ABCTF - 50 - Java Madness - Reverse Engineering

Information#

Version#

By	Version	Comment
noraj	1.0	Creation

CTF#

• Name : ABCTF 2016
• Website : http://abctf.xyz/
• Type : Online
• Format : Jeopardy - Student
• CTF Time : link

Description#

Hey if you can get [this][this] to pass some tests you could probably have the flag. [this]:https://mega.nz/#!DpVk3QII!0q_VFSxC2v0bjwodRIPT2UhJTTK3qvYsDMD7JL3qfPo

Solution#

1. Here is the java source code:

public class what_the_hack {

	public static void main(String[] args) {

		String check = "";

		if(args.length != 5){
			System.out.println("Almost! (;");
		}

		else {
			for(int i = args.length - 1; i >= 0; i--){
				System.out.println(i);
				for(int j = args[i].length() - 1; j >= 0; j--){
					check += args[i].charAt(j);
					System.out.println(args[i].charAt(j));
				}
			}

			if(check.equals("abctf is the coolest ctf")){
				System.out.println("Flag: " + "ABCTF{" + args[0] + args[1] + args[2] +args[3] + args[4] + "}");
			}
			else{
				System.out.println(check);
			}
		}
	}
}

2. Compile the code: javac what_the_hack.java
3. Run it:

java what_the_hack
Almost! (;

4. Analyse the code, there must be 5 strings args:

java what_the_hack a b c d e
4
e
3
d
2
c
1
b
0
a
edcba

5. By analysing the code we understand that the software display the position of the arg and is content in the reverse order and that the valid check must be abctf is the coolest ctf
6. So write abctf is the coolest ctf to reverse it:

java what_the_hack abctf is the coolest ctf
4
f
t
c
3
t
s
e
l
o
o
c
2
e
h
t
1
s
i
0
f
t
c
b
a
ftctseloocehtsiftcba

7. ftctseloocehtsiftcba is the reverse but we need to space it:

java what_the_hack 'ftc' ' tselooc' ' eht' ' si' ' ftcba'
4
a
b
c
t
f

3
i
s

2
t
h
e

1
c
o
o
l
e
s
t

0
c
t
f
Flag: ABCTF{ftc tselooc eht si ftcba}

8. We got it!