Guess-the-Number  su-ctf-quals-2014

使用jd-gui 反编译jar

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) {//可求得guess_number为309137378，
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);
}
}
}

java -jar guess.jar 309137378

your flag is: a7b08c546302cc1fd2a4d48bf2bf2ddb