catflag-easy-java

拿到题目使用jad把reverse.class编译成reverse.java

jad -o -r -s java -d ./ ./Reverse.class

用jd-gui打开java源文件

// Decompiled by Jad v1.5.8g. Copyright 2001 Pavel Kouznetsov.
// Jad home page: http://www.kpdus.com/jad.html
// Decompiler options: packimports(3) 
// Source File Name:   Reverse.java

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

public class Reverse
{

    public Reverse()
    {
    }

    public static void main(String args[])
    {
        Scanner scanner = new Scanner(System.in);
        System.out.println("Please input the flag uFF1A");
        String s = scanner.next();
        System.out.println("Your input is uFF1A");
        System.out.println(s);
        char ac[] = s.toCharArray();
        Encrypt(ac);
    }

    public static void Encrypt(char ac[])
    {
        ArrayList arraylist = new ArrayList();
        for(int i = 0; i < ac.length; i++)
        {
            int j = ac[i] + 64 ^ 0x20;
            arraylist.add(Integer.valueOf(j));
        }

        int ai[] = {
            131, 129, 148, 134, 140, 129, 135, 155, 138, 129, 
            150, 129, 191, 133, 129, 147, 153, 191, 133, 129, 
            147, 153, 157
        };
        ArrayList arraylist1 = new ArrayList();
        for(int k = 0; k < ai.length; k++)
            arraylist1.add(Integer.valueOf(ai[k]));

        System.out.println("Result:");
        if(arraylist.equals(arraylist1))
            System.out.println("CongratulationsuFF01");
        else
            System.err.println("ErroruFF01");
    }
}

我们查看关键代码

for(int i = 0; i < ac.length; i++)
        {
            int j = ac[i] + 64 ^ 0x20;
            arraylist.add(Integer.valueOf(j));
        }

        int ai[] = {
            131, 129, 148, 134, 140, 129, 135, 155, 138, 129, 
            150, 129, 191, 133, 129, 147, 153, 191, 133, 129, 
            147, 153, 157
        };

这里是一个简单的加密函数，我们反向操作即可

a = [131, 129, 148, 134, 140, 129, 135, 155, 138, 129,
            150, 129, 191, 133, 129, 147, 153, 191, 133, 129,
            147, 153, 157]
for i in a:
    s = i - 64 ^ 0x20
    print(chr(s),end='')

catflag{java_easy_easy}