AES / ECB / PKCS5Padding如何正确填充最后一块？

Question

I am getting this exception:

javax.crypto.BadPaddingException: Given final block not properly padded

The weird thing is that this only happens for certain strings, in a kind of unpredictable manner, so I don't know how to fix it.

Here is the full Exception:

javax.crypto.BadPaddingException: Given final block not properly padded
    at com.sun.crypto.provider.CipherCore.doFinal(CipherCore.java:966)
    at com.sun.crypto.provider.CipherCore.doFinal(CipherCore.java:824)
    at com.sun.crypto.provider.AESCipher.engineDoFinal(AESCipher.java:436)
    at javax.crypto.Cipher.doFinal(Cipher.java:2165)

And here is my code:

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Arrays;

import javax.crypto.Cipher;
import javax.crypto.spec.SecretKeySpec;

public class Crypt {

    private Crypt() {
    }

    public static String encrypt(String key, String toEncrypt) {
        byte[] toEncryptBytes = toEncrypt.getBytes();
        try {
            Cipher cipher = Cipher.getInstance("AES/ECB/PKCS5Padding");
            cipher.init(Cipher.ENCRYPT_MODE, getKey(key));
            byte[] encrypted = cipher.doFinal(toEncryptBytes);
            return new String(encrypted);
        } catch (Exception ex) {
            ex.printStackTrace();
        }
        return null;
    }

    public static String decrypt(String key, String toDecrypt) {
        try {
            Cipher cipher = Cipher.getInstance("AES/ECB/PKCS5Padding");
            cipher.init(Cipher.DECRYPT_MODE, getKey(key));
            byte[] decrypted = cipher.doFinal(toDecrypt.getBytes());
            return new String(decrypted);
        } catch (Exception ex) {
            ex.printStackTrace();
        }
        return null;
    }

    private static SecretKeySpec getKey(String str) {
        byte[] bytes = str.getBytes();
        try {
            MessageDigest sha = MessageDigest.getInstance("SHA-256");
            bytes = sha.digest(bytes);
            bytes = Arrays.copyOf(bytes, 16);
            return new SecretKeySpec(bytes, "AES");
        } catch (NoSuchAlgorithmException e) {
            e.printStackTrace();
        }
        return null;
    }

    public static final void main(String[] args) {
        String key = "KEY123";
        String toEncrypt = "0";
        boolean[] b = new boolean[128];
        for (int i = 0; i < 128; i++) {
            if (test(key, toEncrypt) == false) {
                System.err.println("ERROR with size " + toEncrypt.length());
            } else {
                b[i] = true;
            }
            toEncrypt += "0";
        }

        System.out.println("Following sizes don't work:");
        for (int i = 0; i < b.length; i++) {
            if (!b[i]) {
                System.out.println(i + 1);
            }
        }
    }

    // true = success
    public static boolean test(String key, String toEncrypt) {
        try {
            System.out.print(toEncrypt.length() + ": ");
            String encrypted = encrypt(key, toEncrypt);
            System.out.print(encrypted + "; ");

            String decrypted = decrypt(key, encrypted);
            System.out.println(decrypted);

            if (decrypted == null) {
                return false;
            }

            if (toEncrypt.equals(decrypted)) {
                return true;
            }
            return false;
        } catch (Throwable t) {
            return false;
        }
    }
}

After running this code I get the following output:

Following sizes don't work:
3
6
10
11
15
18
22
23
27
30
34
35
39
42
46
47
51
54
58
59
63
66
70
71
75
78
82
83
87
90
94
95
99
102
106
107
111
114
118
119
123
126

However, these numbers change when I change the String I want to encrypt., eg when I use "1" instead of "0".

How do I get this to work? How do I properly pad the final block?

Thanks in advance.

Answer 1

You are not feeding the decrypt function the exact output of the encrypt function.

The encryption produces a byte array. You however convert this to a String, which will interpret the byte array in some fashion (depending on the platform charset).

Either keep the byte array itself, or convert the byte array to a hex String or Base64 String. For example:

        byte[] encrypted = cipher.doFinal(toEncryptBytes);
        return Base64.getEncoder().encode(encrypted);  // Uses Java 8 java.util.Base64