The Pirate Bay un-SSL

Theory

Recently, the world saw The Pirate Bay offering SSL encryption on their server. This means that your ISP won't know anymore which torrent you are downloading, right? Wrong.
HTTPS is quite useless for protecting static and public content. By static, I do mean the .torrent file itself. It is always the same. By public, I do mean than one doesn't need any kind of authentication to pick up the content. It's always the same, for everyone. For crawlers, too.
So, one could easily index (a portion of) The Pirate Bay torrent database by the Content-Length. Then, one could intercept some encrypted traffic between some machine(s) within his/her network and the torrents.thepiratebay.org server. Knowing both (encrypted) request and response lengths, it is possible to get a quite reliable list of matches from the previously indexed torrent list.

Practice

Don't try this at work, or you might hurt yourself Eye-wink

  1. Use Wireshark to capture some torrent downloads. Torrents are hosted on a separate server, which makes the task easier yet. Just use the following capture filter: "tcp and port 443 and host torrents.thepiratebay.org"
  2. Now, just go with the stream Smiling ("Follow TCP Stream" for the packet you suspect belongs to the torrent download. This will create another filter, just like "(ip.addr eq 192.168.0.10 and ip.addr eq 83.140.176.156) and (tcp.port eq 2157 and tcp.port eq 443)")
  3. Just save the displayed stream anywhere else (pcap1.pcap sounds nice)
  4. Now, use my quick&dirty TPB-TLSlen.pl Perl script to get the request/response lengths:
    perl TPB-TLSlen.pl pcap1.pcap
    Yeah, I know, it is nasty. It only supports the TLS cypher. And it simply calls the tshark (the command line version of Wireshark) to parse it's output.
  5. Now, just paste the REQ and RES values below Laughing out loud
    (note that the REQ value is optional, setting it to 0 simply ignores the request size for matching)
Note that you are able to fine-tune the maximum and minimum header sizes. For the response, the headers are almost the same all the time. The only thing that varies is the decimal representation of the file length and age. (Un)fortuately, the request headers do vary for different browsers and referring pages. However, knowing the request size still helps a bit, specially if the torrent's filename was huge Smiling

Precision

The following size distribution chart was generated using the database with ~165K torrents:

torrent size distribution

The most common torrent size is ~14 KB, and it's easy to figure out that such torrents represent the shared 700 MB files Smiling
There's also a major peak for the 454 bytes torrents. However, bigger torrents are less common, thus, the size detection technique becomes more precise. Now, the average "distance" between torrent sizes is ~44 bytes (at least for the sample I've collected). So, adding a cookie with the random size up to 128 bytes will disrupt the size matching detection a lot. The request size disruption is even easier: the largest torrent URI I've found was 150 bytes-wide. Thus, padding every request URI to match 150 characters is enough to make the requests completely indistinguishable. Joining the pieces (the padding add-on strings are bold):
GET /4319199/[a4e]Ghost_in_the_Shell_TV_01-26.4319199.TPB.torrent?nVM2UGfcG533un4ym70eT2
9r0WwBLYdmFCNN+UTV/hiJ7EAXdFU5KfdWHpkB5lXaCmITsACKOPVyjmpbaOB+CrI5
HTTP/1.1 Host: torrents.thepiratebay.org User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.1) Gecko/2008070208
Firefox/3.0.1 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 Accept-Language: en-us,en;q=0.5 Accept-Encoding: gzip,deflate Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7 Keep-Alive: 300 Connection: keep-alive Referer: https://thepiratebay.org/recent Cookie: language=pt_BR; country=BR; PHPSESSID=ad6cb7e414c8dc88e0c2444f6215165a
HTTP/1.1 200 OK Content-Type: application/x-bittorrent Etag: "2198642509" Last-Modified: Mon, 28 Jul 2008 22:28:59 GMT Server: lighttpd Content-Length: 91601 Date: Mon, 28 Jul 2008 22:37:56 GMT X-Varnish: 108010229 107999438 Age: 253 Via: 1.1 varnish Connection: keep-alive Set-Cookie: p=68eOfxOC7JwBYcMe1RJWC4Z5PV/lJzqJORW8KROPMH9zQhszSjFnRp2tsNWEoyabWAloneUaoz
MxYtx4hoM9MZUKE/7wGzC3ZKLEZdppG4og3W; expires=Mon, 28-Jul-2008 22:37:56 GMT; path=/;
domain=torrents.thepiratebay.org
(binary torrent data)

Solution

  1. Use a constant padding in the .torrent files. This messes things a bit, but stills ineffective. The only advantage is not messing up with the server Sad
  2. Patch the lighttpd server so it sends a non-lasting cookie with a random size.

Thanks


Filename/TitleSize
TPB-TLSlen.pl.txt997 bytes

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stas's picture
stas » July 31, 2008 » 11:05
database » hack » music » network » perl » php » video » web

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shit

what stupid fuck downloads .torrent files without tor?

Anonymous (not verified) » August 11, 2008 » 07:53

People who aren't douches.

People who aren't douches. Do not use tor for torrents, you screw it up for everyone else.

Anonymous (not verified) » July 1, 2009 » 00:31

publication

stas's picture

Now also at the milw0rm papers Laughing out loud

stas » September 11, 2008 » 09:04

another one

stas's picture
stas » October 14, 2008 » 15:34

Can't download your Perl

Can't download your Perl file.

Anonymous (not verified) » September 12, 2008 » 20:14

fixed!

stas's picture

Bad, bad server!

stas » September 15, 2008 » 10:34

ghaghuuwQQrHYJafRN

A minute saved is a minute eraend, and this saved hours!

Jaycee (not verified) » July 30, 2011 » 11:59

Good on your

Good on your

Lida (not verified) » August 8, 2009 » 06:01

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