Tor (network)

Tor is a free overlay network for enabling anonymous communication. It is built on free and open-source software run by over seven thousand volunteer-operated relays worldwide, as well as by millions of users who route their internet traffic via random paths through these relays. This technique is called onion routing. Using Tor makes it more difficult to trace a user's internet activity by preventing any single point on the internet (other than the user's device) from being able to view both where traffic originated from and where it is ultimately going to at the same time. This conceals a user's location and usage from anyone performing network surveillance or traffic analysis from any such point, protecting the user's freedom and ability to communicate confidentially.

Tor is a free overlay network for enabling anonymous communication. It is built on free and open-source software run by over seven thousand volunteer-operated relays worldwide, as well as by millions of users who route their internet traffic via random paths through these relays. This technique is called onion routing. Using Tor makes it more difficult to trace a user's internet activity by preventing any single point on the internet (other than the user's device) from being able to view both where traffic originated from and where it is ultimately going to at the same time. This conceals a user's location and usage from anyone performing network surveillance or traffic analysis from any such point, protecting the user's freedom and ability to communicate confidentially. ==History== The core principle of Tor, known as onion routing, was developed in the mid-1990s by United States Naval Research Laboratory employees, mathematician Paul Syverson, and computer scientists Michael G. Reed and David Goldschlag, to protect American intelligence communications online. Onion routing is implemented by means of encryption in the application layer of the communication protocol stack, nested like the layers of an onion. The alpha version of Tor, developed by Syverson and computer scientists Roger Dingledine and Nick Mathewson and then called The Onion Routing project (which was later given the acronym "Tor"), was launched on 20 September 2002. The first public release occurred a year later. In 2004, the Naval Research Laboratory released the code for Tor under a free license, and the Electronic Frontier Foundation (EFF) began funding Dingledine and Mathewson to continue its development. In 2006, Dingledine, Mathewson, and five others founded The Tor Project, a Massachusetts-based 501(c)(3) research-education nonprofit organization responsible for maintaining Tor. The EFF acted as The Tor Project's fiscal sponsor in its early years, and early financial supporters included the U.S. Bureau of Democracy, Human Rights, and Labor and International Broadcasting Bureau, Internews, Human Rights Watch, the University of Cambridge, Google, and Netherlands-based Stichting NLnet. illustrating Tor usage in 2015]] Over the course of its existence, various Tor vulnerabilities have been discovered and occasionally exploited. Attacks against Tor are an active area of academic research that is welcomed by The Tor Project itself. ==Operation== Tor aims to conceal its users' identities and their online activity from surveillance and traffic analysis by separating identification and routing. It is an implementation of onion routing, which encrypts and then randomly bounces communications through a network of relays run by volunteers around the globe. These onion routers employ encryption in a multi-layered manner (hence the onion metaphor) to ensure perfect forward secrecy between relays, thereby providing users with anonymity in a network location. That anonymity extends to the hosting of censorship-resistant content by Tor's anonymous onion service feature. Furthermore, by keeping some of the entry relays (bridge relays) secret, users can evade Internet censorship that relies upon blocking public Tor relays. Because the IP address of the sender and the recipient are not both in cleartext at any hop along the way, anyone eavesdropping at any point along the communication channel cannot directly identify both ends. Furthermore, to the recipient, it appears that the last Tor node (called the exit node), rather than the sender, is the originator of the communication. ===Originating traffic=== A Tor user's SOCKS-aware applications can be configured to direct their network traffic through a Tor instance's SOCKS interface, which is listening on TCP port 9050 (for standalone Tor) or 9150 (for Tor Browser bundle) at localhost. Tor periodically creates virtual circuits through the Tor network through which it can multiplex and onion-route that traffic to its destination. Once inside a Tor network, the traffic is sent from router to router along the circuit, ultimately reaching an exit node at which point the original (potentially cleartext) packet is available and is forwarded on to its original destination. Viewed from the destination, the traffic appears to originate at the Tor exit node. Tor's application independence sets it apart from most other anonymity networks: it works at the Transmission Control Protocol (TCP) stream level. Applications whose traffic is commonly anonymized using Tor include Internet Relay Chat (IRC), instant messaging, and World Wide Web browsing. ===Onion services=== .]] Tor can also provide anonymity to websites and other servers. Servers configured to receive inbound connections by connecting to Tor are called onion services (formerly, hidden services). Rather than revealing a server's IP address (and thus its network location), an onion service is accessed through its onion address, usually via the Tor Browser or some other software designed to use Tor. The Tor network understands these addresses by looking up their corresponding public keys and introduction points from a distributed hash table within the network. It can route data to and from onion services, even those hosted behind firewalls or network address translators (NAT), while preserving the anonymity of both parties. Tor is necessary to access these onion services. Because the connection never leaves the Tor network, and is handled by the Tor application on both ends, the connection is always end-to-end encrypted. Onion services were first specified in 2003 and have been deployed on the Tor network since 2004. They are unlisted by design, and can only be discovered on the network if the onion address is already known, though a number of sites and services do catalog publicly known onion addresses. Popular sources of .onion links include Pastebin, Twitter, Reddit, other Internet forums, and tailored search engines. While onion services are often discussed in terms of websites, they can be used for any TCP service, and are commonly used for increased security or easier routing to non-web services, such as secure shell remote login, chat services such as IRC and XMPP, or file sharing. They have also become a popular means of establishing peer-to-peer connections in messaging and file sharing applications. Web-based onion services can be accessed from a standard web browser without client-side connection to the Tor network using services like Tor2web, which remove client anonymity. In 2023, the Tor Project unveiled a new defense mechanism to safeguard onion services against denial-of-service (DoS) attacks. With the release of Tor 0.4.8, this proof-of-work (PoW) defense promises to prioritize legitimate network traffic while deterring malicious attacks. ==Attacks and limitations== Like all software with an attack surface, Tor's protections have limitations, and Tor's implementation or design have been vulnerable to attacks at various points throughout its history. While most of these limitations and attacks are minor, either being fixed without incident or proving inconsequential, others are more notable. ===End-to-end traffic correlation=== Tor is designed to provide relatively high performance network anonymity against an attacker with a single vantage point on the connection (e.g., control over one of the three relays, the destination server, or the user's internet service provider). Like all current low-latency anonymity networks, Tor cannot and does not attempt to protect against an attacker performing simultaneous monitoring of traffic at the boundaries of the Tor network—i.e., the traffic entering and exiting the network. While Tor does provide protection against traffic analysis, it cannot prevent traffic confirmation via end-to-end correlation. There are no documented cases of this limitation being used at scale; as of the 2013 Snowden leaks, law enforcement agencies such as the NSA were unable to perform dragnet surveillance on Tor itself, and relied on attacking other software used in conjunction with Tor, such as vulnerabilities in web browsers. However, targeted attacks have been able to make use of traffic confirmation on individual Tor users, via police surveillance or investigations confirming that a particular person already under suspicion was sending Tor traffic at the exact times the connections in question occurred. The relay early traffic confirmation attack also relied on traffic confirmation as part of its mechanism, though on requests for onion service descriptors, rather than traffic to the destination server. ===Consensus attacks=== Like many decentralized systems, Tor relies on a consensus mechanism to periodically update its current operating parameters. For Tor, these include network parameters like which nodes are good and bad relays, exits, guards, and how much traffic each can handle. Tor's architecture for deciding the consensus relies on a small number of directory authority nodes voting on current network parameters. Currently, there are nine directory authority nodes, and their health is publicly monitored. The IP addresses of the authority nodes are hard coded into each Tor client. The authority nodes vote every hour to update the consensus, and clients download the most recent consensus on startup. A compromise of the majority of the directory authorities could alter the consensus in a way that is beneficial to an attacker. Alternatively, a network congestion attack, such as a DDoS, could theoretically prevent the consensus nodes from communicating, and thus prevent voting to update the consensus (though such an attack would be visible). ===Server-side restrictions=== , notifying that the website is blocking Tor traffic]] Tor makes no attempt to conceal the IP addresses of exit relays, or hide from a destination server the fact that a user is connecting via Tor. Operators of Internet sites therefore have the ability to prevent traffic from Tor exit nodes or to offer reduced functionality for Tor users. For example, Wikipedia generally forbids all editing when using Tor or when using an IP address also used by a Tor exit node, and the BBC blocks the IP addresses of all known Tor exit nodes from its iPlayer service. Apart from intentional restrictions of Tor traffic, Tor use can trigger defense mechanisms on websites intended to block traffic from IP addresses observed to generate malicious or abnormal traffic. Because traffic from all Tor users is shared by a comparatively small number of exit relays, tools can misidentify distinct sessions as originating from the same user, and attribute the actions of a malicious user to a non-malicious user, or observe an unusually large volume of traffic for one IP address. Conversely, a site may observe a single session connecting from different exit relays, with different Internet geolocations, and assume the connection is malicious, or trigger geo-blocking. When these defense mechanisms are triggered, it can result in the site blocking access, or presenting captchas to the user. ===Vulnerable applications=== Many attacks targeted at Tor users result from flaws in applications used with Tor, either in the application itself, or in how it operates in combination with Tor. E.g., researchers with Inria in 2011 performed an attack on BitTorrent users by attacking clients that established connections both using and not using Tor, then associating other connections shared by the same Tor circuit. ====Fingerprinting==== When using Tor, applications may still provide data tied to a device, such as information about screen resolution, installed fonts, language configuration, or supported graphics functionality, reducing the set of users a connection could possibly originate from, or uniquely identifying them. This information is known as the device fingerprint, or browser fingerprint in the case of web browsers. Applications implemented with Tor in mind, such as Tor Browser, can be designed to minimize the amount of information leaked by the application and reduce its fingerprint. ====Eavesdropping==== Tor cannot encrypt the traffic between an exit relay and the destination server. If an application does not add an additional layer of end-to-end encryption between the client and the server, such as Transport Layer Security (TLS, used in HTTPS) or the Secure Shell (SSH) protocol, this allows the exit relay to capture and modify traffic. Attacks from malicious exit relays have recorded usernames and passwords, and modified Bitcoin addresses to redirect transactions. Some of these attacks involved actively removing the HTTPS protections that would have otherwise been used. To attempt to prevent this, Tor Browser has since made it so only connections via onion services or HTTPS are allowed by default. ====Onion service configuration==== Like client applications that use Tor, servers relying on onion services for protection can introduce their own weaknesses. Servers that are reachable through Tor onion services and the public Internet can be subject to correlation attacks, and all onion services are susceptible to misconfigured services (e.g., identifying information included by default in web server error responses), leaking uptime and downtime statistics, intersection attacks, or various user errors. The OnionScan program, written by independent security researcher Sarah Jamie Lewis, comprehensively examines onion services for such flaws and vulnerabilities. ==Software== The main implementation of Tor is written primarily in C. Starting in 2020, the Tor Project began development of a full rewrite of the C Tor codebase in Rust. The project, named Arti, was publicly announced in July 2021. ===Tor Phone=== In 2016, Tor developer Mike Perry announced a prototype Tor-enabled smartphone based on CopperheadOS. It was meant as a direction for Tor on mobile. The project was called 'Mission Improbable'. Copperhead's then lead developer Daniel Micay welcomed the prototype. ===Third-party applications=== The Vuze (formerly Azureus) BitTorrent client, Bitmessage anonymous messaging system, and TorChat instant messenger include Tor support. The Briar messenger routes all messaging via Tor by default. OnionShare allows users to share files using Tor. The Guardian Project is actively developing a free and open-source suite of applications and firmware for the Android operating system to improve the security of mobile communications. The applications include the ChatSecure instant messaging client, Orbot Tor implementation (also available for iOS), Orweb (discontinued) privacy-enhanced mobile browser, Orfox, the mobile counterpart of the Tor Browser, ProxyMob Firefox add-on, and ObscuraCam. Onion Browser is an open-source, privacy-enhancing web browser for iOS, which uses Tor. It is available in the iOS App Store, and source code is available on GitHub. Brave added support for Tor into its desktop browser's private-browsing mode in 2018, with the release of version 0.23. File:Orbot-logo.svg|Orbot logo File:Orbot, running under Firefox.png|Orbot 17.5.0, running under Firefox for Android File:Onion Browser on iPad.png|Onion Browser on iPad ===Security-focused operating systems=== In September 2024, it was announced that Tails, a security-focused operating system, had become part of the Tor Project. Other security-focused operating systems that make or made extensive use of Tor include Hardened Linux From Scratch, Incognito, Liberté Linux, Qubes OS, Subgraph, Parrot OS, Tor-ramdisk, and Whonix. In July 2015, the Tor Project announced an alliance with the Library Freedom Project to establish exit nodes in public libraries. The pilot program, which established a middle relay running on the excess bandwidth afforded by the Kilton Library in Lebanon, New Hampshire, making it the first library in the U.S. to host a Tor node, was briefly put on hold when the local city manager and deputy sheriff voiced concerns over the cost of defending search warrants for information passed through the Tor exit node. Although the Department of Homeland Security (DHS) had alerted New Hampshire authorities to the fact that Tor is sometimes used by criminals, the Lebanon Deputy Police Chief and the Deputy City Manager averred that no pressure to strong-arm the library was applied, and the service was re-established on 15 September 2015. U.S. Rep. Zoe Lofgren (D-Calif) released a letter on 10 December 2015, in which she asked the DHS to clarify its procedures, stating that "While the Kilton Public Library's board ultimately voted to restore their Tor relay, I am no less disturbed by the possibility that DHS employees are pressuring or persuading public and private entities to discontinue or degrade services that protect the privacy and anonymity of U.S. citizens." In a 2016 interview, Kilton Library IT Manager Chuck McAndrew stressed the importance of getting libraries involved with Tor: "Librarians have always cared deeply about protecting privacy, intellectual freedom, and access to information (the freedom to read). Surveillance has a very well-documented chilling effect on intellectual freedom. It is the job of librarians to remove barriers to information." The second library to host a Tor node was the Las Naves Public Library in Valencia, Spain, implemented in the first months of 2016. In August 2015, an IBM security research group, called "X-Force", put out a quarterly report that advised companies to block Tor on security grounds, citing a "steady increase" in attacks from Tor exit nodes as well as botnet traffic. In September 2015, Luke Millanta created OnionView (now defunct), a web service that plots the location of active Tor relay nodes onto an interactive map of the world. The project's purpose was to detail the network's size and escalating growth rate. In December 2015, Daniel Ellsberg (of the Pentagon Papers), Cory Doctorow (of Boing Boing), Edward Snowden, and artist-activist Molly Crabapple, amongst others, announced their support of Tor. ==See also== Anonymous P2P Anonymous web browsing Crypto-anarchism Darknet Deep web Freedom of information Freenet GNUnet I2P Internet censorship circumvention Internet privacy Signal Privacy software Privoxy Proxy server Psiphon Tor Phone ZeroNet ==External links==

Text derived from https://en.wikipedia.org/w/index.php?oldid=1375292344 under CC BY-SA 4.0; modifications have been made.