I2P

The Invisible Internet Project (I2P) is an anonymous network layer (implemented as a Mix Network) that allows for censorship resistant, peer to peer communication. Anonymous connections are achieved by encrypting the user's traffic (by using end-to-end encryption), and sending it through a volunteer-run network of roughly 55,000 computers distributed around the world. Given the high number of possible paths the traffic can transit, a third party watching a full connection is unlikely. The software that implements this layer is called an "I2P router", and a computer running I2P is called an "I2P node". I2P is free and open source, and is published under multiple licenses.
Technical design.
I2P is beta software since 2003, when it started as a fork of Freenet. The software's developers emphasize that there are likely to be bugs in the beta version and that there has been insufficient peer review to date. However, they believe the code is now reasonably stable and well-developed, and more exposure can help the development of I2P.
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) through garlic routing, and even the end points ("destinations") are cryptographic identifiers (essentially a pair of public keys), so that neither sender nor recipient of a message need to reveal their IP address to the other side or to third-party observers.
Although many developers had been a part of the "Invisible IRC Project (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. I2P is an anonymous peer-to-peer distributed communication layer designed to run "any" traditional internet service (e.g. Usenet, email, 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, or DNS).
Many developers of I2P are known only under pseudonyms. While the previous main developer, "jrandom", is currently on hiatus, others, such as "zzz", "killyourtv", and "Complication" have continued to lead development efforts, and are assisted by numerous contributors.
I2P uses 2048bit ElGamal/AES256/SHA256+Session Tags encryption and Ed25519 EdDSA/ECDSA signatures.
Releases.
I2P has had a stable release every six to eight weeks. Updates are distributed via I2P torrents and are signed by the release manager (generally "zzz" or "str4d").
Software.
Since I2P is an anonymous network layer, it is designed so other software can use it for anonymous communication. As such, there are a variety of tools currently available for I2P or in development.
The I2P router is controlled through the router console, which is a web frontend accessed through a web browser.
File sharing.
Bridging to Clearnet.
Currently, Vuze and BiglyBT are the only torrent clients that makes clearnet (connections not through I2P) torrents available on I2P and vice versa, by using a plugin that connects them to the I2P network. Depending on the client settings, torrents from the internet can be made available on I2P (via announcements to I2P's DHT network) and torrents from I2P can be made available to the internet. For this reason, torrents previously published only on I2P can be made available to the entire Internet, and users of I2P can often download popular content from the Internet while maintaining the anonymity of I2P.
The Privacy Solutions project.
The Privacy Solutions project, a new organization that develops and maintains I2P software, launched several new development efforts designed to enhance the privacy, security, and anonymity for users, based on I2P protocols and technology.
These efforts include:
The code repository and download sections for the i2pd and Abscond project is available for the public to review and download.
Effective January, 2015 i2pd is operating under PurpleI2P.
Cryptocurrency.
Some cryptocurrencies that support I2P are listed below.
Vulnerabilities.
Denial of service attacks are possible against websites hosted on the network, though a site operator may secure their site against certain versions of this type of attack to some extent.
A zero-day vulnerability was discovered for I2P in 2014, and was exploited to de-anonymize at least 30,000 users. This included users of the operating system Tails. This vulnerability was later patched.
A 2017 study examining how forensic investigators might exploit vulnerabilities in I2P software to gather useful evidence indicated that a seized machine which had been running I2P router software may hold unencrypted local data that could be useful to law enforcement. Records of which eepsites a user of a later-seized machine was interested in may also be inferred. The study identified a "trusted" I2P domain registrar ("NO.i2p") which appeared to have been abandoned by its administrator, and which the study identified as a potential target for law enforcement takeover. It alternatively suggested waiting for NO.i1p's server to fail, only to social engineer the I2P community into moving to a phony replacement. Another suggestion the study proposed was to register a mirror version of a target website under an identical domain.
I2PCon.
From the first I2P convention was held in Toronto, Ontario. The conference was hosted by a local hackerspace, Hacklab. The conference featured presentations from I2P developers and security researchers.
 mainly had presentations on the past growth of the I2P network, a talk on what happens when companies sell people's personal information, and a round-table discussion on general privacy and security topics. The day ended with a CryptoParty, which helped to introduce new users to installing I2P, sending secure emails with I2P-Bote, and using I2P along with Vuze.
 had more technical discussions than the previous day. The talks focused on how to dissuade bad-actors from using the network, how I2P has worked computer connection limits, how to do application development using I2P, and the development of the Android version. This day ended with a development meeting.
Cultural references.
In "House of Cards" season 2 episode 2, I2P is referenced.