用错误的密码解密，最后的块没有正确填充

Question

我有以下代码来定义密码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);
    */

我对用不正确的密码解密密文很感兴趣，为此我定义了以下测试 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;
                            }
                    }
            }
    }
}

使用正确的密码进行加密和解密效果很好。但是，当我尝试使用不正确的密码解密时，出现以下错误：

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)

我猜这与我的 IV 有关，但我不确定如何修复它。有人有什么建议吗？

Answer 1

AES 是一种分组密码，以 16 字节为单位进行加密。填充用于在加密之前填充最后一个块，因此它适合偶数个块。

您正在指定 PKCS5Padding：

   AESCipher = Cipher.getInstance("AES/CBC/PKCS5Padding");

填充的创建方式使其可以在解密过程中被识别和删除。如果您使用错误的密钥解密，Cipher 将无法识别有效的 pad，从而给您 BadPaddingException

如果您在没有填充的情况下实例化您的解密 Cipher（因此您自己承担了该责任），您可以避免此异常。

用错误的密码解密，最后的块没有正确填充

Decrypting with the wrong password, final block not properly padded

encryption

aes

password-encryption