【Android 逆向】【攻防世界】人民的名义-抓捕赵德汉1-200

1. 这一题下载下来是个jar文件，感觉很android关系不大，但还是放在了mobile这个分类下了

2. 直接java jar运行，提示需要输入密码

# java -jar 169e139f152e45d5ae634223fe53e6be.jar 
Enter password:
1234
Incorrect password
Enter password:

3. jadx 打开jar文件

public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException, IOException, NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException, IllegalBlockSizeException, BadPaddingException {
        CheckInterface checkerObject = loadCheckerObject();
        BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in));
        while (true) {
            System.out.println("Enter password:");
            String line = stdin.readLine();
            if (checkerObject.checkPassword(line)) {
                System.out.println("Well done, that is the correct password");
                System.exit(0);
            } else {
                System.out.println("Incorrect password");
            }
        }
    }

    private static CheckInterface loadCheckerObject() throws IOException, NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException, IllegalBlockSizeException, BadPaddingException, ClassFormatError, InstantiationException, IllegalAccessException {
        CheckPassword mycl = new CheckPassword();
        InputStream in = CheckPassword.class.getClass().getResourceAsStream("/ClassEnc");
        ByteArrayOutputStream bout = new ByteArrayOutputStream();
        byte[] bytes = new byte[512];
        while (true) {
            int len = in.read(bytes);
            if (len > -1) {
                bout.write(bytes, 0, len);
            } else {
                byte[] myClassBytesEnc = bout.toByteArray();
                in.close();
                SecretKeySpec secretKeySpec = new SecretKeySpec(hexStringToByteArray(hexKey), "AES");
                Cipher decAEScipher = Cipher.getInstance("AES");
                decAEScipher.init(2, secretKeySpec);
                byte[] myClassBytes = decAEScipher.doFinal(myClassBytesEnc);
                CheckInterface passCheckObject = (CheckInterface) mycl.defineClass(null, myClassBytes, 0, myClassBytes.length).newInstance();
                return passCheckObject;
            }
        }
    }

从这里可以看出，checkerObject 对象是通过classloader加载外部文件来获得的，外部文件还需要通过AES进行解密，那么我们只需要获得这个解密后的文件，就可以查看他的逻辑了

4 将代码复制出来，放到IDEA里运行

import java.io.*;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.NoSuchPaddingException;
import javax.crypto.spec.SecretKeySpec;

/* renamed from: CheckPassword  reason: default package */
/* loaded from: 169e139f152e45d5ae634223fe53e6be.jar:CheckPassword.class */
public class CheckPassword extends ClassLoader {

    static String hexKey = "bb27630cf264f8567d185008c10c3f96";

    public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException, IOException, NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException, IllegalBlockSizeException, BadPaddingException {
        CheckInterface checkerObject = loadCheckerObject();
        BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in));
        while (true) {
            System.out.println("Enter password:");
            String line = stdin.readLine();
            if (checkerObject.checkPassword(line)) {
                System.out.println("Well done, that is the correct password");
                System.exit(0);
            } else {
                System.out.println("Incorrect password");
            }
        }
    }

    private static CheckInterface loadCheckerObject() throws IOException, NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException, IllegalBlockSizeException, BadPaddingException, ClassFormatError, InstantiationException, IllegalAccessException {
        CheckPassword mycl = new CheckPassword();
        InputStream in = CheckPassword.class.getClass().getResourceAsStream("/ClassEnc");
        ByteArrayOutputStream bout = new ByteArrayOutputStream();
        byte[] bytes = new byte[512];
        while (true) {
            int len = in.read(bytes);
            if (len > -1) {
                bout.write(bytes, 0, len);
            } else {
                byte[] myClassBytesEnc = bout.toByteArray();
                in.close();
                SecretKeySpec secretKeySpec = new SecretKeySpec(hexStringToByteArray(hexKey), "AES");
                Cipher decAEScipher = Cipher.getInstance("AES");
                decAEScipher.init(2, secretKeySpec);
                byte[] myClassBytes = decAEScipher.doFinal(myClassBytesEnc);
                writeToFile(myClassBytes);
                CheckInterface passCheckObject = (CheckInterface) mycl.defineClass(null, myClassBytes, 0, myClassBytes.length).newInstance();
                return passCheckObject;
            }
        }
    }

    private static void writeToFile(byte[] bytes) throws IOException {
        Files.write(Paths.get("./file.class"), bytes);
    }

    private static byte[] hexStringToByteArray(String s) {
        int len = s.length();
        byte[] data = new byte[len / 2];
        for (int i = 0; i < len; i += 2) {
            data[i / 2] = (byte) ((Character.digit(s.charAt(i), 16) << 4) + Character.digit(s.charAt(i + 1), 16));
        }
        return data;
    }
}

直接得到file.class, 使用IDEA打开会自动反编译成源代码，获得了MD5后的字符串fa3733c647dca53a66cf8df953c2d539
md5解密工具 https://www.cmd5.com/ 中查询得到flag monkey99