javax.crypto.BadPaddingException as I try to convert hex string to byte array. Why do I get it?

Question

As I try to convert a hex string to a byte array I get this exception :

Aug 15, 2013 10:17:32 PM Tester main
SEVERE: null
javax.crypto.BadPaddingException: Given final block not properly padded
        at com.sun.crypto.provider.CipherCore.doFinal(CipherCore.java:811)
        at com.sun.crypto.provider.CipherCore.doFinal(CipherCore.java:676)
        at com.sun.crypto.provider.BlowfishCipher.engineDoFinal(BlowfishCipher.java:319)
        at javax.crypto.Cipher.doFinal(Cipher.java:1978)
        at Tester.main(Tester.java:21)

Following is the code that attempted so :

try {
        KeyGenerator keyGenerator = KeyGenerator.getInstance("Blowfish");
        SecretKey secretKey = keyGenerator.generateKey();
        Cipher cipher = Cipher.getInstance("Blowfish"); 
        cipher.init(Cipher.DECRYPT_MODE, secretKey);
        String decryptSt = new String(cipher.doFinal(DatatypeConverter.parseHexBinary("f250d7a040859d66541e2ab4a83eb2225d4fff880f7d2506")));
        System.out.println(decryptSt);
    } catch (NoSuchAlgorithmException ex) {
        Logger.getLogger(Tester.class.getName()).log(Level.SEVERE, null, ex);
    } catch (NoSuchPaddingException ex) {
        Logger.getLogger(Tester.class.getName()).log(Level.SEVERE, null, ex);
    } catch (InvalidKeyException ex) {
        Logger.getLogger(Tester.class.getName()).log(Level.SEVERE, null, ex);
    } catch (IllegalBlockSizeException ex) {
        Logger.getLogger(Tester.class.getName()).log(Level.SEVERE, null, ex);
    } catch (BadPaddingException ex) {
        Logger.getLogger(Tester.class.getName()).log(Level.SEVERE, null, ex);
    }

What is the problem ? Why am I getting an exception ?

Answer 1

You can never decrypt with a random key. If you do, you will get plaintext consisting of random bytes. The cipher however tries to unpad the message. As it does not find a valid padding, you will get this exception. Note that - by "luck", about once in 256 - the padding may be correct, in which case you simply retrieve random bytes as plaintext.