Decrypting with the wrong password, final block not properly padded - encryption

Question

I have the following code to define a cipher class
import java.util.*;
import javax.crypto.Cipher;
import java.security.SecureRandom;
import java.security.NoSuchAlgorithmException;
import java.security.AlgorithmParameters;
import javax.crypto.*;
import javax.crypto.SecretKeyFactory;
import javax.crypto.SecretKey;
import javax.crypto.spec.PBEKeySpec;
import javax.crypto.spec.SecretKeySpec;
import javax.crypto.spec.IvParameterSpec;
public class Cipher{
private SecureRandom rand;
private SecretKeyFactory kFact;
private Cipher AESCipher;
private SecretKey key;
public Cipher(char[] mpw, byte[] salt){
try{
kFact = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA1");
rand = SecureRandom.getInstance("SHA1PRNG");
AESCipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
PBEKeySpec spec = new PBEKeySpec(mpw, salt, 1024, 256);
key = new SecretKeySpec(kFact.generateSecret(spec).getEncoded(),"AES");
}catch(Exception e){
System.out.println("no such algorithm");
}
}
/*Henc[k,m] will return c such that Hdec[k,HEnc[k,m]] = m
*/
public ArrayList<byte[]> HEnc(byte[] message) throws Exception{
ArrayList<byte[]> res = new ArrayList<byte[]>(2);
AESCipher.init(Cipher.ENCRYPT_MODE ,key);
AlgorithmParameters params = AESCipher.getParameters();
byte[] iv = params.getParameterSpec(IvParameterSpec.class).getIV();
byte[] ctxt = AESCipher.doFinal(message);
res.add(0,iv);
res.add(1,ctxt);
return res;
}
public byte[] HDec(byte[] iv, byte[] cipher) throws Exception{
AESCipher.init(Cipher.DECRYPT_MODE, key, new IvParameterSpec(iv) );
System.out.println("decrypting");
return AESCipher.doFinal(cipher);
}
/*public abstract byte[] HDec(SecretKey k, byte[] cipher);
*/
I am interested in decrypting cipher-text with incorrect passwords, to do so i defined the following test class,
import java.util.*;
import java.io.*;
public class testCipher{
public static void main(String[] args) throws Exception{
while(true){
Scanner sc = new Scanner(System.in);
System.out.println("Enter master password");
String pass = sc.nextLine();
System.out.println("Enter incorrect password");
String fakepass = sc.nextLine();
System.out.println("Enter message to encrypt");
String message = sc.next();
String salt = "123";
HCipher goodEnc = new HCipher(pass.toCharArray(),salt.getBytes());
HCipher badEnc = new HCipher(fakepass.toCharArray(),salt.getBytes());
byte[] toEncrypt = message.getBytes();
ArrayList<byte[]> cipher = goodEnc.HEnc(toEncrypt);
byte[] ciphertxt = cipher.get(1);
byte[] iv = cipher.get(0);
while(true){
System.out.println("Enter 1 to decrypt with correct pass, 2 to decrypt with incorrect pass and 0 to end simulation");
int i = sc.nextInt();
if(i == 1){
System.out.println("Decrypting with correct password");
byte[] plaintxt = goodEnc.HDec(iv, ciphertxt);
System.out.println(new String(plaintxt));
}
if(i == 2){
System.out.println("Decrypting with incorrect password");
byte[] plaintxt = badEnc.HDec(iv, ciphertxt);
System.out.println(new String(plaintxt));
}
if(i == 0){
break;
}
}
}
}
}
Encrypting and Decrypting using the correct password works well. However, when I try to decrypt using an incorrect password, I get the follwing error:
Exception in thread "main" 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.AESCipher.engineDoFinal(AESCipher.java:420)
at javax.crypto.Cipher.doFinal(Cipher.java:1966)
at HCipher.HDec(HCipher.java:54)
at testCipher.main(testCipher.java:52)
I am guessing it has something to do with my IV but im not sure how to fix it. Does anyone have any suggestions?

Answer 1

AES is a block cipher that encrypts in blocks of 16 bytes. Padding is used to fill up your last block before encryption so it fits in an even number of blocks.
You are specifying PKCS5Padding with this:
AESCipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
The padding is created in such a way that it can be recognized and removed during decrypt. If you decrypt with the wrong key the Cipher won't be able to identify a valid pad and thus give you the BadPaddingException
If you instantiate your decrypt Cipher without padding (and thus take on that responsibility yourself) you can avoid this exception.