My First CTF: A Cybersecurity Challenge

As someone who spent most of their time in cybersec staring at assembly code and decompiled functions to create mods for my favorite game, CTFs never really caught my attention. I had a vague idea of what they were, some kind of digital puzzles people did for fun or to flex their skills, but it just never interested me enough to try it out.

However, after being invited to a cybersec conference in the US and having an amazing time, I grew eager to attend more events like it. Shortly after, I came across a CTF challenge organized by my own country’s intelligence agencies, who announced that top participants would be invited to a physical event. I figured if any CTF worth attempting, it would be this one. So, I registered.

The first thing I learned is that the challenges were organized into categories: forensics, crypto, pwn, rev, web, misc, etc. The second thing I realized was that I was completely unprepared tool-wise. Pretty much all of these challenges required specific tools to solve them. So, even though reverse engineering (rev) was probably the category I was best at, attempting those challenges would have required installing a bunch of software, and I was doing this on a fresh copy of Windows.

Then, I discovered a challenge that was basically a text adventure game played over raw TCP via TLS, where you had to solve various puzzles to progress. This was perfect because it was quick and easy to set up.

The Challenge

The goal of this text adventure was to break into an underwater laboratory, solve several puzzles to collect four clues, and then find the ’red thread’ tying them all together.

Puzzle 1: The Circuit

After navigating into the laboratory, the first challenge was a circuit with transistor-like components. The clues I found was a bunch of ’blocks’ with values assigned to them and circuit structure with input values, a desired output of 7.

Block	Value
A	8
B	3
C	12
D	3
G	7
H	12
I	3

The circuit (with the correct blocks placed):

 5   3     2   6     4   1     7   2
  \ /       \ /       \ /       \ / 
  [A]       [C]       [B]       [D] 
    \       /           \       /   
     8    12             3     3    
      \  /                \   /     
       [H]                 [I]       
         \                 /          
          12             3           
            \           /            
                 [G]                 
                  |                  
                  7  ← desired output

After realizing each component was doing an operation these blocks were the ones that made sense, so I inserted them into the circuit in the order above and that solved the puzzle. The clue that this puzzle gave me was the word ’FOSTER’ in Swedish, which means fetus.

Puzzle 2: Reverse DNS

The second puzzle gave us an IP (46.248.163.200) and a regex (^(?:.{5})\K.{5}) and instructed the player to search in reverse. I realized what they wanted me to was a reverse DNS on the IP address. The result was: http://ns2.eskada.com.pl/

And after doing a regex on this URL we get the world ’SKADA’ which means damage in Swedish.

Puzzle 3: Coordinates and Google Maps

The third puzzle gave us 4 coordinates and hinted the player to search on Google and that ’X marks the spot’. My intuition was that I needed to calculate the in where these 4 coordinates intersect, thinking that they together would form an X, which turned out to be the correct solution. And sure enough, if you actually go there on Google Maps, you see a large trailer with the word ’KRAFT’ which means power in Swedish.

Puzzle 4: Brainfuck

In the fourth challenge we were given a bunch of symbols (>, <, +, -, ., ,, [, ]) which I realized was just code in the programming language called Brainfuck. After compiling that code in an online interpreter I got the output SJUK which means sick in Swedish, and was the final clue.

The Solution

So putting it together our clues were:

Swedish	English
FOSTER	fetus
SKADA	damage
KRAFT	power
SJUK	sick

And the final part required me to input the word which was the ’red thread’ tying them all together. This took a long while to figure out... But I finally found the solution, which was the word ’water’. The first 3 words make sense, fetus water, water damage, water power(plant), but the last one is based on a rarely used Swedish word vattensjuk, which directly translated would be water sick (waterlogged in english).

After inputting the word ’water’ it finally gave the flag which I submitted to clear the challenge.