I2P (originally short for Invisible Internet Project, though it is not frequently referred to by this name anymore) is a free and open source project building an anonymous network (or, more accurately, a pseudonymous overlay network). The network is a simple layer that applications can use to anonymously and securely send messages to each other. Possible uses include anonymous surfing, chatting, blogging and file transfers.

The developers emphasize that there are likely to be bugs in the software and there has been insufficient peer review to date;thereforethose seeking strong anonymity should look elsewhere.

The network itself is strictly message-based (like IP), but there is a library available to allow reliable streaming communication on top of it (similar to TCP, although from version 0.6 there is a new UDP-based SSU transport). All communication is end-to-end encrypted (in total there are four layers of encryption used when sending a message), and even the end points ("destinations") are cryptographic identifiers (essentially a pair of public keys).

Although many of the developers had been a part of the IIP and Freenet communities, there are significant differences between their designs and concepts. IIP was an anonymous centralized IRC server, Freenet is a censorship-resistant distributed data store, but I2P is an anonymous peer-to-peer distributed communication layer designed to run any traditional internet service (e.g. Usenet, E-mail, IRC, file sharing, Web hosting and HTTP, Telnet), as well as more traditional distributed applications (e.g. a distributed data store, a web proxy network using Squid cache, and DNS).

I2P is currently in the pre-alpha development stage and not yet ready for general anonymity use. Therefore the developers have asked the user community not to mention the software in high publicity websites like Slashdot.

The main developer of I2P is jrandom (see J. Random), with support from numerous others.

I2P software tools

Since I2P is an anonymous network layer, it is designed so other software programs can use it for anonymous communication, the so-called application layer. As such there are a variety of tools currently available for I2P or in development.

I2PTunnel

I2PTunnel is an application embedded into I2P that allows arbitrary TCP/IP applications to communicate over I2P by setting up "tunnels" that can be accessed by connecting to pre-determined ports on localhost.

SAM

SAM is a protocol which allows a client application written in any language to communicate over I2P, by using a socket-based interface to the I2P router.

BitTorrent

Several programs provide BitTorrent functionality for use within the I2P network. Each relies on the user being able to access the I2P network with a web browser to download the .torrent files.

I2PSnark, included in the I2P install package, is a port of the BitTorrent client named Snark.

Azureus is a BitTorrent client that includes a plugin for I2P, allowing anonymous swarming through this network. This plugin is still in an early stage of development, however it is already rather stable.

I2P-BT was a BitTorrent client for I2P that allowed anonymous swarming for file sharing. This client was a modified version of the original BitTorrent 3.4.2 program which ran on Windows and most dialects of Unix in a GUI and command-line environment. It was developed by the individual known as 'duck' on I2P in cooperation with 'smeghead'. It is no longer being actively developed; however, there is a small effort to upgrade the I2P-BT client up to par with the BitTorrent 4.0 release.

I2PRufus is an I2P port of the Rufus BitTorrent client.

Gnutella I2Phex

I2Phex is a port of the popular Gnutella client Phex to I2P. It is stable and rather functional to work anonymously. It is the first full functional client for an anonymous file transfer, because the client is based on the developed client Phex.

Susimail

I2P has a free pseudonymous e-mail service, run by an individual called 'Postman'. The mail transfer servers are pop.mail.i2p (POP3) and smtp.mail.i2p (SMTP).

Susimail was created to address privacy concerns in using these servers directly using traditional email clients, such as leaking the user's hostname while communicating with the SMTP server. Susimail is a Web-based email client intended primarily for use with Postman's mail servers, designed with security and anonymity in mind. It is currently included in the default I2P distribution, and can be accessed through your I2P router console web interface. (Note that this is only used to read and send e-mail, not to create or manage your mail.i2p account; the latter must be done at www.mail.i2p.)

Syndie

Syndie is a blogging application for I2P, which is also usable through the Tor network; it is currently at an alpha release.

I2P terminology

I2P users will see references to the following terms on the I2P home page and on the router console.

Eepsite

Eepsites are websites that are hosted anonymously within the I2P network. Eepsite names end in .i2p, such as ugha.i2p or orion.i2p. EepProxy can locate these sites through the cryptographic identifier keys stored in the hosts.txt file found within the I2P program directory. Typically, I2P is required to access these eepsites.

EepProxy

The EepProxy program handles all communication between your browser and any eepsite. It functions as a proxy server that can be used by any web browser.

Peers

Other machines using I2P that are connected to your machine within the network. Each machine within the network shares the routing and forwarding of encrypted packets.

Tunnels

Every ten minutes, a connection is established between your machine and another peer. Data to and from your machine, along with data for other users, passes through these tunnels, and are forwarded such that the packets eventually reach their final destination.