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.

For a more detailed overview of ScrambleSuit, please have a look at the project page available at http://veri.nymity.ch/scramblesuit/. The research paper is available at http://veri.nymity.ch/pdf/wpes2013.pdf. Finally, the directory "doc/" in this repository contains ScrambleSuit's protocol specification.

The following instructions were tested on Debian wheezy but they should work just fine on other GNU/Linux distributions as well.

1. 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

2. 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

3. 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

4. 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.

Contact: Philipp Winter [email protected]
OpenPGP fingerprint: B369 E7A2 18FE CEAD EB96 8C73 CF70 89E3 D7FD C0D0