BadPaddingException in AES

[Nitin Dubey]

Guys, I am new to crypto world. I need to encrypt some numbers to a String and then from String back to the same number. Hence, I grabbed the following code from net did some modifications and trying to run but this is throwing exception.

public class StringEncryptDecryptAES {
 
// Get the KeyGenerator
static KeyGenerator kgen; 
 
// Generate the secret key specs.
static SecretKey skey;
 
static SecretKeySpec skeySpec;
 
 // Instantiate the cipher
static Cipher cipher;
 
 
static{
 try {
kgen = KeyGenerator.getInstance("AES");
kgen.init(128); // 192 and 256 bits may not be available
 
skey = kgen.generateKey();
byte[] raw = skey.getEncoded();
skeySpec = new SecretKeySpec(raw, "AES");
 
cipher = Cipher.getInstance("AES");
} catch (NoSuchAlgorithmException e) {
e.printStackTrace();
} catch (NoSuchPaddingException e) {
e.printStackTrace();
}
 }
 
 
/**
     * Turns array of bytes into string
     *
     * @param bufArray of bytes to convert to hex string
     * @returnGenerated hex string
     */
     public static String asHex (byte buf[]) {
      StringBuffer strbuf = new StringBuffer(buf.length * 2);
      int i;
 
      for (i = 0; i < buf.length; i++) {
       if (((int) buf[i] & 0xff) < 0x10)
    strbuf.append("0");
 
       strbuf.append(Long.toString((int) buf[i] & 0xff, 16));
      }
 
      return strbuf.toString();
     }
 
     /**
     * @param args
     * @throws Exception
     */
    public static void main(String[] args) throws Exception {
 
    System.out.println("Number\t\tEncrypted\t\tDecrypted");
    String enc;
    String dec;
    String str;
    
// The commented code runs properly for what reason I don't know.
/*       for(int counter = 0; counter <= 10; counter++){
       str = "" + counter;
       enc = encryptAES(str);
       dec = decryptAES(enc);
       
       if(counter % 100000 == 0)
       System.out.println(counter);
       
       System.out.println(counter + "\t\t" + enc + "\t\t" + dec);
 
       if(!dec.equals(str)){
           System.out.println(counter + "\t\t" + enc + "\t\t" + dec);
       System.out.println("ERROR");
       System.exit(0);
       }
       }
*/    
// The two encrypted strings I am using in main method are encrypted "0" and I was trying to decrypt both of them back to "0".
 
       String str1 = "f2677f13a98e83d1543df8a56520f376";
    String decrypted = decryptAES(str1);
    System.out.println("Decrypted: " + decrypted);
 
       decrypted = decryptAES("565a581fe3841b6990d62d05a01791e6");
       System.out.println("Decrypted: " + decrypted);
       
     }
 
    
     public static String encryptAES(String str){
 
    byte[] encrypted = new byte[0];

try {
cipher.init(Cipher.ENCRYPT_MODE, skeySpec);
encrypted = cipher.doFinal(str.getBytes());
} catch (InvalidKeyException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IllegalBlockSizeException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (BadPaddingException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
         
         return asHex(encrypted);
     }
 
     public static String decryptAES(String encrypted){
byte []original = null;

     try {
 cipher.init(Cipher.DECRYPT_MODE, skeySpec);
 byte [] arr = toBinArray(encrypted);
 original = cipher.doFinal(arr);
} catch (InvalidKeyException e) {
e.printStackTrace();
} catch (IllegalBlockSizeException e) {
e.printStackTrace();
} catch (BadPaddingException e) {
e.printStackTrace();
}
 
return new String(original);
     }
 
     public static byte[] toBinArray(String hexStr) {
byte bArray[] = new byte[hexStr.length() / 2];
for (int i = 0; i < (hexStr.length() / 2); i++) {
byte firstNibble = Byte.parseByte(hexStr
.substring(2 * i, 2 * i + 1), 16); // [x,y)
byte secondNibble = Byte.parseByte(hexStr.substring(2 * i + 1,
2 * i + 2), 16);
int finalByte = (secondNibble) | (firstNibble << 4); // bit-operations only with numbers, not bytes.
bArray[i] = (byte) finalByte;
}
return bArray;
}
 
}
    

[Ulf Dittmer]

In which line does the exception occur?

[Joe Ess]

That code looks familiar. I just worked through a similar example. It turns out that you get the "bad padding exception" for one of two reasons. One, if your data is not the correct size for the encryption algorithm (this is fixed by "padding" the data to get the correct size). The other reason is if you attempt to use different keys to encrypt and decrypt the data.
Notice that your code generates a new key in the static block each time it runs. This is why the commented code works. It is using the same instance of skeySpec to encrypt and decrypt. If you take the encoded data from one run and try to decrypt it in another, this code generates a new skeySpec and you get an exception.

[Nitin Dubey]

Thanks for the help Joe. I will be using this in a web application, I am planning to put this in startup and serialize the seckey so that it can be retrieved again. This is coz in our application the users will have the URL's all the time even if the server is down.

I hope this will work.

[Aryan Khan]

Do not use Strings...You will run into trouble even if it works for some encrypted messages.
Always use base46 encoding.
Encode your string into base64 encrypt it, decrypt it and decode to string.

Aryan