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I need to decrypt an file in Java.
The file was encrypted in C# using RijndaelManaged. This are the C# settings for the encryption (summarized and not mentioned are default value):

var crypto = new RijndaelManaged();
crypto.GenerateKey();
crypto.BlockSize = crypto.KeySize;
crypto.IV = crypto.Key;
crypto.Padding = PaddingMode.Zeros;

my java code so far, which produces an error:

java.security.InvalidAlgorithmParameterException: Wrong IV length: must be 16 bytes long:

Key aesKey = new SecretKeySpec(key, "AES");
Cipher cipher2 = Cipher.getInstance("AES/CBC/noPadding");
byte[] iv = key;
IvParameterSpec ivspec = new IvParameterSpec(iv);
cipher2.init(Cipher.DECRYPT_MODE, aesKey, new IvParameterSpec(iv));
byte[] decryptedFile = cipher2.doFinal(buildFile);

I see 2 Problems concerning the C# code:

  1. the generated Key is 256bit. Then the block size is set to to 256bit, but the standard is 128bit. As far as I know, Java is only able to use a 128bit block size, as the Error message suggests. At least I didn't find out how to set the block size.
  2. Rijndael uses PaddingMode.Zeros, whats the equivalent Padding for Cipher?

please be aware that I can't change the C# code

Is there even a way to decrypt this in Java?

Update - Solution
I finally managed to decrypt the file using BouncyCastle suggested by Topaco in the comments.
BouncyCastle includes RijndaelEngine, same as the C# part. It allows to set the block size to 256bit.

To answer my 2. question: PaddinMode.Zeros equals new ZeroBytePadding()

Full code:

import org.bouncycastle.crypto.BlockCipher;
import org.bouncycastle.crypto.CipherParameters;
import org.bouncycastle.crypto.engines.RijndaelEngine;
import org.bouncycastle.crypto.modes.CBCBlockCipher;
import org.bouncycastle.crypto.paddings.PaddedBufferedBlockCipher;
import org.bouncycastle.crypto.paddings.ZeroBytePadding;
import org.bouncycastle.crypto.params.KeyParameter;
import org.bouncycastle.crypto.params.ParametersWithIV;
...

private byte[] decryptPart(byte[] part, byte[] key) throws Exception {
    BlockCipher engine = new RijndaelEngine(key.length * 8);
    PaddedBufferedBlockCipher cipher = new PaddedBufferedBlockCipher(new CBCBlockCipher(engine),
            new ZeroBytePadding());

    KeyParameter keyParam = new KeyParameter(key);
    CipherParameters cipherParams = new ParametersWithIV(keyParam, key);
    cipher.init(false, cipherParams);
    byte[] output = new byte[cipher.getOutputSize(part.length)];
    int tam = cipher.processBytes(part, 0, part.length, output, 0);
    try {
        cipher.doFinal(output, tam);
    } catch (Exception ce) {
        ce.printStackTrace();
    }
    return output;
}
Niklas Meier
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    The C# code you provide, don't provide any information about the implementation. How do you deduct that the block size is 256 ? If the Block Size is 256 bit, then it' ain't AES. – Ebbe M. Pedersen Jul 10 '20 at 11:22
  • I assume the `GenerateKey()` generates an 256bit key. I got an example key – Niklas Meier Jul 10 '20 at 11:47
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    AES blocksize is always 128 bit. Rijndael do support blocksize of 256 bit, but then it ain't AES anymore .. – Ebbe M. Pedersen Jul 10 '20 at 11:55
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    The C# code uses Rijndael with a block size of 256 bits and no AES (AES is a subset of Rijndael, Rijndael defines different block sizes, Aes only 128 bits). The SunJCE Provider of Java does not support Rijndael with a block size of 256 bits, a third party provider must be used, e.g. BouncyCastle, [here](https://stackoverflow.com/a/8087178/9014097). – Topaco Jul 10 '20 at 12:00
  • Could you kindly provide a test dataset (plaintext, encrypted data, key, iv [seems to be same as key]) to test a Java decryption implementation ? – Michael Fehr Jul 10 '20 at 15:10
  • could solve the question. Updated my post. Thanks for the help – Niklas Meier Jul 24 '20 at 09:06

0 Answers0