。

我用的class是这样的：

package my.company.domain;

import android.content.Context;
import android.content.SharedPreferences;
import android.security.keystore.KeyProperties;
import android.security.keystore.KeyProtection;
import android.support.annotation.NonNull;
import android.util.Base64;
import android.util.Log;

import java.io.DataInputStream;
import java.io.FileInputStream;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.charset.StandardCharsets;
import java.security.GeneralSecurityException;
import java.security.InvalidKeyException;
import java.security.KeyFactory;
import java.security.KeyStore;
import java.security.KeyStoreException;
import java.security.NoSuchAlgorithmException;
import java.security.NoSuchProviderException;
import java.security.PublicKey;
import java.security.SecureRandom;
import java.security.spec.X509EncodedKeySpec;

import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.NoSuchPaddingException;
import javax.crypto.SecretKey;
import javax.crypto.spec.GCMParameterSpec;
import javax.crypto.spec.SecretKeySpec;

public class CryptoHelper {

    public static final String TAG = CryptoHelper.class.getSimpleName();

    private static final String KEY_ALIAS = "OI1lTI1lLI1l0";
    private static final char[] KEY_PASSWORD = "Il0VELI1lO".toCharArray();

    private static final String PREF_NAME = "CryptoPrefs";
    private static final String KEY_ENCRYPTED_SECRET = "encryptedSecret";

    private static final String ANDROID_KEY_STORE = "AndroidKeyStore";

    private static final int    IV_SIZE = 12;
    private static final int    IV_BIT_LEN = IV_SIZE * 8;


    //generate 128 bit key (16), other possible values 192(24), 256(32)
    private static final int    AES_KEY_SIZE = 16;
    private static final String AES = KeyProperties.KEY_ALGORITHM_AES;
    private static final String AES_MODE = AES + "/" + KeyProperties.BLOCK_MODE_GCM + "/" + KeyProperties.ENCRYPTION_PADDING_NONE;

    private static final String RSA = KeyProperties.KEY_ALGORITHM_RSA;
    private static final String RSA_MODE = KeyProperties.KEY_ALGORITHM_RSA + "/" + KeyProperties.BLOCK_MODE_ECB + "/" + KeyProperties.ENCRYPTION_PADDING_NONE;
    private static final String RSA_PROVIDER = "AndroidOpenSSL";

    private final Context mContext;
    private final SharedPreferences mPrefs;

    private SecureRandom mSecureRandom;
    private KeyStore mAndroidKeyStore;
    private PublicKey mPublicKey;
    private byte[] mEncryptedSecretKey;

    public CryptoHelper(Context context) {
        mContext = context;
        mSecureRandom = new SecureRandom();
        mPrefs = mContext.getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE);
        try {
            mAndroidKeyStore = KeyStore.getInstance(ANDROID_KEY_STORE);
            mAndroidKeyStore.load(null);

        } catch (KeyStoreException e) {
            Log.wtf(TAG, "Could not get AndroidKeyStore!", e);
        } catch (Exception e) {
            Log.wtf(TAG, "Could not load AndroidKeyStore!", e);
        }
    }

    public void reset() throws KeyStoreException {
        mAndroidKeyStore.deleteEntry(KEY_ALIAS);
    }

    public byte[] encrypt(byte[] message){
        SecretKey secretKey = getSecretKey();
        try {
            Cipher cipher = Cipher.getInstance(AES_MODE);
            cipher.init(Cipher.ENCRYPT_MODE, getSecretKey());

            byte[] cryptedBytes = cipher.doFinal(message);
            byte[] iv = cipher.getIV();
            byte[] encryptedSecretKey = getEncryptedSecretKey();
            ByteBuffer buffer = ByteBuffer.allocate(IV_BIT_LEN + encryptedSecretKey.length + cryptedBytes.length);
            buffer
                .put(iv)
                .put(encryptedSecretKey)
                .put(cryptedBytes);
            return buffer.array();
        } catch (GeneralSecurityException e) {
            e.printStackTrace();
        }
        return null;
    }

    public byte[] encrypt(String message){
        return encrypt(message.getBytes(StandardCharsets.UTF_8));
    }

    public byte[] decrypt(byte[] bytes){
        ByteBuffer buffer = ByteBuffer.wrap(bytes);
        byte[] iv = new byte[IV_SIZE];
        buffer.get(iv);
        byte[] unused = getEncryptedSecretKey();
        buffer.get(unused);
        byte[] encryptedMessage = new byte[bytes.length - IV_SIZE - unused.length];
        buffer.get(encryptedMessage);
        try {
            Cipher cipher = Cipher.getInstance(AES_MODE);
            GCMParameterSpec parameterSpec = new GCMParameterSpec(IV_BIT_LEN, iv);
            cipher.init(Cipher.DECRYPT_MODE, getSecretKey(), parameterSpec);
            byte[] decryptedMessage = cipher.doFinal(encryptedMessage);
            return decryptedMessage;
        } catch (GeneralSecurityException e) {
            e.printStackTrace();
        }
        return null;
    }

    public String decryptToString(byte[] bytes){
        return new String(decrypt(bytes), StandardCharsets.UTF_8);
    }

    public byte[] decrypt(FileInputStream fileToDecrypt){
        byte[] buffer = null;
        try {
            buffer = new byte[fileToDecrypt.available()];
            fileToDecrypt.read(buffer);
            buffer = decrypt(buffer);
        } catch (IOException e) {
            e.printStackTrace();
        }
        return buffer;
    }


    public PublicKey getPublicKey() {
        if (null == mPublicKey) {
            mPublicKey = readPublicKey();
        }
        return mPublicKey;
    }

    public byte[] getEncryptedSecretKey() {
        if (null == mEncryptedSecretKey){
            mEncryptedSecretKey = Base64.decode(mPrefs.getString(KEY_ENCRYPTED_SECRET, null), Base64.NO_WRAP);
        }
        return mEncryptedSecretKey;
    }

    private void saveEncryptedSecretKey(byte[] encryptedSecretKey){
        String base64EncryptedKey = Base64.encodeToString(encryptedSecretKey, Base64.NO_WRAP);
        mPrefs.edit().putString(KEY_ENCRYPTED_SECRET, base64EncryptedKey).apply();
    }

    protected SecretKey getSecretKey(){
        SecretKey secretKey = null;
        try {
            if (!mAndroidKeyStore.containsAlias(KEY_ALIAS)){
               generateAndStoreSecureKey();
            }
            secretKey = (SecretKey) mAndroidKeyStore.getKey(KEY_ALIAS, KEY_PASSWORD);
        } catch (KeyStoreException e) {
            Log.wtf(TAG, "Could not check AndroidKeyStore alias!", e);
            secretKey = null;
        } catch (GeneralSecurityException e) {
            e.printStackTrace();
            secretKey = null;
        }
        return secretKey;
    }

    private void generateAndStoreSecureKey()
            throws NoSuchPaddingException, NoSuchAlgorithmException, NoSuchProviderException, InvalidKeyException, KeyStoreException, BadPaddingException, IllegalBlockSizeException {
        SecretKey secretKey = generateSecureRandomKey();
        PublicKey publicKey = getPublicKey();
        Cipher keyCipher = Cipher.getInstance(RSA_MODE, RSA_PROVIDER);
        keyCipher.init(Cipher.ENCRYPT_MODE, publicKey);
        byte[] encryptedSecretKeyBytes = keyCipher.doFinal(secretKey.getEncoded());

        saveEncryptedSecretKey(encryptedSecretKeyBytes);

        KeyProtection keyProtection = new KeyProtection.Builder(KeyProperties.PURPOSE_DECRYPT | KeyProperties.PURPOSE_ENCRYPT | KeyProperties.PURPOSE_VERIFY)
                .setBlockModes(KeyProperties.BLOCK_MODE_GCM)
                .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_NONE)
                .build();
        mAndroidKeyStore.setEntry(KEY_ALIAS, new KeyStore.SecretKeyEntry(secretKey), keyProtection);
    }


    protected PublicKey readPublicKey() {
        DataInputStream dis = null;
        PublicKey key = null;
        try {
            dis = new DataInputStream(mContext.getResources().getAssets().open("public_key.der"));
            byte[] keyBytes = new byte[dis.available()];
            dis.readFully(keyBytes);

            X509EncodedKeySpec spec = new X509EncodedKeySpec(keyBytes);
            KeyFactory facotory = KeyFactory.getInstance(RSA);
            key = facotory.generatePublic(spec);
        } catch (Exception e) {
            key = null;
        } finally {
            if (null != dis) {
                try {
                    dis.close();
                } catch (IOException e) {
                    Log.wtf(TAG, "Cannot Close Stream!", e);
                }
            }
        }
        return key;
    }

    @NonNull
    protected SecretKey generateSecureRandomKey() {
        return new SecretKeySpec(generateSecureRandomBytes(AES_KEY_SIZE), AES);
    }

    @NonNull
    protected byte[] generateSecureRandomBytes(int byteCount) {
        byte[] keyBytes = new byte[byteCount];
        mSecureRandom.nextBytes(keyBytes);
        return keyBytes;
    }
}

我是这样测试的：

@Test
public void testCrypto() throws Exception {
    CryptoHelper crypto = new CryptoHelper(InstrumentationRegistry.getTargetContext());
    crypto.reset();
    String verySecretOpinion = "we're all doomed";
    byte[] encrypt = crypto.encrypt(verySecretOpinion);
    Assert.assertNotNull("Encrypted secret is Null!", encrypt);
    Assert.assertFalse("encrypted Bytes are the same as Input!", new String(encrypt, StandardCharsets.UTF_8).equals(verySecretOpinion));
    String decryptedString = crypto.decryptToString(encrypt);
    Assert.assertNotNull("Decrypted String must be Non-Null!", decryptedString);
    Assert.assertEquals("Decrypted String doesn't equal encryption input!", verySecretOpinion, decryptedString);
}

顺便说一句，minSdkVersion 是 25，比 Marshmallow 高

更新:

修复了 Cipher.DECRYPT_MODE 至 ENCRYPT_MODE 保存 SecretKey 的问题，感谢 James K Polk 的评论
如果我从 BlockMode GCM 切换到 BlockMode CBC （并将 GCMParameterSpec 更改为 IvParamterSpec 但松开 GCM 验证，它会起作用模式。

Answer 1

操作界面至少有两个问题。首先，您使用错误的密码模式对密钥进行 RSA 加密：您本应使用加密模式却使用了解密模式。其次，您使用的是没有任何填充的 RSA。需要使用real padding模式，推荐OEAP padding模式之一

调整用于保存结果的缓冲区大小时，加密端发生错误：

ByteBuffer buffer = ByteBuffer.allocate(IV_BIT_LEN + encryptedSecretKey.length + cryptedBytes.length);

分配太多 space。 IV_BIT_LEN 可能应该更改为 IV_SIZE 以获得正确大小的 ByteBuffer。

最后一个错误是在解密端设置GCMParameterSpec时没有考虑到GCM认证标签长度。您在这一行中初始化了标签长度

GCMParameterSpec parameterSpec = new GCMParameterSpec(IV_BIT_LEN, iv);

但这是不正确的，标签长度与 IV 无关。由于您没有在加密端明确设置 GCMParameterSpec，因此您获得了默认标签长度，恰好是 128。

您可以通过调用在加密端检索标签长度 cipher.getParameters().getParameterSpec(GCMParameterSpec.class) 获取参数规范。从这里您可以检索标签长度和 iv。您可能应该将标记长度（16 字节 = 128 位）视为硬编码常量而不传输它。接收方应该类似地假设标签长度为 128 位。

尝试在 GCM 模式下解密消息时出现 AEADBadTagException

got AEADBadTagException while trying to decrypt message in GCM mode

java

encryption

android

cryptography

android-keystore