Psiphon uses many code repositories for different parts of our system. This is a meta-repo with general information about the Psiphon Circumvention System.

For downloads and more info about Psiphon and Psiphon Inc., please visit our web site at psiphon.ca

These are the repos that will likely be of primary interest:

All of our public repositories can be found at https://github.com/Psiphon-Inc and https://github.com/Psiphon-Labs.

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

The system consists of a client application, which configures a user's 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.

Features:

• Automatic discovery. Psiphon 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 system takes special measures to control how many servers may be discovered by a single client.

• Cross-platform. Psiphon clients are available for Windows, Android, and iOS.

• Zero install. Psiphon 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 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 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 Psiphon log files.

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

Our legacy design document is outdated, but many of the fundamentals still apply.

Psiphon 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 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. 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 equivalent 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.

Please see the LICENSE files in the individual code repos. Most code is GPLv3; some libraries have more permissive licenses.