BitTorrent protocol encryption

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Protocol encryption (PE), Message stream encryption (MSE), or Protocol header encrypt (PHE)[1] are related features of some peer-to-peer file-sharing clients, including BitTorrent. They attempt to make traffic harder to identify by third parties including internet service providers (ISPs).

MSE/PE is implemented in Azureus, BitComet, BitTornado, KTorrent, Mainline (incoming only), µTorrent, and rTorrent. PHE was implemented in old versions of BitComet. Similar protocol obfuscation is supported in up-to-date versions of some other (non-BitTorrent) systems including eMule.



Peer-to-peer file-sharing traffic makes up more than a third of total internet traffic.[2] Some ISPs deal with this traffic by increasing their capacity whilst others use specialised systems to throttle (i.e. slow down) BitTorrent traffic. Obfuscation and encryption make traffic harder to detect and therefore harder to throttle. These systems are not designed to provide anonymity.


Early approach

Protocol header encryption (PHE) was conceived by RnySmile and first implemented in BitComet version 0.60 on 8 September, 2005. Some software like IPP2P claims BitComet traffic is detectable even with PHE.[3] PHE is detectable because only part of the stream is encrypted. Since there are no open specifications to this protocol implementation the only possibility to support it in other clients would have been via reverse engineering.

Development of MSE/PE

In late January 2006 the Azureus developers decided to design and simultaneously implement a new, open protocol obfuscation method, called message stream encryption (MSE). It was included in Azureus CVS snapshot 2307-B29 on 19 January, 2006.[4]

This first draft was heavily criticized since it lacked several key features. After negotiations between different BitTorrent developers a new proposal was written and then implemented into the Azureus and µTorrent betas within days. The developers were ludde, uau, The 8472, Parg and Nolar. In µTorrent, the new protocol was called protocol encryption (PE).

MSE/PE in BitTorrent client versions

Azureus supports the final spec since 25 January, 2006 (CVS snapshot 2307-B33)[5]. Azureus version was released 10 February, 2006, and was the first stable version of a client to support MSE/PE. However, glitches in Azureus' implementation resulted in improperly encrypted pieces that failed hash checking. The glitches were rectified as of version[6]

µTorrent premiered MSE/PE 4 days after Azureus with beta 1.4.1 build 407.[7]. µTorrent version 1.5 (build 436) was released on 7 March, 2006; it was the first stable version of µTorrent with PE.

BitComet version 0.63 was released 7 March, 2006. It removed the old protocol header encryption and implemented the new MSE/PE to be compatible with Azureus and µTorrent.[8]

KTorrent implemented MSE/PE in SVN version 535386[9] on April 29, 2006.[10]

Mainline supports MSE/PE since version 4.9.2-beta on May 2, 2006.[11]

BitTornado supports MSE/PE as of build T-0.3.18. As of January 5, 2007, this build is still marked "experimental" on the Download page.[12]

rTorrent supports MSE/PE as of rTorrent-0.7.0.[13]

Deluge supports MSE/PE as of Deluge-0.5.1.[14]


The BitComet PHE method used in versions 0.60 to 0.62 is neither published, nor is it compatible with MSE/PE.

MSE/PE uses a D-H key exchange combined with the infohash of the torrent to establish the key, then it uses RC4 to encrypt the data. The D-H key exchange helps to minimize the risk of passive listeners, and the infohash helps avoid man-in-the-middle attacks. RC4 is chosen for its speed. The first kilobyte of the RC4 output is discarded to prevent a particular attack.

The specification allows the users to choose between encrypting the headers only or the full connection. Encrypting the full connection provides more obfuscation but uses more CPU time. However, only Azureus and µTorrent beta 1.4.1 build 413 or older lets the user choose. All other clients default to full encryption.

To ensure compatibility with other clients that don't support this specification, users may also choose whether unencrypted incoming or outgoing connections are still allowed.

All supported clients will enable encryption automatically if they receive an encrypted incoming connection even if outgoing encryption is disabled.

Supported clients propagate the fact that they have MSE/PE enabled through PEX and DHT. Other clients will then connect to them with encryption even if outgoing encryption is disabled.


The estimated strength of the encryption corresponds to about 60–80 bits for common symmetrical ciphers[15]. This is quite low for today's standards but one has to keep in mind that this protocol wasn't designed as a secure transport protocol but as a fast and efficient obfuscation method. AES was proposed as the encryption method but not adopted because it consumed too much CPU time and the required D-H keys to achieve a security equal to AES would have been much bigger or require elliptic curve cryptography, making the handshake more expensive in terms of used CPU time.


Many ISPs are now using more sophisticated measures (e.g. pattern/timing analysis or categorizing ports based on side-channel data) to detect BitTorrent traffic. This means that even encrypted BitTorrent traffic can be throttled. However, whilst most ISPs use the simpler, less costly, methods to identify and throttle BitTorrent, the current solution remains extremely effective.

A fatal flaw where encryption doesn't matter

The Sandvine net appliance uses a different approach to throttling BitTorrent traffic that makes seeding impossible, by sending counterfeit protocol hangup packets. Multiple secure VPNs between clients have become necessary (in order for the Sandvine hack to be disabled).

Using Tor as a secure tunnel (as it is not a true VPN) to exit one's traffic from one's ISP is the only semi-workable solution at the moment. It must be assumed that Sandvine throttles Tor traffic less severely than BitTorrent traffic.

This link gives a more technical explaination of the above


Bram Cohen, the inventor of BitTorrent, opposed adding encryption to the BitTorrent protocol. Cohen stated he was worried that encryption could create incompatibility between clients. He also stressed the point that the majority of ISPs don't block the torrent protocol. Cohen wrote "I rather suspect that some developer has gotten rate limited by his ISP, and is more interested in trying to hack around his ISP's limitations than in the performance of the internet as a whole".[16] Many BitTorrent community users responded strongly against Cohen's accusations.[17] Cohen later added the ability to receive encrypted connections but not send out encrypted data on his Mainline client.[18]

BBC claims

In 2006, a Newsnight episode claimed that the use of encrypted BitTorrent helps terrorists and paedophiles because any increase in encrypted traffic makes it more difficult for law enforcement to monitor the InternetTemplate:Fact.

Many BitTorrent users were outraged and complained.[19] BBC stood by its claim about paedophiles and terrorists, but it did admit that such references are often used by others as a way to sell copy, and that traditional media like television are under threat from new media like BitTorrent.Template:Fact BBC apologised for saying that peer to peer file sharing was "theft"[20]

As the UK does not have a program like Australia's ABC Mediawatch -- there is no neutral entity to challenge the quality of journalism in this case. It must also be pointed out that probably some 5% of global BitTorrent traffic (where the end content is in English) may indeed by BBC programmes.

Notes and references

  1. It is usually referred to as the more correct protocol header encryption instead.
  2. Wired: The Bittorrent Effect
  3. IPP2P homepage - News
  4. CVS Snapshot Azureus2307-B29.jar has been released !
  5. CVS Snapshot Azureus2307-B33.jar has been released !
  6. Azureus : Java BitTorrent Client - Changelog
  7. µTorrent 1.4.2 beta 435
  8. BitComet Client Release Notes
  9. The SVN server is at svn://
  10. Encryption has been added !
  11. Version Notes
  12. BitTornado T-0.3.18
  13. Announcement of rTorrent-0.7.0
  14. Deluge 0.5.1 Release Announcement
  15. RFC 3526 chapter 8
  16. Obfuscating BitTorrent
  17. Debate over Protocol Encryption
  18. BitTorrent Mainline Version History
  19. Feedback - February 2006 (2)
  20. A bit of BitTorrent bother

External links