Attacks against Tor are an active area of academic research   which is welcomed by the Tor Project itself. Tor enables its users to surf the Internet, chat and send instant messages anonymously, and is used by a wide variety of people for both licit and illicit purposes. Tor is not meant to completely solve the issue of anonymity on the web. Tor is not designed to completely erase tracks but instead to reduce the likelihood for sites to trace actions and data back to the user.
Tor is also used for illegal activities. These can include privacy protection or censorship circumvention,  as well as distribution of child abuse content, drug sales, or malware distribution. Tor has been described by The Economist , in relation to Bitcoin and Silk Road , as being "a dark corner of the web". They hear how somebody got away with downloading child porn. The Tor Project states that Tor users include "normal people" who wish to keep their Internet activities private from websites and advertisers, people concerned about cyber-spying, users who are evading censorship such as activists, journalists, and military professionals.
As of November [update] , Tor had about four million users. In March the Parliamentary Office of Science and Technology released a briefing which stated that "There is widespread agreement that banning online anonymity systems altogether is not seen as an acceptable policy option in the U. Criminals can already do bad things. Tor aims to provide protection for ordinary people who want to follow the law.
Only criminals have privacy right now, and we need to fix that So yes, criminals could in theory use Tor, but they already have better options, and it seems unlikely that taking Tor away from the world will stop them from doing their bad things. At the same time, Tor and other privacy measures can fight identity theft, physical crimes like stalking, and so on.
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.
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.
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 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 can also provide anonymity to websites and other servers. Servers configured to receive inbound connections only through Tor are called onion services formerly, hidden services.
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. Onion services were first specified in  and have been deployed on the Tor network since Because onion services route their traffic entirely through the Tor network, connection to an onion service is encrypted end-to-end and not subject to eavesdropping.
There are, however, security issues involving Tor onion services. For example, services that are reachable through Tor onion services and the public Internet are susceptible to correlation attacks and thus not perfectly hidden. Other pitfalls include poorly configured services e. Onion services can also be accessed from a standard web browser without client-side connection to the Tor network, using services like Tor2web.
It runs on any platform supported by curses including Linux , macOS , and other Unix-like variants. The project began in the summer of ,   and since 18 July it has been an official part of the Tor Project. Like all current low-latency anonymity networks , Tor cannot and does not attempt to protect against monitoring of traffic at the boundaries of the Tor network i. While Tor does provide protection against traffic analysis , it cannot prevent traffic confirmation also called end-to-end correlation.
In spite of known weaknesses and attacks listed here, a study revealed Tor and the alternative network system JonDonym Java Anon Proxy, JAP are considered more resilient to website fingerprinting techniques than other tunneling protocols. The reason for this is conventional single-hop VPN protocols do not need to reconstruct packet data nearly as much as a multi-hop service like Tor or JonDonym.
Currently, there are ten 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. If an autonomous system AS exists on both path segments from a client to entry relay and from exit relay to destination, such an AS can statistically correlate traffic on the entry and exit segments of the path and potentially infer the destination with which the client communicated.
In , LASTor proposed a method to predict a set of potential ASes on these two segments and then avoid choosing this path during the path selection algorithm on the client side. In this paper, they also improve latency by choosing shorter geographical paths between a client and destination. In September , Dan Egerstad, a Swedish security consultant, revealed he had intercepted usernames and passwords for email accounts by operating and monitoring Tor exit nodes.
While this may not inherently breach the anonymity of the source, traffic intercepted in this way by self-selected third parties can expose information about the source in either or both of payload and protocol data. Who would pay for this and be anonymous? In October a tor researcher revealed that since at least there were a more than one hundred of highly suspicious relay nodes that were running on previously unprecedented scale by an unknown group.
In October , a research team from ESIEA claimed to have discovered a way to compromise the Tor network by decrypting communication passing over it. Then, using these known keys and seeds, they claim the ability to decrypt two encryption layers out of three. They claim to break the third key by a statistical attack. In order to redirect Tor traffic to the nodes they controlled, they used a denial-of-service attack.
There are two methods of traffic-analysis attack, passive and active. In the passive traffic-analysis method, the attacker extracts features from the traffic of a specific flow on one side of the network and looks for those features on the other side of the network. In the active traffic-analysis method, the attacker alters the timings of the packets of a flow according to a specific pattern and looks for that pattern on the other side of the network; therefore, the attacker can link the flows in one side to the other side of the network and break the anonymity of it.
Steven Murdoch and George Danezis from University of Cambridge presented an article at the IEEE Symposium on security and privacy on traffic-analysis techniques that allow adversaries with only a partial view of the network to infer which nodes are being used to relay the anonymous streams. Murdoch and Danezis have also shown that otherwise unrelated streams can be linked back to the same initiator.
This attack, however, fails to reveal the identity of the original user. Operators of Internet sites have the ability to prevent traffic from Tor exit nodes or to offer reduced functionality for Tor users. For example, it is not generally possible to edit Wikipedia when using Tor or when using an IP address also used by a Tor exit node. The BBC blocks the IP addresses of all known Tor guards and exit nodes from its iPlayer service, although relays and bridges are not blocked. One method of attack depends on control of an exit node or hijacking tracker responses, while a secondary attack method is based in part on the statistical exploitation of distributed hash table tracking.
The results presented in the bad apple attack research paper are based on an attack launched against the Tor network by the authors of the study. The attack targeted six exit nodes, lasted for twenty-three days, and revealed a total of 10, IP addresses of active Tor users. This study is significant because it is the first documented attack designed to target P2P file-sharing applications on Tor. The study was conducted by monitoring six exit nodes for a period of twenty-three days.
Researches used three attack vectors : . With this technique, researchers were able to identify other streams initiated by users, whose IP addresses were revealed. Jansen et al. The attack works using a colluding client and server, and filling the queues of the exit node until the node runs out of memory, and hence can serve no other genuine clients. By attacking a significant proportion of the exit nodes this way, an attacker can degrade the network and increase the chance of targets using nodes controlled by the attacker.
On 30 July the Tor Project issued the security advisory "relay early traffic confirmation attack" in which the project discovered a group of relays that tried to de-anonymize onion service users and operators. The attacking relays were stable enough to achieve being designated as "suitable as hidden service directory" and "suitable as entry guard"; therefore, both the onion service users and the onion services might have used those relays as guards and hidden service directory nodes.
The attacking nodes joined the network early in the year on 30 January and the project removed them on 4 July. The project mentioned the following mitigations besides removing the attacking relays from the network:. In November there was speculation in the aftermath of Operation Onymous , resulting in 17 arrests internationally, that a Tor weakness had been exploited. A representative of Europol was secretive about the method used, saying: " This is something we want to keep for ourselves.
Andrew Lewman—a Tor Project representative—downplayed this possibility, suggesting that execution of more traditional police work was more likely. In November court documents on the matter  addressed concerns about security research ethics  [ non-primary source needed ] and the right of not being unreasonably searched as guaranteed by the US Fourth Amendment.
In , the administrators of Agora , a darknet market , announced they were taking the site offline in response to a recently discovered security vulnerability in Tor. They did not say what the vulnerability was, but Wired speculated it was the "Circuit Fingerprinting Attack" presented at the Usenix security conference.
A study showed "anonymization solutions protect only partially against target selection that may lead to efficient surveillance" as they typically "do not hide the volume information necessary to do target selection".
In April , the Tor Project shut down the Tor Messenger project because the developers of Instantbird discontinued support for their own software. OnionShare allows 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.
Onion Browser  is open-source, privacy-enhancing web browser for iOS , which uses Tor. Several security-focused operating systems make extensive use of Tor. Tor has been praised for providing privacy and anonymity to vulnerable Internet users such as political activists fearing surveillance and arrest, ordinary web users seeking to circumvent censorship, and people who have been threatened with violence or abuse by stalkers.
National Security Agency NSA has called Tor "the king of high-secure, low-latency Internet anonymity",  and BusinessWeek magazine has described it as "perhaps the most effective means of defeating the online surveillance efforts of intelligence agencies around the world". Advocates for Tor say it supports freedom of expression , including in countries where the Internet is censored, by protecting the privacy and anonymity of users. The mathematical underpinnings of Tor lead it to be characterized as acting "like a piece of infrastructure , and governments naturally fall into paying for infrastructure they want to use".
The project was originally developed on behalf of the U. State Department , the Broadcasting Board of Governors , and the National Science Foundation as major contributors,  aiming "to aid democracy advocates in authoritarian states". Naval Research Laboratory , and the Government of Sweden. Tor executive director Andrew Lewman said that even though it accepts funds from the U. Critics say that Tor is not as secure as it claims,  pointing to U.
The citation read, "Using free software, Tor has enabled roughly 36 million people around the world to experience freedom of access and expression on the Internet while keeping them in control of their privacy and anonymity. Its network has proved pivotal in dissident movements in both Iran and more recently Egypt.
In , Foreign Policy magazine named Dingledine, Mathewson, and Syverson among its Top Global Thinkers "for making the web safe for whistleblowers". In , Jacob Appelbaum described Tor as a "part of an ecosystem of software that helps people regain and reclaim their autonomy.
It helps to enable people to have agency of all kinds; it helps others to help each other and it helps you to help yourself. It runs, it is open and it is supported by a large community spread across all walks of life. In September , in response to reports that Comcast had been discouraging customers from using the Tor Browser, Comcast issued a public statement that "We have no policy against Tor, or any other browser or software.
In October , The Tor Project hired the public relations firm Thomson Communications to improve its public image particularly regarding the terms "Dark Net" and "hidden services," which are widely viewed as being problematic and to educate journalists about the technical aspects of Tor. In July , the Tor Project announced an alliance with the Library Freedom Project to establish exit nodes in public libraries.
Although the 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 Surveillance has a very well-documented chilling effect on intellectual freedom.
It is the job of librarians to remove barriers to information. In August , 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 , Luke Millanta created OnionView, a web service that plots the location of active Tor relay nodes onto an interactive map of the world.
In March , New Hampshire state representative Keith Ammon introduced a bill  allowing public libraries to run privacy software. The bill specifically referenced Tor. The text was crafted with extensive input from Alison Macrina , the director of the Library Freedom Project.
On 16 May , CNN reported on the case of core Tor developer isis agora lovecruft,  who had fled to Germany under the threat of a subpoena by the FBI during the Thanksgiving break of the previous year. The Electronic Frontier Foundation legally represented lovecruft.
On 2 December , The New Yorker reported on burgeoning digital privacy and security workshops in the San Francisco Bay Area , particularly at the hackerspace Noisebridge , in the wake of the United States presidential election ; downloading the Tor browser was mentioned. Tor and Bitcoin was fundamental to the operation of the darkweb marketplace AlphaBay , which was taken down in an international law enforcement operation in July The Russian-speaking underground offering a crime-as-a-service model is regarded as being particularly robust.
In June , Venezuela blocked access to the Tor network. The block affected both direct connections to the network and connections being made via bridge relays. On 20 June , Bavarian police raided the homes of the board members of the non-profit Zwiebelfreunde, a member of torservers. The hardware and documentation seized had been kept under seal, and purportedly were neither analyzed nor evaluated by the Bavarian police.
Since October , Chinese online communities within Tor have begun to dwindle due to increased efforts to stop them by the Chinese government. In November , Edward Snowden called for a full, unabridged simplified Chinese translation of his autobiography, Permanent Record , as the Chinese publisher had violated their agreement by expurgating all mentions of Tor and other matters deemed politically sensitive by the Communist Party of China. Tor responded to earlier vulnerabilities listed above by patching them and improving security.
In one way or another, human user errors can lead to detection. The Tor Project website provides the best practices instructions on how to properly use the Tor browser. When improperly used, Tor is not secure. For example, Tor warns its users that not all traffic is protected; only the traffic routed through the Tor browser is protected.
Users are also warned to use HTTPS versions of websites, not to torrent with Tor, not to enable browser plugins, not to open documents downloaded through Tor while online, and to use safe bridges. The best tactic of law enforcement agencies to de-anonymize users appears to remain with Tor-relay adversaries running poisoned nodes, as well as counting on the users themselves using the Tor browser improperly.
When properly used, odds of being de-anonymized through Tor are said to be extremely low. At present the Tor network offers, out of a total of around 7, relays, around 2, guard entry nodes and around 1, exit nodes.
From Wikipedia, the free encyclopedia. This is the latest accepted revision , reviewed on 28 January Free and open-source anonymity network based on onion routing. This article is about the software and anonymity network. For other uses, see Tor disambiguation. This article may require copy editing for grammar, style, cohesion, tone, or spelling. You can assist by editing it. December Learn how and when to remove this template message. The Tor Project logo.
Further information: Dark web. Infographic about how Tor works, by EFF. See also: List of Tor onion services. This section needs to be updated. Please help update this article to reflect recent events or newly available information. September The neutrality of this section is disputed. Relevant discussion may be found on the talk page.
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Stephen Shankland. May 21, p. Tor is an "onion-routing" network that protects your privacy online.
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Browse Privately. Explore Freely. Defend yourself against tracking and surveillance. Circumvent censorship. Download Tor Browser. Private Browser Incognito is an advanced browser designed to tunnel your web traffic via the TOR Network. The in-built TOR Browser uses. Onion Browser is the original free and open-source Tor-powered web browser for iOS. Onion Browser helps you access the internet with more safety and privacy.