CrossCTF_2018: Choose Your Own Adventure

Category: Misc Points: 964 Description:

Maybe your question was too long. You reduced the length of your question and asked the oracle again: "Where's the flag?" As expected, she had no reaction again. You sigh... maybe the solution really was in there after all. You close your eyes and started willing yourself back into the enemy's base... Non-standard flag format: [A-Z0-9]+ nc ctf.pwn.sg 1400 Creator - prokarius (@prokarius)

Write-up

This challenge was one of the more complex challenges in this CTF. Essentially, it's a maze of 4 levels, 3x4 rooms.

# nc ctf.pwn.sg 1400
After a long time of trying to will yourself, you are finally in the maze.
You take a deep breath. The clue to the flag is in here somewhere.
Non standard flag format.

00 00 00, Score: 00
The room is just empty space. It is a void.
Scribbled into the wall are the numbers 9 and 12.
Below in red, the number 0 was written
Where would you like to move?
2 -- South
3 -- East
5 -- Up
9 -- Quit
2

00 01 00, Score: 02
A think blanket of fog covers this room.
Scribbled into the wall is the number 15.
Below in red, the number 4 was written.
Where would you like to move?
1 -- North
2 -- South
5 -- Up
9 -- Quit

As a miscellanous challenge, it is more of a guessing game than technical challenge so to guess we go. Well, one of the more important hints given from the start is to take note of everything given. So after scrambling around trying to play detective, suddenly, I had a flash of ingeniuity, could it be, chemistry?

With that in mind, we try to piece things together. First, we mapped out all the rooms,

Map 1

Now, we proceed to think about how things are. In the earlier example, in room 00 01 00, we get A thick blanket of fog covers this room., perhaps it's water? H2O? Well, we have a score of 2, could the score refer to the hydrogen and the Y coordinate refer to oxygen? Moving on to other rooms,

01 01 02, Score: 04
You contribute to the puddle of urine in the corner of this room.
Scribbled into the wall is the number 17.
Below in red, the number 0 was written.
Where would you like to move?
6 -- Down
9 -- Quit

In the room 01 01 02 with a score of 04, we get a puddle of urine. This leads to urea, the chemical in urine with a formula of CH4N2O. Suddenly things begun to fall in place, with CH4N2O, we can conclude that the format is as follows, C O N and the score being H.

Moving on, we also found that only certain rooms had scribbles, so we decided to ignore those,

Map 2

With that, we can extract all the chemicals in every room with a scribble and plot them according to the coordinates and score, with C O N Score: H respectively.

Now that we have sorted all the chemicals into a table by the scribbling number, we can then get the chemical name and using the red number as a character position, we can extract a hidden message.

No. Red Formula Name Letter
0 1 H6C3O Acetone. C
1 0 C2H4O2 Acetic Acid A
2 0 C3H6 Propene P
3 6 N2O Nitrous Oxide S
4 0 C2H4O2 Acetic Acid A
5 1 N2 Nitrogen I
6 1 C3H6O Acetone C
7 1 N2 Nitrogen I
8 3 O3 Ozone N
10 1 N2 Nitrogen I
11 3 O3 Ozone N
13 0 CH2O Formaldehyde F
14 3 NH3 Ammonia O
15 4 H2O Water R
16 0 CH4 Methane M
17 0 CH4N2O Urea U
18 1 C3H8O3 Glycerol L
19 0 C2H4O2 Acetic Acid A

Putting that together, we get the words

CAPSAICIN IN FORMULA

Looking capsaicin's chemical formula up on Google gives us C18H27NO3.

Therefore, the flag is C18H27NO3.

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