Since the academic community seems to be far more aware of Tor than I2P, it may be helpful to compare the two systems and cover some of the basics concerning how I2P works. Both Tor and I2P use layered cryptography so that intermediates cannot decipher the contents of connections beyond what they need to know to forward the connection on to the next hop in the chain. Rather than focusing on anonymous access to the public Internet, I2P's core design goal is to allow the anonymous hosting of services (similar in concept to Tor Hidden Services). It does provide proxied access to the public Internet via what are referred to as "out proxies", as well as various internal services to proxy out onto the Tor and Freenet systems, but that is not its core design goal.
Every I2P node is also generally a router (and you can use the terms somewhat interchangeably when it comes to I2P) so there is not a clear distinction between a server and a mere client like there is with the Tor network. Some I2P nodes do take on more responsibility than others, such as floodfill routers that participate in NetDB to handle routing information. Unlike Tor, I2P does not use centralized directory servers to connect nodes, but instead utilizes a DHT (Distributed Hash Table), based on Kademlia, referred to as NetDB. This distributed system helps to eliminate a single point of failure, and stems off blocking attempts similar to what happened to Tor when China blocked access to the core directory servers on September 25th 2009. I2P's reliance on a peer to peer system for distributing routing information does open up more avenues for Sybil attacks and rogue peers, but steps have been taken to help mitigate this and are covered in the documentation.
Instead of referring to other routers and services by their IP, I2P uses cryptographical identifiers to specify both routers and end point services. For example the identifier for "www.i2p2.i2p", the project's main website internal to the I2P network, is:
-KR6qyfPWXoN~F3UzzYSMIsaRy4udcRkHu2Dx9syXSz
UQXQdi2Af1TV2UMH3PpPuNu-GwrqihwmLSkPFg4fv4y
QQY3E10VeQVuI67dn5vlan3NGMsjqxoXTSHHt7C3nX3
szXK90JSoO~tRMDl1xyqtKm94-RpIyNcLXofd0H6b02
683CQIjb-7JiCpDD0zharm6SU54rhdisIUVXpi1xYgg
2pKVpssL~KCp7RAGzpt2rSgz~RHFsecqGBeFwJdiko-
6CYW~tcBcigM8ea57LK7JjCFVhOoYTqgk95AG04-hfe
hnmBtuAFHWklFyFh88x6mS9sbVPvi-am4La0G0jvUJw
9a3wQ67jMr6KWQ~w~bFe~FDqoZqVXl8t88qHPIvXelv
Ww2Y8EMSF5PJhWw~AZfoWOA5VQVYvcmGzZIEKtFGE7b
gQf3rFtJ2FAtig9XXBsoLisHbJgeVb29Ew5E7bkwxvE
e9NYkIqvrKvUAt1i55we0Nkt6xlEdhBqg6xXOyIAAAA
This is the base64 representation of the destination. Obviously having a user type in this 516 byte chunk of data as an Identifier would be somewhat less than user-friendly, and it would not be valid in some protocols anyway (HTTP for example). I2P provides some workarounds for naming identifiers; one is called "Base 32 Names", similar in many ways to Tor's .onion naming convention. Essential the 516 byte Identifier is decoded (with some character replacements) into its raw value, the value is hashed with SHA256, then this hash is base 32 encoded and ".b32.i2p" is concatenated onto the end. The results for the "www.i2p2.i2p" identifier shown above would be:
rjxwbsw4zjhv4zsplma6jmf5nr24e4ymvvbycd3swgiinbvg7oga.b32.i2p
This form is much easier to work with. For most eepSite users the common naming solution is to just use the local I2P address book that maps a simple name like "www.i2p2.i2p" to its much longer Base 64 identifier. There is no official DNS like service to do this lookup as that would be a single point of failure that the I2P project wishes to avoid. Each I2P node has its own series of text files that contain the name mappings in much the same way that the Internet used to use just HOSTS files to translate names to IPs before DNS was invented. There are however naming subscription services inside of I2P that can be synced to if the user wishes, though this means the user is putting some level of trust in these services not to hijack the name mappings.
A router's ID is not the same as a service's ID, so even if the service happens to be running on a particular router the two identifiers cannot be easily tied together. I2P also uses a few techniques to help mitigate traffic correlation attacks. While the Tor network uses a single changing path for communications, I2P uses the concept of "in" and "out" tunnels so requests and responses are not necessarily using the same paths for exchanging information. I2P also uses an Onion routing variant referred to as Garlic routing, where more than one message is bundled together into a "clove". This mixing of messages using Garlic routing can lead to confusion for attackers attempting to correlate transmission sizes and timings, and if "cloves" are composed of messages from both high latency tolerant applications (e.g. email) and low latency applications (e.g. web traffic) correlation could become even harder. More comparisons between I2P, Tor and other anonymity networks can be found on I2P's "I2P Compared to Other Anonymous Networks" page.
Many services can be hosted inside of the I2P overlay network (IRC, Bittorent, eDonkey, Email, etc.), and the I2P team has provided an API for creating new applications that ride on top of the I2P overlay network. As the developers note on their page, many standard Internet applications are not designed with anonymity in mind, so caution should be taken when adapting an existing application to run on top of I2P. While many applications exist and could be researched for application data leaks, this paper will be concentrating on eepSites which are websites internal to I2P. Some measures are taken by the default I2P install to help filter revealing information at the application level, but service providers do make mistakes that can lead to too much information being revealed.
