javatar

CTCTF 2015: Javatar

Problem

Points: 40

Description:

C++, JavaScript, Python, Java. Long ago four languages worked together then, everything changed when annoying developers started complaining. Only the Javatar can master all four languages. Javatar

Hint:

Check out jd-gui

Solution

Java, unlike other languages, is very friendly towards decompilers, such as jd-gui, which is mentioned in the hint. After we downloaded jd-gui as well as some graphics libraries needed to run it, we got this source:

import java.io.PrintStream;

public class Javatar
{
  public static void main(String[] paramArrayOfString)
  {
    String str1 = "KYLIIWIBTKEQAAAOAW";
    String str2 = "PEQVVNRSAHFJKVWTLR";
    String str3;
    int i;
    if (paramArrayOfString[0].equals("WHOISTHEJAVATAR")) {
      str3 = "";
      for (i = 0; i < str1.length(); i++) {
        if (i % 2 == 0) str3 = str3 + str1.charAt(i); else
          str3 = str3 + str2.charAt(i);
      }
      System.out.println(str3);
    }
    else
    {
      str3 = "";
      for (i = 0; i < str1.length(); i++) {
        if (i % 2 != 0) str3 = str3 + str1.charAt(i); else
          str3 = str3 + str2.charAt(i);
      }
      System.out.println(str3);
    }
  }

A complete understanding of the program is not even necessary to solve this challenge. We got the flag by running the code in the if block by passing in WHOISTHEJAVATAR as the first additional argument.

[!] mv Javatar.class JavatarOriginal.class
[!] javac Javatar.java
[!] java Javatar WHOISTHEJAVATAR
KELVINISTHEJAVATAR

Note: We had to store the original Javatar.class in JavatarOriginal.class and to recompile the program because of conflicting Java versions.

Flag: KELVINISTHEJAVATAR

Other Resources

• None.