picoCTF 2022 – Torrent Analyze

This is a small write-up of the “Torrent Analyze” challenge from picoCTF 2022 (Forensics Category). The challenge is now available in picoGym here !

Description :

SOS, someone is torrenting on our network. One of your colleagues has been using torrent to download some files on the company’s network. Can you identify the file(s) that were downloaded? The file name will be the flag, like picoCTF{filename}

Hint 1 : Download and open the file with a packet analyzer like Wireshark.

Hint 2 : You may want to enable BitTorrent protocol (BT-DHT, etc.) on Wireshark. Analyze -> Enabled Protocols

Hint 3 : Try to understand peers, leechers and seeds. Article

Hint 4 : The file name ends with .iso

Solution :

As explained in the hints : download the capture file (.pcap) and open it with Wireshark, enable BitTorrent related protocols.

Enabled Protocols in Wireshark

First, use filters to only display bt-dht traffic :

Traffic filtered : bt-dht

We see now that the local IP address from which we are looking for more details is, let’s adjust the filters :

Traffic filtered : bt-dht and ip.src_host == ""

Navigate through the packets until you find one packet where “Message type: Request” and “Request type: get_peers“. The first packet of this type is the packet #51080

Request to retrieve peers

Unfold the “Request arguments” of the middle panel to find the info_hash value e2467cbf021192c241367b892230dc1e05c0580e

Find the info_hash value

Right click on the value -> Copy -> Value, and search it in Google :

Google search for hash e2467cbf021192c241367b892230dc1e05c0580e

The flag we are looking for is then : picoCTF{ubuntu-19.10-desktop-amd64.iso}

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