AES/CBC/PKCS5Padding issue

Question

I am trying to encrypt and decrypt some simple text. But for some reason I am getting a strange error: javax.crypto.BadPaddingException. Why would JCE generates bytes that are not properly padded?

I have the following code:

import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;
import javax.crypto.spec.IvParameterSpec;
import java.security.SecureRandom;

public class SimplestTest {
    public static void main(String[] args) throws Exception {
        SecureRandom rnd = new SecureRandom();

        String text = "Hello, my dear! ... " + System.getProperty("user.home");
        byte[] textData = text.getBytes();

        IvParameterSpec iv = new IvParameterSpec(rnd.generateSeed(16));

        KeyGenerator generator = KeyGenerator.getInstance("AES");
        generator.init(128);
        SecretKey k = generator.generateKey();

        Cipher c = Cipher.getInstance("AES/CBC/PKCS5Padding");
        c.init(Cipher.ENCRYPT_MODE, k, iv);
        c.update(textData);
        byte[] data = c.doFinal();

        System.out.println("E: " + data.length);

        c = Cipher.getInstance("AES/CBC/PKCS5Padding");
        c.init(Cipher.DECRYPT_MODE, k, iv);
        c.update(data);

        System.out.println("R: " + c.doFinal().length);
    }

}

But for some reason it does not work. It fails with this exception:

E: 16
Exception in thread "main" javax.crypto.BadPaddingException: Given final block not properly padded
        at com.sun.crypto.provider.SunJCE_f.b(DashoA13*..)
        at com.sun.crypto.provider.SunJCE_f.b(DashoA13*..)
        at com.sun.crypto.provider.AESCipher.engineDoFinal(DashoA13*..)
        at javax.crypto.Cipher.doFinal(DashoA13*..)
        at SimplestTest.main(SimplestTest.java:31)

What's going wrong? The data size is 16 bytes in length but is still "not properly padded" ?

Answer 1

Cipher.update returns a byte[] as well. So you are missing part of the encrypted data when you go to decrypt. Update the last section to read as follows:

Cipher c = Cipher.getInstance("AES/CBC/PKCS5Padding");
c.init(Cipher.ENCRYPT_MODE, k, iv);
byte[] someData = c.update(textData);
byte[] moreData = c.doFinal();

System.out.println("E: " + (someData.length + moreData.length));

c = Cipher.getInstance("AES/CBC/PKCS5Padding");
c.init(Cipher.DECRYPT_MODE, k, iv);
byte[] someDecrypted = c.update(someData);
byte[] moreDecrypted = c.doFinal(moreData);

System.out.println("R: " + (someDecrypted.length + moreDecrypted.length));

Answer 2

You can forgo the calls to update and just pass the byte[] data directly to doFinal performing the operation of encrypting or decrypting in one step.