PlaidCTF 2012 Challenge Torrent - Practical Packets writeup (200 points)

This weekend PPP organized its second PlaidCTF which was a lot of fun. Below is a quick writeup for the bittorrent forensics challenge.

Description:

It turns out that robots, like humans, are cheap and do not like paying for their movies and music. We were able to intercept some torrent downloads but are unsure what the file being downloaded was. Can you figure it out?

Provided was a file torrent.pcap, we used tshark (the command line tool for wireshark) to extract data from the packet capture. The only interesting data points are bittorrent.piece, from those we only need index, begin and data. By printing them in this order we can run a simple sort to make sure the file contents are in order.

Next we strip everything but the data field and the colons. Finally we use translate and sed to turn the hex representation into binary. After running the below script we have a file binout.


tshark -r torrent.pcap -R 'bittorrent.piece.data and ip.dst_host == 128.237.112.101' -T fields -e bittorrent.piece.index -e bittorrent.piece.begin -e bittorrent.piece.length -e bittorrent.piece.data -E separator=+ | sort | sed -re 's!.*\+!!' | sed -re 's!:!!g' | echo -n -e $(tr -d '[:space:]' | sed 's/../\\x&/g') > binout

By using the file command and consequently unpacking we figure out its a bz2-ed tar file. Inside we find the files key.mp3 and key.txt. key.txt contains "t0renz0_v0n_m4tt3rh0rn", which turned out to be the valid key. We couldn't extract any hidden information from key.mp3 :-)

Note: if you are trying to reconstruct a file from a bittorrent pcap you might want to check for retransmits, missing indices, multiple files in one capture etc. It would make sense not to strip the headers directly with sed but keep them and run some script to analyze them.