J2ME text encryption - javax.crypto.IllegalBlockSizeException

Question

I use DES algorithm to encrypt/decrypt my text. And it works perfect with latin texts.

But when I start encrypt/decrypt cyrillic text, decrypted one is shown as ????? ???????? in my TextField form and in console. How can i fix it?

After Joachim Sauer advice I changed inputBytes = textToEnrypt.getBytes(); to inputBytes = textToEnrypt.getBytes("UTF-8"); and now I have javax.crypto.IllegalBlockSizeException. Help me, please...

package crypting;

import javax.crypto.Cipher;
import javax.crypto.spec.SecretKeySpec;
import javax.microedition.midlet.*;

public class Encryptor extends MIDlet {

    String buffer;

    public void startApp() {
        String keyString = "testtest";
//        encrypt("Text for encrypting", keyString);
        encrypt("Привет", keyString);
        decrypt(buffer, keyString);
    }

    public void pauseApp() {
    }

    public void destroyApp(boolean unconditional) {
    }

    public void encrypt(String textToEnrypt, String keyString) {
        Cipher cipher;
        try {
            cipher = Cipher.getInstance("DES");
        } catch (Exception ex) {
            System.out.println(ex.toString());
            return;
        }

        byte[] keyData = keyString.getBytes();
        SecretKeySpec key = new SecretKeySpec(keyData, 0, keyData.length, "DES");

        try {
            cipher.init(Cipher.ENCRYPT_MODE, key);
        } catch (Exception ex) {
            System.out.println(ex.toString());
            return;
        }

        int cypheredBytes = 0;

        byte[] inputBytes;
        try {
            inputBytes = textToEnrypt.getBytes("UTF-8");
//            inputBytes = textToEnrypt.getBytes();
        } catch (Exception ex) {
            System.out.println(ex.toString());
            return;
        }

        byte[] outputBytes = new byte[100];

        try {
            cypheredBytes = cipher.doFinal(inputBytes, 0, inputBytes.length,
                    outputBytes, 0);
        } catch (Exception ex) {
            System.out.println(ex.toString());
            return;
        }

        String str = new String(outputBytes, 0, cypheredBytes);
        buffer = str;
        System.out.println("Encrypted string = " + str);
    }


    public void decrypt(String textToDecrypt, String keyString) {
        Cipher cipher;
        try {
            cipher = Cipher.getInstance("DES");
        } catch (Exception ex) {
            System.out.println(ex.toString());
            return;
        }

        byte[] keyData = keyString.getBytes();
        SecretKeySpec key = new SecretKeySpec(keyData, 0, keyData.length, "DES");

        try {
            cipher.init(Cipher.DECRYPT_MODE, key);
        } catch (Exception ex) {
            System.out.println("2. " + ex.toString());
            return;
        }

        int cypheredBytes = 0;

        byte[] inputBytes;
        try {
            inputBytes = textToDecrypt.getBytes("UTF-8");
//            inputBytes = textToDecrypt.getBytes();
        } catch (Exception ex) {
            System.out.println("3. " + ex.toString());
            return;
        }

        byte[] outputBytes = new byte[100];

        try {
            cypheredBytes = cipher.doFinal(inputBytes, 0, inputBytes.length,
                    outputBytes, 0);
        } catch (Exception ex) {
            System.out.println("4. " + ex.toString());
            return;
        }

        String str = new String(outputBytes, 0, cypheredBytes);
        System.out.println("Decrypted string = " + str);
    }
}

Answer 1

I was able to make it work with the following changes.

Change the return type of encrypt method, from void to byte[]:

static public byte[] encrypt(String textToEnrypt, String keyString)
  throws Exception
{
    //at the end
    //write this down:
    byte[] newResponse = new byte[cypheredBytes];
    for(int i=0;i < cypheredBytes;i++)
    {
        newResponse[i] = outputBytes[i];
    }
    return newResponse;
}

Instead of:

String str = new String(outputBytes, 0, cypheredBytes);
buffer = str;
System.out.println("Encrypted string = " + str);

Answer 2

My guess (and I can't do more without you showing some code) is that you're using getBytes() without a parameter and construct the String from the byte[] without a parameter as well. That means using the platform default encoding and if that can't represent cyrillic characters, then that's what you get.

It's better to use UTF-8 for both transformations, this way you can represent every Unicode character.

Answer 3

One problem is the line

String str = new String(outputBytes, 0, cypheredBytes);

in your encrypt method. You cannot use String as a container for binary data. Encrypted data should not be converted to a String unless it is a requirement, and then you need to use an appropriate codec such as base64.

Answer 4

DES uses a 64-bit (8-byte) block size. You have to make sure that the data you're encrypting is a multiple of 8 bytes, i.e. pad it out with zero bytes or whatever to ensure that it is. Otherwise you'll get an IllegalBlockSizeException. You'll need to do this after you convert the string to UTF-8, of course....