Editing Onion routing (section)

== Data ==

[[File:Tor Circuit Diagram.svg|thumb|alt=A client, represented as a phone, sending traffic to an onion labelled "Guard" with four nested lines, then going to a "Middle" onion with three nested lines, then going to an "Exit" onion with two nested lines, and finally going to the Server with one line.|A diagram of an onion routed connection, using [[Tor (network)|Tor]]'s terminology of guard, middle, and exit relays.]]

Metaphorically, an onion is the data structure formed by "wrapping" a message with successive layers of encryption to be decrypted ("peeled" or "unwrapped") by as many intermediary computers as there are layers before arriving at its destination. The original message remains hidden as it is transferred from one node to the next, and no intermediary knows both the origin and final destination of the data, allowing the sender to remain anonymous.<ref name=tor-design>{{cite web|title=Tor: The Second-Generation Onion Router |url=http://www.onion-router.net/Publications/tor-design.pdf |author=Roger Dingledine |author2=Nick Mathewson |author3=Paul Syverson  |access-date=26 February 2011}}</ref>

=== Onion creation and transmission ===

To create and transmit an onion, the originator selects a set of nodes from a list provided by a "directory node". The chosen nodes are arranged into a path, called a "chain" or "circuit", through which the message will be transmitted. To preserve the anonymity of the sender, no node in the circuit is able to tell whether the node before it is the originator or another intermediary like itself. Likewise, no node in the circuit is able to tell how many other nodes are in the circuit and only the final node, the "exit node", is able to determine its own location in the chain.<ref name=tor-design/>

Using asymmetric key cryptography, the originator obtains a [[public key]] from the directory node to send an encrypted message to the first ("entry") node, establishing a connection and a [[shared secret]] ("session key"). Using the established encrypted link to the entry node, the originator can then relay a message through the first node to a second node in the chain using encryption that only the second node, and not the first, can decrypt. When the second node receives the message, it establishes a connection with the first node. While this extends the encrypted link from the originator, the second node cannot determine whether the first node is the originator or just another node in the circuit. The originator can then send a message through the first and second nodes to a third node, encrypted such that only the third node is able to decrypt it. The third, as with the second, becomes linked to the originator but connects only with the second. This process can be repeated to build larger and larger chains but is typically limited to preserve performance.<ref name=tor-design/>

When the chain is complete, the originator can send data over the Internet anonymously. When the final recipient of the data sends data back, the intermediary nodes maintain the same link back to the originator, with data again layered, but in reverse such that the final node this time adds the first layer of encryption and the first node adds the last layer of encryption before sending the data, for example a web page, to the originator, who is able to decrypt all layers.<ref name=tor-design/>