.. title: HackIM 2013 - Programming 400 WriteUp
.. slug: hackim-2013-programming-400-writeup
.. date: 2013/03/02 05:08:00
.. tags: CTF, nullcon, HackIM, WriteUps 
.. link: 
.. description: WriteUp for Nullcon HackIM CTF Programming 400

**Programming 4 - 400 points**

**Description:** interpret and give answer. start reading from left if digit then add the digit. if x then remove it and go back 2 places if y then remove and go front 2 places 

    example : 12y34x56 answer is 14
    
    how : 1 + 2 + 5 + 6 = 14

    12y34x5612345678910xy0981235432x4765893x219532y875439664y3x
    2345688x754312x2456x7876554x43y324x6778y7643223457789494x98696763y15348798y765341y1878979

::

    public class FunkyText {

    /** * @param args the command line arguments */
    public static void main(String[] args) {
        // TODO code application logic here
        String num="12y34x5612345678910xy0981235432x4765893x219532y875439664y3x2345688x754312x2456x7876554x43y324x6778y7643223457789494x98696763y15348798y765341y1878979";

        int total=0;
        int num_length=num.length();
        for(int index=0;index<num_length;index++)
        {
            System.out.println("index:"+index);
           switch(num.charAt(index))
           {    
               case 'x':
               {
                   num=num.substring(0, index)+num.substring(index+1,num.length());
                   num_length=num.length();
                   index=index-2;
                   break;
               }

               case 'y':
               {
                   num=num.substring(0, index)+num.substring(index+1,num.length());
                   num_length=num.length();
                   index=index+2;
                   break; 
               }
               default:
               {
                 total=total+(Integer.parseInt(num.substring(index, index+1)));  
               }
           }
            System.out.println("The string:"+num);
            System.out.println("Total:"+total); 
        }
    }

By using the example of  : 12y34x56 answer is 14 how : 1 + 2 + 5 + 6 = 14. We are able to observe and create a logic by manipulating the character position of the statement itself such in this way that I have decided to use the String data type to manipulate the logic to produce the FLAG.

    +--------------------------+-+-+-+-+-+-+-+-+
    |Character position/number:|0|1|2|3|4|5|6|7|
    +--------------------------+-+-+-+-+-+-+-+-+
    |Character:                |1|2|y|3|4|x|5|6|
    +--------------------------+-+-+-+-+-+-+-+-+

Based on figure above, when the switch-case encounters a non-x or y character the program will execute the default statement of
::
    
    "total=total+(Integer.parseInt(num.substring(index, index+1)));" 
    
where by converting the character within the string at the character position into an integer data type and add the following digit into the total variable. Meanwhile, if the switch-case either meets an x character the following statement occurs::

    num=num.substring(0, index)+num.substring(index+1,num.length());
    num_length=num.length();
    index=index-2;

Translation, the program will create a new string that will override the previous string with the deletion of the "x" character at that particular character position and at the same time moves back 2 spaces in the character position index. Same situation occurs in "y" except the character position is moved 2 spaces front. Thus once the program has reached the end, the FLAG sum is printed.

The flag is: **557**