Psiphon 3 Circumvention System README

Note: source code for our cross-platform "tunnel core" client engine and libraries can now be found here.


The Psiphon 3 Circumvention System is a relay-based Internet censorship circumventer.

The system consists of a client application, which configures a users computer to direct Internet traffic; and a set of servers, which proxy client traffic to the Internet. As long as a client can connect to a Psiphon server, it can access Internet services that may be blocked to the user via direct connection.


  • Automatic discovery. Psiphon 3 clients ship with a set of known Psiphon servers to connect to. Over time, clients discover additional servers that are added to a backup server list. As older servers become blocked, each client will have a reserve list of new servers to draw from as it reconnects. To ensure that an adversary cannot enumerate and block a large number of servers, the Psiphon 3 system takes special measures to control how many servers may be discovered by a single client.

  • Mobile ready. A Psiphon 3 client Android app will be available as part of the beta launch, and other mobile platforms are in the works.

  • Zero install. Psiphon 3 is delivered to users as a minimal footprint, zero install application that can be downloaded from any webpage, file sharing site or shared by e-mail and passed around on a USB key. We keep the file size small and avoid the overhead of having to install an application.

  • Custom branding. Psiphon 3 offers a flexible sponsorship system which includes sponsor-branded clients. Dynamic branding includes graphics and text on the client UI; and a region-specific dynamic homepage mechanism that allows a different home page to open depending on where in the world the client is run.

  • Chain of trust. Each client instance may be digitally signed to certify its authenticity. Embedded server certificates certify that Psiphon servers the client connects to are the authentic servers for that client.

  • Privacy. Psiphon 3 is designed to respect user privacy. User statistics are logged in aggregate, but no individual personally identifying information, such as user IP addresses, are retained in PsiphonV log files.

  • Agile transport. Psiphon 3 features a pluggable architecture with multiple transport mechanisms, including VPN and SSH tunneling. In the case where one transport protocol is blocked by a censor, Psiphon automatically switches over to another mechanism.

Coming soon:

  • IPv6 compatibility. Psiphon 3 is designed to be IPv6 compatible. This ensures the system is ready for the next generation Internet, and in the immediate term offers some additional circumvention capabilities as IPv6-based censorship lags behind the tools used to censor IPv4 traffic.

Security Properties

Psiphon 3 is a circumvention system. To accomplish its design goals, it uses computer security technology including encryption and digital signatures. Using these algorithms does not mean Psiphon 3 offers additional security properties such as privacy or authentication of destination sites for users' Internet traffic.

  • Confidentiality. Traffic routed between a user's computer and a Psiphon proxy is encrypted and authenticated (using standard SSH and L2TP/IPSec VPN algorithms). The purpose of this encryption is to evade censorship based on deep-packet inspection of traffic, not to add confidentiality to the user's Internet traffic. The user's traffic is plaintext to the Psiphon proxy and to the Internet at large as it egresses from the Psiphon proxy. Put simply, Psiphon does not add HTTPS or equivilent security where it is not already in place at the application level.

  • Anonymity. Psiphon is not an anonymity solution such as Tor. If a user connects to a Psiphon proxy which is beyond the monitoring of the censor he or she is circumventing, then the censor will only see that the user is sending encrypted traffic to a Psiphon proxy. The censor will know the user is using Psiphon. Psiphon does not defend against traffic analysis attacks the censor may deploy against traffic flowing to Psiphon proxies. The Psiphon proxy will know where the user is coming from, what their unencrypted traffic is, and what their destination is, and so the user is necessarily putting trust in the entity running the Psiphon proxy.

  • Integrity. Psiphon was not designed to add integrity to Internet traffic. However, in the case where a censor is intercepting SSL/TLS traffic using compromised root CA keys, Psiphon adds integrity; but only if the user has secured a trusted client out of band and is using a Psiphon proxy beyond the control of the censor. Simply, the user's HTTPS traffic happens to bypass the censors man-in-the-middle attack, and the Psiphon authentication system does not rely on the commercial Certificate Authority for most use cases. See the design paper for details on Psiphon PKI.

  • Availability. Psiphon is designed to make available Internet content that's otherwise censored. This is its primary design goal.


IMPORTANT: See the design paper, included with this software distribution, for important information regarding security limitations and user privacy issues.


Supported transport mechanisms:

  • L2TP/IPSec VPN
  • HTTP/SOCKS Proxy over SSH Tunnel

Planned transport mechanisms:

  • DNS tunnel

Supported client platforms:

  • Windows XP/Vista/7
  • Android [TODO: version]

Planned client platforms:

  • Mac OS X

Server platform:

  • Debian 6.0


Please see the INSTALL file.

Source Tree

 Automation\...             Build, Install, & Deploy Scripts
 Automation\Tools\...       3rd party Build Tools
 Client\...                 Clients
 Client\psiclient\...       Windows Client
 Data\...                   Network Database
 Data\Banners\...           Branding Assets
 Data\CodeSigning\...       Authenticode Keys
 EmailResponder\...         Email Autoresponder Server
 Server\...                 Web Server and Server Scripts


Please see the LICENSE file.


For more information on Psiphon Inc, please visit our web site at: