I2P is an effort to build, deploy, and maintain a network to support secure and anonymous communication. People using I2P are in control of the tradeoffs between anonymity, reliability, bandwidth usage, and latency. There is no central point in the network on which pressure can be exerted to compromise the integrity, security, or anonymity of the system. The network supports dynamic reconfiguration in response to various attacks, and has been designed to make use of additional resources as they become available. Of course, all aspects of the network are open and freely available.
Unlike many other anonymizing networks, I2P doesn't try to provide anonymity by hiding the originator of some communication and not the recipient, or the other way around. I2P is designed to allow peers using I2P to communicate with each other anonymously – both sender and recipient are unidentifiable to each other as well as to third parties. For example, today there are both in-I2P web sites (allowing anonymous publishing / hosting) as well as HTTP proxies to the normal web (allowing anonymous web browsing). Having the ability to run servers within I2P is essential, as it is quite likely that any outbound proxies to the normal internet will be monitored, disabled, or even taken over to attempt more malicious attacks.
The network itself is message oriented - it is essentially a secure and anonymous IP layer, where messages are addressed to cryptographic keys (Destinations) and can be significantly larger than IP packets. Some example uses of the network include "eepsites" (webservers hosting normal web applications within I2P), a BitTorrent port ("I2PSnark"), or a distributed data store. With the help of mihi's I2PTunnel application, we are able to stream traditional TCP/IP applications over I2P, such as SSH, irc, a squid proxy, and even streaming audio. Most people will not use I2P directly, or even need to know they're using it. Instead their view will be of one of the I2P enabled applications, or perhaps as a little controller app to turn on and off various proxies to enable the anonymizing functionality.
An essential part of designing, developing, and testing an anonymizing network is to define the threat model, since there is no such thing as "true" anonymity, just increasingly expensive costs to identify someone. Briefly, I2P's intent is to allow people to communicate in arbitrarily hostile environments by providing militant grade anonymity, mixed in with sufficient cover traffic provided by the activity of people who require less anonymity. This includes letting Joe Sixpack chat with his buddies without anyone listening in, Jane Filetrader to share files knowing that large corporations cannot identify her, as well as Will Whistleblower to publish sensitive documents - all on the same network, where each one's messages are essentially indistinguishable from the others.
There are a multitude of fantastic reasons why we need a system to support anonymous communication, and everyone has their own personal rationale. There have are many other efforts working on finding ways to provide varying degrees of anonymity to people through the Internet, but we could not find any that met our needs or threat model.
The network at a glance is made up of a set of nodes (“routers”) with a number of unidirectional inbound and outbound virtual paths ("tunnels", as outlined on Tunnel Routing). Each router is identified by a cryptographic RouterIdentity which is typically long lived. These routers communicate with each other through existing transport mechanisms (e.g. TCP, UDP, PHTTP), passing various messages. Client applications have their own cryptographic identifier ("Destination") which enables it to send and receive messages. These clients can connect to any router and authorize the temporary allocation ("lease") of some tunnels that will be used for sending and receiving messages through the network. I2P has its own internal network database (using a modification of the Kademlia algorithm) for distributing routing and contact information.
The following illustration is a simplistic view regarding how tunnels, routers, and destinations are associated, with a pair of inbound tunnels (pink lines) leading towards the router which the destination is connected to (little "a"), with a set of outbound tunnels (green lines) leading away from that router. The gateway for each of the inbound tunnels (big "A"s) are identified in the lease as published in the network database. Additional topological information can be found here.
Messages send from one Destination to another are (typically) sent through a pair of tunnels - first they go out one of the local router's outbound tunnels with instructions specifying that the outbound tunnel's end point forward the message to one of the target Destination's inbound tunnel gateways, which, on receiving the message, passes it down the tunnel to the recipient. Various mechanisms are used to secure these tunnel routed messages, as described tunnel routing. In addition, they can be sent in parallel down multiple tunnels, with the last router discarding duplicates.
Some of the messages sent in the network (such as those used to manage tunnels, publish some Leases, and deliver long lived end to end messages) may be instead are sent via garlic routing. Inspired by a subsection for future works written by Michael Freedman within Roger Dingledine's freehaven thesis, and with some similarities to onion routing. I2P's garlic routing allows multiple messages ("cloves") to be wrapped inside a single message ("garlic") so that only the intended recipient can determine how many cloves are contained as well as how those cloves should be processed.
To deal with a wide range of attacks, I2P is fully distributed with no centralized resources - and hence there are no directory servers keeping statistics regarding the performance and reliability of routers within the network. As such, each router must keep and maintain profiles of various routers and is responsible for selecting appropriate peers to meet the anonymity, performance, and reliability needs of the users, as described in the peer selection pages
The network itself makes use of a significant number of cryptographic techniques and altorithms - a full laundry list includes 2048bit ElGamal encryption, 256bit AES in CBC mode with PKCS#5 padding, 1024bit DSA signatures, SHA256 hashes, 2048bit Diffie-Hellman negotiated connections with station to station authentication, and ElGamal / AES+SessionTag.
Content sent over I2P is encrypted through three or four layers - end to end encryption (absolutely no routers get the plaintext, ever), garlic encryption (used to verify the delivery of the message to the recipient), tunnel encryption (all messages passing through a tunnel is encrypted by the tunnel gateway to the tunnel endpoint), and interrouter transport layer encryption (e.g. the TCP transport uses AES256 with ephemeral keys):
The specific use of these algorithms are outlined elsewhere
The two main mechanisms for allowing people who need militant grade anonymity use the network are explicitly delayed garlic routed messages and more comprehensive tunnels to include support for pooling and mixing messages. These are currently planned for release 3.0, but garlic routed messages with no delays and FIFO tunnels are currently in place. Additionally, the 2.0 release will allow people to set up and operate behind restricted routes (perhaps with trusted peers), as well as the deployment of more flexible and anonymous transports.
Some questions have been raised with regards to the scalability of I2P, and reasonably so. There
will certainly be more analysis over time, but peer lookup and integration should be bounded by
O(log(N)) due to the network database's algorithm, while end to end
messages should be O(log(1)) (scale free), since messages go out K hops through the outbound
tunnel and another K hops through the inbound tunnel - the size of the network (N) bears no impact.
I2P initially began in Feb 2003 as a proposed modification to freenet to allow it to use alternate transports, such as JMS, then grew into its own as an 'anonCommFramework' in April 2003, turning into I2P in July, with code being cut in earnest in August, reaching the 0.2 release in September and 0.3 in March. I2P is currently moving forward according to the roadmap.
The network itself is not ready for general use, and should not be used by those who need anonymity until it has been met with sufficient peer review.
We have a small team spread around several continents, working to advance different aspects of the project. We are very open to other developers who want to get involved and anyone else who would like to contribute in other ways, such as critiques, peer review, testing, writing I2P enabled applications, or documentation. The entire system is open source - the router and most of the SDK are outright public domain with some BSD and Cryptix licensed code, while some applications like I2PTunnel and I2PSnark are GPL. Almost everything is written in Java (1.3+), though some third party applications are being written in Python. The code generally works on Kaffe, and we are hoping to get it working on GCJ sooner rather than later.
Anyone interested should subscribe to the mailing list and join us on the irc channel #i2p (hosted concurrently on IIP, irc.freenode.net, irc.duck.i2p, and irc.baffled.i2p). Weekly development meetings are held there every Tuesday at 9pm GMT with archives available.
The current source is available through an anonymous CVS repository
cvs -d :pserver:anoncvs@i2p.net/cvsroot co i2p
with the password anoncvs.