C# / Java | AES256 encrypt/decrypt

Question

I want to encrypt all the data I send through the Java/C# sockets (Java server, C# client). I would like to use AES256, but I can't get the Java and C# to generate the same encrypted code. Can anyone give me two examples, 1 in Java and 1 in C# that generate the same results and decrypts the results properly?

What I tried so far:

public Encrypt(AOBCore instance){
    try {
        String message="This is just an example";

           // Get the KeyGenerator

           KeyGenerator kgen = KeyGenerator.getInstance("AES");
           kgen.init(256); // 192 and 256 bits may not be available


           // Generate the secret key specs.
           SecretKey skey = kgen.generateKey(); //Cantget 'test' in here...
           byte[] raw = skey.getEncoded();

           SecretKeySpec skeySpec = new SecretKeySpec(raw, "AES");


           // Instantiate the cipher

           Cipher cipher = Cipher.getInstance("AES");

           cipher.init(Cipher.ENCRYPT_MODE, skeySpec);

           byte[] encrypted =
             cipher.doFinal(message.getBytes());
           System.out.println("encrypted string: " + asHex(encrypted));

           cipher.init(Cipher.DECRYPT_MODE, skeySpec);
           byte[] original =
             cipher.doFinal(encrypted);
           String originalString = new String(original);
           System.out.println("Original string: " +
             originalString + " " + asHex(original));
    } catch (Exception e) {
        instance.logMessage(e.getMessage());
    }
}

public static String asHex (byte buf[]) {
      StringBuffer strbuf = new StringBuffer(buf.length * 2);
      int i;

      for (i = 0; i < buf.length; i++) {
       if (((int) buf[i] & 0xff) < 0x10)
        strbuf.append("0");

       strbuf.append(Long.toString((int) buf[i] & 0xff, 16));
      }

      return strbuf.toString();
     }

}

static void Main(string[] args)
    {
        while (true)
        {
            var plain = Console.ReadLine();
            var key = GenerateKey(256);
            var encoded = Encrypt(plain, key, 256);
            Console.WriteLine("Encoded: " + encoded);
            Console.WriteLine(Decrypt(encoded, key, 256));
        }
    }

    private static string GenerateKey(int keySize)
    {
        return "test";
    }

    private static string Encrypt(string plainStr, string completeEncodedKey, int keySize)
    {
        RijndaelManaged aesEncryption = new RijndaelManaged();
        aesEncryption.KeySize = keySize;
        aesEncryption.BlockSize = 256;
        aesEncryption.Mode = CipherMode.CBC;
        aesEncryption.Padding = PaddingMode.PKCS7;
        aesEncryption.IV = Convert.FromBase64String(ASCIIEncoding.UTF8.GetString(Convert.FromBase64String(completeEncodedKey)).Split(',')[0]);
        aesEncryption.Key = Convert.FromBase64String(ASCIIEncoding.UTF8.GetString(Convert.FromBase64String(completeEncodedKey)).Split(',')[1]);
        byte[] plainText = ASCIIEncoding.UTF8.GetBytes(plainStr);
        ICryptoTransform crypto = aesEncryption.CreateEncryptor();
        // The result of the encryption and decryption            
        byte[] cipherText = crypto.TransformFinalBlock(plainText, 0, plainText.Length);
        return Convert.ToBase64String(cipherText);
    }

    private static string Decrypt(string encryptedText, string completeEncodedKey, int keySize)
    {
        RijndaelManaged aesEncryption = new RijndaelManaged();
        aesEncryption.KeySize = keySize;
        aesEncryption.BlockSize = 128;
        aesEncryption.Mode = CipherMode.CBC;
        aesEncryption.Padding = PaddingMode.PKCS7;
        aesEncryption.IV = Convert.FromBase64String(ASCIIEncoding.UTF8.GetString(Convert.FromBase64String(completeEncodedKey)).Split(',')[0]);
        aesEncryption.Key = Convert.FromBase64String(ASCIIEncoding.UTF8.GetString(Convert.FromBase64String(completeEncodedKey)).Split(',')[1]);
        ICryptoTransform decrypto = aesEncryption.CreateDecryptor();
        byte[] encryptedBytes = Convert.FromBase64CharArray(encryptedText.ToCharArray(), 0, encryptedText.Length);
        return ASCIIEncoding.UTF8.GetString(decrypto.TransformFinalBlock(encryptedBytes, 0, encryptedBytes.Length));
    }

You should show what you have tried so far - this is not a code factory and the Internet provides tons of examples. — home, Mar 06 '12 at 13:01
Added what I tried... And no, I can't find alot of info on C#/Java examples. — Basaa, Mar 06 '12 at 13:05
Looks like you have the same problem http://stackoverflow.com/q/5295110/55209 — Artem Koshelev, Mar 06 '12 at 13:14
Indeed. with that code I get uZ▀pE÷R~JÙ‗(tÄÒ$f6~∟\¸¸(À}5ÒZêL♣ with plain testtesttesttest and pass testtesttesttest. That's not right I guess :P — Basaa, Mar 06 '12 at 13:28

score 5 · Accepted Answer · edited May 23 '17 at 12:06

The problem is that you aren't specifying the ciphermode or the padding in the Java code. This will use the algorithm defaults, which is never something you want to do when interoperability with other libraries is required. Initialize your Cipher like this:

Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");

PKCS5 in Java should be compatible with PKCS7 in .Net according to this answer. Since you are wisely using CBC you are going to need to modify the code to use the same initialization vector for both encryption and decryption. You should NOT use the secret key for that. The IV should be randomly generated. You can use the IV that the Java Cipher generated for encryption by calling cipher.getIV().

Also, take care to be consistent with character encodings as has been mentioned in the comments.

C# / Java | AES256 encrypt/decrypt

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