The Invisible Internet Project (originally abbreviated IIP, later shortened to I2P, and usually referred to as I2P) is a decentralized anonymization network similar to The Onion Router (Tor). Contrasted with Tor, however, I2P encrypts all communications end-to-end, does not rely on a centralized database of server nodes, and employs “garlic routing” rather than Tor’s “onion routing”.
Rather than rely on a centralized node database, users connected to the I2P network effectively turn their machines into nodes, or “routers”, that each have a unique cryptographic “Router identity”. Each router has a series of inbound and outbound virtual tunnels, and all messages sent over the I2P network through these tunnels are addressed to cryptographic destinations. The term “garlic routing” in the case of I2P refers to both its layered encryption of messages as well as its bundling together of multiple messages into a “clove”. This system of cryptographic addressing and bundled messages allows users of the I2P network to communicate anonymously, as none of the sender, the receiver, or a third party is able to enumerate I2P users’ IP address. Furthermore, the mix messaging combined with the overall traffic from users not requiring significant anonymity allows those who do require significant anonymity to remain well-hidden.
Just as with Tor, users can run anonymous websites, called “Eepsites” (for I2P). Eepsites have an .i2p domain extension, and typically require I2P to connect too (although users hosting .i2p sites can enable the viewing of their sites by non-I2P users). Hosting these sites is accomplished by using a built-in tunneling application, I2PTunnel, to create a tunnel to a web server. Besides I2PTunnel, other built-in I2P applications include SAM (Simple Anonymous Messaging - a chat application), Susimail (a web-based email client) and I2PSnark (a Bitorrent client) among others.