ScrambleSuit is a pluggable transport protocol for obfsproxy. It is written in pure Python and solves two problems:
Protection against active probing attacks by requiring a shared secret between the client and the server. This secret is communicated out-of-band via Tor's BridgeDB.
Rudimentary defence against traffic analysis attacks by altering flow features. In particular, ScrambleSuit alters its inter-arrival times and packet length distribution.
Besides, ScrambleSuit's payload is computationally indistinguishable from randomness. Clients can authenticate themselves towards a ScrambleSuit bridge by conducting a modified UniformDH handshake or by redeeming a session ticket.
The following instructions were tested on Debian wheezy but they should work just fine on other GNU/Linux distributions as well.
On a Debian wheezy system, the following packages are needed: git, python-dev, python-gmpy, python-yaml, python-setuptools, automake, libevent-dev, libssl-dev,asciidoc
Clone and compile the current Tor-git (or use a version of your choice as long as it is >= v0.2.5.0-alpha-dev): git clone https://git.torproject.org/tor.git
Clone the current version of pyptlib (or use a version of your choice as long as it is >= 0.0.5): git clone https://git.torproject.org/pluggable-transports/pyptlib.git
Clone a modified version of obfsproxy which contains the scramblesuit branch: git clone -b scramblesuit_integration https://git.torproject.org/user/phw/obfsproxy.git
The directory "test/" in this repository contains two configuration files for tor which provide a local (i.e., using the loopback interface) ScrambleSuit setup. Furthermore, the script generate_secret.py can be used to generate shared secrets for Tor's configuration file.
Check out obfs4 which is an enhancement of ScrambleSuit.