How to use Bouncy Castle lightweight API with AES and PBE

I have a block of ciphertext that was created using the JCE algorithim “PBEWithSHA256And256BitAES-CBC-BC”. The provider is BouncyCastle. What I’d like to do it decrypt this ciphertext using the BouncyCastle lightweight API. I don’t want to use JCE because that requires installing the Unlimited Strength Jurisdiction Policy Files.

Documentation seems to be thin on the ground when it comes to using BC with PBE and AES.

Here’s what I have so far. The decryption code runs without exception but returns rubbish.

The encryption code,

String password = "qwerty";
String plainText = "hello world";

byte[] salt = generateSalt();
byte[] cipherText = encrypt(plainText, password.toCharArray(), salt);

private static byte[] generateSalt() throws NoSuchAlgorithmException {
    byte salt[] = new byte[8];
    SecureRandom saltGen = SecureRandom.getInstance("SHA1PRNG");
    saltGen.nextBytes(salt);
    return salt;
}

private static byte[] encrypt(String plainText, char[] password, byte[] salt) throws NoSuchAlgorithmException, InvalidKeySpecException, NoSuchPaddingException, InvalidKeyException, InvalidAlgorithmParameterException, IllegalBlockSizeException, BadPaddingException {
    Security.addProvider(new BouncyCastleProvider());

    PBEParameterSpec pbeParamSpec = new PBEParameterSpec(salt, 20);

    PBEKeySpec pbeKeySpec = new PBEKeySpec(password);
    SecretKeyFactory keyFac = SecretKeyFactory.getInstance("PBEWithSHA256And256BitAES-CBC-BC");
    SecretKey pbeKey = keyFac.generateSecret(pbeKeySpec);

    Cipher encryptionCipher = Cipher.getInstance("PBEWithSHA256And256BitAES-CBC-BC");
    encryptionCipher.init(Cipher.ENCRYPT_MODE, pbeKey, pbeParamSpec);

    return encryptionCipher.doFinal(plainText.getBytes());
}

The decryption code,

byte[] decryptedText = decrypt(cipherText, password.getBytes(), salt);

private static byte[] decrypt(byte[] cipherText, byte[] password, byte[] salt) throws DataLengthException, IllegalStateException, InvalidCipherTextException, InvalidKeyException, NoSuchAlgorithmException, NoSuchPaddingException, IllegalBlockSizeException, BadPaddingException {
    BlockCipher engine = new AESEngine();
    CBCBlockCipher cipher = new CBCBlockCipher(engine);

    PKCS5S1ParametersGenerator keyGenerator = new PKCS5S1ParametersGenerator(new SHA256Digest());
    keyGenerator.init(password, salt, 20);

    CipherParameters keyParams = keyGenerator.generateDerivedParameters(256);
    cipher.init(false, keyParams);

    byte[] decryptedBytes = new byte[cipherText.length];
    int numBytesCopied = cipher.processBlock(cipherText, 0, decryptedBytes, 0);

    return decryptedBytes;
}

Answer

I tried this and it seemed to work. Borrowed heavily from the BC class org.bouncycastle.jce.provider.test.PBETest

private byte[] decryptWithLWCrypto(byte[] cipher, String password, byte[] salt, final  int iterationCount)
        throws Exception
{
    PKCS12ParametersGenerator pGen = new PKCS12ParametersGenerator(new SHA256Digest());
    char[] passwordChars = password.toCharArray();
    final byte[] pkcs12PasswordBytes = PBEParametersGenerator
            .PKCS12PasswordToBytes(passwordChars);
    pGen.init(pkcs12PasswordBytes, salt, iterationCount);
    CBCBlockCipher aesCBC = new CBCBlockCipher(new AESEngine());
    ParametersWithIV aesCBCParams = (ParametersWithIV) pGen.generateDerivedParameters(256, 128);
    aesCBC.init(false, aesCBCParams);
    PaddedBufferedBlockCipher aesCipher = new PaddedBufferedBlockCipher(aesCBC,
            new PKCS7Padding());
    byte[] plainTemp = new byte[aesCipher.getOutputSize(cipher.length)];
    int offset = aesCipher.processBytes(cipher, 0, cipher.length, plainTemp, 0);
    int last = aesCipher.doFinal(plainTemp, offset);
    final byte[] plain = new byte[offset + last];
    System.arraycopy(plainTemp, 0, plain, 0, plain.length);
    return plain;
}

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