Simple text encyption - why BadPaddingException?

Question

Hi
I'm trying out how to encrypt and descrypt a text. The key must be the same since I decrypt in another program. With the program below, I get this exception:
javax.crypto.BadPaddingException: Given final block not properly padded
     at com.sun.crypto.provider.DESedeCipher.engineDoFinal(DashoA6275)
     at com.sun.crypto.provider.DESedeCipher.engineDoFinal(DashoA6275)
     at javax.crypto.Cipher.doFinal(DashoA6275)
     at endecryptor.main(endecryptor.java:49)
What's the problem here?
import java.security.*;
import javax.crypto.*;
import java.io.*;
public class endecryptor {
public static void main(String[] args) throws Exception {
String text = "test eins";
System.out.println("Generating a DESede (TripleDES) key ... ");
//add the provider
Provider sunJce = new com.sun.crypto.provider.SunJCE();
Security.addProvider(sunJce);
//create a triple DES key
KeyGenerator keyGenerator = KeyGenerator.getInstance("DESede");
keyGenerator.init(168); //initialize with the keysize
Key key = keyGenerator.generateKey();
System.out.println("Key Algorithm :" + key.getAlgorithm());
System.out.println("Key Algorithm :" + key);
System.out.println("Done generating the key.");
//create a cipher using a key to initialize it
Cipher cipher = Cipher.getInstance("DESede/ECB/PKCS5Padding");//DES/ECB/PKCS5Padding
cipher.init(Cipher.ENCRYPT_MODE, key); ;
byte[] plaintext = text.getBytes("UTF8");
byte[] ciphertext = cipher.doFinal(plaintext);
System.out.println("Decrypting the string ...");
sunJce = new com.sun.crypto.provider.SunJCE();
Security.addProvider(sunJce);
//create a triple DES key
keyGenerator = KeyGenerator.getInstance("DESede");
keyGenerator.init(168); //initialize with the keysize
key = keyGenerator.generateKey();
System.out.println("Key Algorithm :" + key.getAlgorithm());
System.out.println("Key Algorithm :" + key);
System.out.println("Done generating the key.");
//create a cipher using a key to initialize it
cipher = Cipher.getInstance("DESede/ECB/PKCS5Padding");
cipher.init(Cipher.DECRYPT_MODE, key);
--> exception is thrown here -->byte[] decryptedText = cipher.doFinal(ciphertext);
String output = new String(decryptedText, "UTF8");
System.out.println("\n\nDecrypted Text:" + output);
}

Answer 1

Your problem is that you use one key to encrypt, and then create a completely different key to decrypt.
Ciphers are just functions that take in a bitstream, operate on it using a sequence of bytes we call a key, and spit out another bitstream. There's no way for the cipher to know that you're using the wrong key. It's often the case that a cipher will be perfectly happy decrypting with an incorrect key, handing you back utter gibberish.
Padding, however, is something that a cipher can recognize, and know when it isn't seeing what it expects at the end of the plaintext. So it is not uncommon, when using an incorrect key to decipher, to get a BadPaddingException, since the key-problem causes gibberish in the pad-bytes.
But the net is, you need to share one generated key - not generate two different ones.
Good luck,
Grant