Ilmu Hisab

The challenge provides us with 2 netcat connections and a non stripped-ELF file

Inspecting the available functions, main() displays a message when connected to the netcat

merdeka() looks like the function that will display the flag

There is one interesting function called addtwonumber(), notably at this part

Let's rename the variables

0x539 = 1337

0x1ca3 = 7331

Basically we need to satisfy one of the two conditions then the we will get the flag. By looking at the first condition it looks illogical, and so is the second condition.

The solution here is to do an Integer Overflow.

In C, integers are stored in a fixed number of bits (32-bit, 64-bit). Each integer type has a maximum and minimum and maximum value it can hold. If the result of an operation exceeds this maximum or minimum, it wraps around to the opposite extreme, causing unexpected behavior.

This also has a bit to do with signed and unsigned integers where

in a 32-bit system:

• Unsigned integer: Values from 0 to 4,294,967,295

• Signed Integer: Values from -2,147,483,648 to 2,147,483,647

Anyways back to the challenge, if you look at the initial lines of the function, it was specfied that it uses int which is a 32-bit data type even though on a system that is 64-bit.

We can satisfy the first condition like this

The sum of the two numbers wrapped around to and became the negative part of the signed integer because of overflow.

Credits go to mhz, pinjam ss bang infra dah tutup hehe.