攻防世界进阶区-Guess-the-Number-writeup

1. 介绍

本题是xctf攻防世界中Reverse的进阶区的题Guess-the-Number
题目来源： su-ctf-quals-2014
题目描述：猜个数字然后找到flag.

2. 分析

文件下载下来是一个jar包。在 http://java-decompiler.github.io/ 下载jd-gui，下载下来双击运行即可，免安装。

代码如下，读取用户输入的数字保存到guess_number，只有当my_number / 5 == guess_number时，才会进入打印正确信息的if块，跟进以后发现if块中的内容和guess_number毫无关系，只是用来计算flag的。。因此最终guess_number应该为：my_number / 5 = 309137378 ，命令行执行下面的命令，得到flag: a7b08c546302cc1fd2a4d48bf2bf2ddb

$ java -jar 6a8bee79ee404194bb8a213a8a0e211d.jar 309137378
your flag is: a7b08c546302cc1fd2a4d48bf2bf2ddb

import java.math.BigInteger;
public class guess {
    static String XOR(String _str_one, String _str_two) {
    BigInteger i1 = new BigInteger(_str_one, 16);BigInteger i2 = new BigInteger(_str_two, 16);BigInteger res = i1.xor(i2);String result = res.toString(16);return result;}public static void main(String[] args) {
    int guess_number = 0;int my_num = 349763335;int my_number = 1545686892;int flag = 345736730;if (args.length > 0) {
    try {
    guess_number = Integer.parseInt(args[0]);if (my_number / 5 == guess_number) {
    String str_one = "4b64ca12ace755516c178f72d05d7061";String str_two = "ecd44646cfe5994ebeb35bf922e25dba";my_num += flag;String answer = XOR(str_one, str_two);System.out.println("your flag is: " + answer);} else {
    System.err.println("wrong guess!");System.exit(1);} } catch (NumberFormatException e) {
    System.err.println("please enter an integer \nexample: java -jar guess 12");System.exit(1);} } else {
    System.err.println("wrong guess!");int num = 1000000;num++;System.exit(1);} }
}