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Hack.lu 2020 CTF write up: Callboy

christalib profile image chris Updated on ・2 min read

Description

Have you ever called a Callboy? No!? Then you should definitely try it. To make it a pleasant experience for you, we have recorded a call with our Callboy to help you get started, so that there is no embarrassing silence between you.

PS: do not forget the to wrap flag{} around the secret

Write up

This was quite a fun challenge, I learned a new way to use Wireshark and I can't stop to be amazed by this tool!

We get a pcap file and after looking a bit through it we can identify quite quickly the communicating IP addresses. Coupling that with the description of the challenge, we know that we have to capture some kind of communication between two clients.

There is a lot of intended noise in the traffic: DNS requests to a lot of the top alexas websites, but we don't care so much about it and it's an obvious rabbit hole.

We know this is a recorded communication, so probably telephony or video. After some quick research, we realise it's possible to extract audio from pcap files!

We tried to fix a bash script for like an hour before realising that Wireshark does it for you thanks to this link.

After that, the challenge is a piece of cake, we need to follow the steps explained in the web page on how to extract the audio, play it and we get the flag!

Final result of the analysis

In itself it's quite an easy challenge when you know where to look. Some interesting conclusions on that would be:

  • situational awareness: how to get information from the noise of the traffic
  • identify quickly protocols between relevant IPs
  • get to know Wireshark much better!

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