Need Help For AES to work with UDP

[Sebastian Wong]

I need some help to make AES work with UDP. I keep getting this error.

Exception in thread "main" javax.crypto.IllegalBlockSizeException: Input length
must be multiple of 16 when decrypting with padded cipher
at com.sun.crypto.provider.CipherCore.doFinal(CipherCore.java:750)
at com.sun.crypto.provider.CipherCore.doFinal(CipherCore.java:676)
at com.sun.crypto.provider.AESCipher.engineDoFinal(AESCipher.java:313)
at javax.crypto.Cipher.doFinal(Cipher.java:2087)
at AES.decrypt(AES.java:31)
at EchoServer.main(EchoServer.java:26)


It seems that upon receiving the UDP AES encrypted message from client, it has problem decrypting due to the byte[] which is being received from UDP transmission. I have also no idea whether am I doing it the right way.
Can Somebody help and recommend a good way to solve this problem?

This is my main driver file (UDP Server):

				socket.receive( packet );
				System.out.println( "Recv: "+new Date()+" "+packet.getAddress()+":"+packet.getPort()+" "+new String(packet.getData(),0,packet.getLength()) );
				socket.send( packet );
				packet = new DatagramPacket(new byte[1024],1024);
				socket.receive( packet );
				rdata2 = new String(packet.getData(),0,packet.getLength());
				rdata = AES.decrypt(rdata2.getBytes("UTF-8"),Key); // the Line that is giving the problem. AES Driver is in another code.
				System.out.print("Received: "+ rdata);

This is my AES Driver.

import java.security.MessageDigest;
import java.util.Arrays;
import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;
import javax.crypto.spec.SecretKeySpec;
import javax.crypto.spec.IvParameterSpec;
 
import javax.crypto.Cipher;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;
 
public class AES 
{ 
    static String IV = "AAAAAAAAAAAAAAAA";

    public static byte[] encrypt(String plainText, String encryptionKey) throws Exception 
    {
        Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding", "SunJCE");
        SecretKeySpec key = new SecretKeySpec(encryptionKey.getBytes("UTF-8"), "AES");
        cipher.init(Cipher.ENCRYPT_MODE, key,new IvParameterSpec(IV.getBytes("UTF-8")));
        return cipher.doFinal(plainText.getBytes("UTF-8"));
    }

    public static String decrypt(byte[] cipherText, String encryptionKey) throws Exception
    {
        Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding", "SunJCE");
        SecretKeySpec key = new SecretKeySpec(encryptionKey.getBytes("UTF-8"), "AES");
        cipher.init(Cipher.DECRYPT_MODE, key,new IvParameterSpec(IV.getBytes("UTF-8")));
        return new String(cipher.doFinal(cipherText),"UTF-8");
    }
}

All help is very much appreciated. Thanks in advance.

[Winston Gutkowski]

Sebastian Wong wrote:I need some help to make AES work with UDP. I keep getting this error.
Exception in thread "main" javax.crypto.IllegalBlockSizeException: Input length
must be multiple of 16 when decrypting with padded cipher

Well, the error message seems pretty clear to me. Is your input length a multiple of 16?

This is my main driver file (UDP Server):...

Please DontWriteLongLines.
I'd break yours up myself, but you have LOTS of them, and it makes your thread very hard to read.
You can use the 'Edit' button to edit your post, and the 'Preview' button to view it before you save,
but please follow the guidelines in the link.

Thanks.

Winston

[Sebastian Wong]

Sorry about that. First time posting here.

What I mean is that, is there any way which I can do to ensure that the decryption input byte is always in multiples of 16? as the input size we receive from the UDP socket is always unpredictable?

[Richard Tookey]

The two lines

               rdata2 = new String(packet.getData(),0,packet.getLength());  
                rdata = AES.decrypt(rdata2.getBytes("UTF-8"),Key); // the Line that is giving the problem. AES Driver is in another code.  

together cause your problem. You should modify your decrypt method so that it takes as arguments the byte array and the number of bytes to decrypt. You can then use these directly in the cipher.doFinal() method (check the Javadoc for Cipher). You can then pass packet.getData() and packet.getLength() as the arguments.

The cipher text is binary data. It is very common for people to use String objects as containers for binary data but not all byte and byte sequences represent valid characters so as you are finding the process is not always reversible.

There is another point to consider. UDP packets can be fragment by the system and your code does not handle the fragmentation!

[Sebastian Wong]

Thanks! Your solution solved the problem.

[Richard Tookey]

Glad to know that but out of interest how are you handling the UDP package fragmentation issue?

[Sebastian Wong]

The code that I show is my template code generated from my code generator for secure network protocols for my degree project. My main priority is my code generator and so I did not touch on that part. Sorry about that and my capability in coding is also limited.

[Clément Inconnu]

Richard Tookey wrote: You should modify your decrypt method so that it takes as arguments the byte array and the number of bytes to decrypt. You can then use these directly in the cipher.doFinal() method (check the Javadoc for Cipher). You can then pass packet.getData() and packet.getLength() as the arguments.!

Thank you so much Mr Tookey.. I've been looking for that answer for two hours. I checked every single link related to UDP, AES, Symmetric encryption. EVERYTHING.
Every single thread , but everytime the same stupid stuff : some random guy posting some non-working code, no answers ...
And then I read this simple post, reminding me to read the doc ; I feel a bit mad I didn't check it earlier. I should have known. At least I can say i've learned a lot, reading all these threads. Haha.
Anyway, you'll probably never see this message. I just had to write it. Thanks a lot again.