not enforced by default. 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. 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. BitTorrent (Mainline) supports MSE/PE since version 4.9.2-beta on May 2, 2006. Deluge supports MSE/PE as of Deluge-0.5.1. KTorrent implemented MSE/PE in SVN version 535386 on April 29, 2006. libtorrent implemented protocol encryption in v0.13 released on 10 December 2011. rTorrent supports MSE/PE as of rTorrent-0.7.0. Transmission supports MSE/PE as of Transmission-0.90. Vuze (formerly Azureus) supports the final spec since 25 January 2006 (CVS snapshot 2307-B33). Azureus version 2.4.0.0 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 2.4.0.2. μTorrent premiered MSE/PE 4 days after Azureus with beta 1.4.1 build 407. μTorrent version 1.5 (build 436) was released on 7 March 2006; it was the first stable version of μTorrent with PE. Operation 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 key exchange combined with the infohash of the torrent to establish an RC4 encryption key. The 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 kibibyte (1024 bytes) of the output is discarded to prevent the Fluhrer, Mantin and Shamir 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. 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. Supported clients propagate the fact that they have MSE/PE enabled through PEX and DHT. Security The estimated strength of the encryption corresponds to about 60–80 bits for common symmetrical ciphers. Cryptographically, this effective key length is quite low, but appropriate in that the protocol was not designed as a secure transport protocol but rather as a fast and efficient obfuscation method. AES was proposed as the encryption method, but not adopted because it consumed too much