Java JCE Problem

Question

I am now making a secure mail application which can encrypt plaintext mail and send to a mail server. Also, decrypt encrypted mail to plaintext. I�m using Java to develop the application.
However, I am facing the following problem:
I tried using AES of JCE to encrypt a message. As you know, the doFinal( ) will return encrypted result as a byte[ ]. Then I turned the result byte[ ] into String (The String will be treated as mail content in further development).
Next, I decrypted the String. First of all, I turned the String back to byte[ ] and decrypted it using doFinal( ). However the program flew this exception:
javax.crypto.BadPaddingException: Given final block not properly padded
If I haven�t changed the resulted byte[] into String, it can be properly decrypted.
Does anyone know how to solve it? I know that the problem may be triggered by the byte[] to String conversion.
Here is my source code:
import javax.crypto.*;
import javax.crypto.spec.*;
public class TestingAES {
   public static void main(String[] args) throws Exception {
      String message = "This is just an example";     
      System.out.println("message : " + message);
      // Get the KeyGenerator          
      KeyGenerator kgen = KeyGenerator.getInstance("AES");
      kgen.init(128);
      // Generate the secret key specs.
      SecretKey skey = kgen.generateKey();
      byte[] raw = skey.getEncoded();          
      SecretKeySpec skeySpec = new SecretKeySpec(raw, "AES");
      // Instantiate the cipher          
      Cipher cipher = Cipher.getInstance("AES");          
      cipher.init(Cipher.ENCRYPT_MODE, skeySpec);
      //get the result enrypted byte[]
      byte[] encrypted = cipher.doFinal(message.getBytes());
      //turn the result enrypted byte[] to String
      //As the result will be store in server, it must be turned to String
      String encryptedString = new String(encrypted);
      //turn the String into byte[] and decrypt it
      byte[] encryptedByte = encryptedString.getBytes();
      cipher.init(Cipher.DECRYPT_MODE, skeySpec);
      byte[] plainByte = cipher.doFinal(encryptedByte);
      String plainText = new String(plainByte);
      System.out.println("plainText : " + plainText);
}

Answer 1

" I know that the problem may be triggered by the byte[] to String conversion."
That is correct, so one answer is to use an encoding (not encrypting) algorithm. The most common encodings I know of for crypto are Base64, ASCIII armor ala PGP, and ASCII hexadecimal. There is a Base64 encoder in Java, but it is not officially supported. However, lots of folks use it. You need to import the sun.misc.BASE64Encoder package,. For example in your code you have the lines
     //get the result enrypted byte[]
      byte[] encrypted = cipher.doFinal(message.getBytes());
      //turn the result enrypted byte[] to String
      //As the result will be store in server, it must be turned to String
      String encryptedString = new String(encrypted);these would become instead
import sun.misc.BASE64Encoder;
import sun.misc.BASE64Decoder;
     //get the result enrypted byte[]
      byte[] encrypted = cipher.doFinal(message.getBytes());
      //turn the result enrypted byte[] to String
      //As the result will be store in server, it must be turned to String
      String encryptedString = new String(new BASE64Encoder().encodeBuffer(encrypted));and then you must reverse the process to decrypt., i.e. you decryption code becomes
      //turn the String into byte[] and decrypt it
      byte[] encryptedByte = new BASE64Decoder().decodeBuffer(encryptedString).getBytes();