Note to fellow-HTBers: Only write-ups of retired HTB machines or challenges are allowed.

Challenge info

Inferno [by yuntao]
Find the flag.

The challenge

We start of by downloading the file and verifying it’s sha256sum with the hash displayed on the challenge page.

$ echo "b178fc8605169bf0d9fd7ba74cd172a193db867d0057be74b5ad6f103d22dc80" | sha256sum -c - OK

We then proceed to unzip this file using the password provided on the challenge page. This will give us a txt file.

$ unzip 
[] inferno.txt password: 
  inflating: inferno.txt

Looking at the content of the file, we see what looks like base64 encoded text.

$ cat inferno.txt 

Decoding the text, we get what looks like random gibberish.

$ base64 -d inferno.txt 

I tried a few things like ROT13, base64 decoding the output again, hexdump, … to no avail.

Getting the flag

Then it struck me: this might be very similar to that Art challenge!

So I went back to the Esoteric programming languages Wikipedia page.
And look at what I found in this list. Do you see the reference with “Inferno”?

Malbolge (the 8th circle of Hell)

The an online interpreter.

Entering the “gibberish” into the interpreter resulted in the flag: