Android Cipher AES/GCM/NoPadding "iv==null"

Question

Trying to encrypt and decrypt some data, on Marshmallow. Encryption works well, but when trying to decrypt, I'm getting a RuntimeException, saying "iv == null".

Basically, cipher.getIV() and cipher.getParameters() returns null. Am I missing something?

  private static final String ALGORITHM_NAME = "AES/GCM/NoPadding";
  private static final int IV_SIZE = 128;
  private static final int ALGORITHM_SIZE = 256;

  private static final String KEYSTORE_PROVIDER = "AndroidKeyStore";
  private static final String SYMMETRIC_ALIAS = "secret_key";

  private static SecretKey getSymmetricKey()
      throws NoSuchProviderException, NoSuchAlgorithmException, InvalidAlgorithmParameterException, IOException,
      CertificateException, KeyStoreException, UnrecoverableEntryException {

    KeyStore ks = KeyStore.getInstance(KEYSTORE_PROVIDER);
    ks.load(null);

    return (SecretKey) ks.getKey(SYMMETRIC_ALIAS, null);
  }

  private static SecretKey createSecretKey()
      throws NoSuchAlgorithmException, NoSuchProviderException, InvalidAlgorithmParameterException {

    KeyGenerator keyGenerator = KeyGenerator.getInstance(
        KeyProperties.KEY_ALGORITHM_AES, KEYSTORE_PROVIDER);

    keyGenerator.init(new KeyGenParameterSpec.Builder(SYMMETRIC_ALIAS,
        KeyProperties.PURPOSE_ENCRYPT | KeyProperties.PURPOSE_DECRYPT)
        .setBlockModes(KeyProperties.BLOCK_MODE_GCM)
        .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_NONE)
        .setKeySize(ALGORITHM_SIZE)
        .setRandomizedEncryptionRequired(true)
        .setUserAuthenticationRequired(false)
        .build());

    return keyGenerator.generateKey();
  }

  public static byte[] encrypt(byte[] data)
      throws CertificateException, NoSuchAlgorithmException, IOException, UnrecoverableEntryException,
      KeyStoreException, NoSuchProviderException, InvalidAlgorithmParameterException, NoSuchPaddingException,
      InvalidKeyException, BadPaddingException, IllegalBlockSizeException {

    SecretKey secretKey = getSymmetricKey();
    if (secretKey == null) {
      secretKey = createSecretKey();
    }

    Cipher cipher = Cipher.getInstance(ALGORITHM_NAME);
    cipher.init(Cipher.ENCRYPT_MODE, secretKey);
    return cipher.doFinal(data);
  }

  public static byte[] decrypt(byte[] data)
      throws CertificateException, NoSuchAlgorithmException, IOException, UnrecoverableEntryException,
      KeyStoreException, NoSuchProviderException, InvalidAlgorithmParameterException, NoSuchPaddingException,
      InvalidKeyException, BadPaddingException, IllegalBlockSizeException {

    SecretKey secretKey = getSymmetricKey();

    Cipher cipher = Cipher.getInstance(ALGORITHM_NAME);
    GCMParameterSpec spec = new GCMParameterSpec(IV_SIZE, cipher.getIV());
    cipher.init(Cipher.DECRYPT_MODE, secretKey, spec);
    return cipher.doFinal(data.getBytes());
  }

Answer 1

You must to provide a non-null IV to AES/GCM/NoPadding Cipher.init when decrypting. However, your code provides a null IV, obtained by invoking Cipher.getIV() on an uninitialized Cipher instance.

The fix is to save the IV used when encrypting, and then use that same saved IV when decrypting.