BUUCTF-RE-JAVA逆向解密

一、

.class文件用jd-gui打开

二、分析

import java.io.PrintStream;
import java.util.ArrayList;
import java.util.Scanner;

public class Reverse
{
  public static void main(String[] args)
  {
    Scanner s = new Scanner(System.in);
    System.out.println("Please input the flag ：");
    String str = s.next();
    System.out.println("Your input is ：");
    System.out.println(str);
    char[] stringArr = str.toCharArray();
    Encrypt(stringArr);
  }
  
  public static void Encrypt(char[] arr)
  {
    ArrayList<Integer> Resultlist = new ArrayList();
    for (int i = 0; i < arr.length; i++)
    {
      int result = arr[i] + '@' ^ 0x20;
      Resultlist.add(Integer.valueOf(result));
    }
    int[] KEY = { 180, 136, 137, 147, 191, 137, 147, 191, 148, 136, 133, 191, 134, 140, 129, 135, 191, 65 };
    ArrayList<Integer> KEYList = new ArrayList();
    for (int j = 0; j < KEY.length; j++) {
      KEYList.add(Integer.valueOf(KEY[j]));
    }
    System.out.println("Result:");
    if (Resultlist.equals(KEYList)) {
      System.out.println("Congratulations！");
    } else {
      System.err.println("Error！");
    }
  }
}

第23行flag做了    arr[i]+'@' ^ 0x20;

第26行可以看到     int[] KEY = { 180, 136, 137, 147, 191, 137, 147, 191, 148, 136, 133, 191, 134, 140, 129, 135, 191, 65 };

key = [180, 136, 137, 147, 191, 137, 147, 191, 148, 136, 133, 191, 134, 140, 129, 135, 191, 65]
flag = ''
for i in key:
     flag+=chr(i-ord('@')^0x20)
print(flag)

*    chr是将ascii码转字符  ord是将字符转ascii码

三、flag

flag{This_is_the_flag_!}