Buuoj Java逆向解密

1. 下载压缩包，解压得到Reverse.class，用IDEA打开，得到以下源码

import java.util.ArrayList;
import java.util.Scanner;

public class Reverse {
    public 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] + 64 ^ 32;
            Resultlist.add(result);
        }

        int[] KEY = new int[]{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(KEY[j]);
        }

        System.out.println("Result:");
        if (Resultlist.equals(KEYList)) {
            System.out.println("Congratulations！");
        } else {
            System.err.println("Error！");
        }

    }
}

2. 大致的意思是：输入的字符串flag会被转成字符串列表，然后对每个元素进行加密。加密的方法是：每个元素加上64后再与32异或。加密完转换成整型需与KEY各元素相等。所以对KEY中各元素异或32后减去64再转char即可得到flag。

3. 解密代码如下：

public class JavaReverseDecypto {
    public static void main(String[] args) {
        int[] KEY = new int[]{180, 136, 137, 147, 191, 137, 147, 191, 148, 136, 133, 191, 134, 140, 129, 135, 191, 65};
        int len = KEY.length;

        char[] flag = new char[len];
        for (int i = 0; i < len; ++i) {
            flag[i] = (char) ((KEY[i] ^ 32) - 64);
            System.out.print(flag[i]);
        }
    }
}

4. 输出结果为This_is_the_flag_!