Quartet [NahamCon]

May 23 2025 · 1 min read · 176 words

Hello everyone!

In this writeup, let’s see how we can solve the Quartet CTF challenge from NahamCon 2025.

We are given four files with strange extensions. Well, they may seem strange for someone who’s seeing them for the first time, but they’re not!

These files with “zX” (where X is a number) extensions are parts of a single zip file (.zip). Basically, combining these zip files will give us one single complete zip file in theory.

Let’s try to combine them with the following command:

cat * > main.zip # Make sure all these files are in an isolated folder!

Now let’s unzip this main.zip file:

unzip main.zip

And alas! We are greeted with an error, which is just a warning 🤷‍♂️

Archive:  main.zip
warning [main.zip]:  zipfile claims to be last disk of a multi-part archive;
  attempting to process anyway, assuming all parts have been concatenated
  together in order.  Expect "errors" and warnings...true multi-part support
  doesn't exist yet (coming soon).
warning [main.zip]:  1526784 extra bytes at beginning or within zipfile
  (attempting to process anyway)
file #1:  bad zipfile offset (local header sig):  1526788
  (attempting to re-compensate)
  inflating: quartet.jpeg

Meh, just ignore them. We can see inflating: quartet.jpeg at the end, which means a JPEG file is extracted from main.zip. Let’s look at it.

Woah, a quartet! No wonder the Problem name is the same. Well, it’s an image. Let’s search for strings and filter them out with the following command:

strings quartet.jpeg | grep "flag"

And there you go—the flag will appear!

Last edit: 24-05-2025