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Tor (The Onion Router) is a free software
Free software

Free Software or software libre is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with minimal restrictions only to ensure that further recipients can also do these things and to prevent consumer-facing hardware...
 implementation of second-generation onion routing
Onion routing

Onion routing is a technique for anonymity communication over a computer network. Messages are repeatedly encrypted and then sent through several network nodes called onion routers....
 – a system enabling its users to communicate anonymously
Anonymity

Anonymity is derived from the Greek word a??????a, meaning "without a name" or "namelessness". In colloquial use, the term typically refers to a person, and often means that the Identity , or personally identifiable information of that person is not known....
 on the Internet. Roger Dingledine, Nick Mathewson, and Paul Syverson presented "Tor: The Second-Generation Onion Router" at the 13th USENIX
USENIX

The USENIX Association is the Advanced Computing Technical Association. It was founded in 1975 under the name "Unix Users Group," focusing primarily on the study and development of Unix and similar systems....
 Security Symposium.

Originally sponsored by the US Naval Research Laboratory
United States Naval Research Laboratory

The United States Naval Research Laboratory is the corporate research laboratory for the United States Navy and the United States Marine Corps and conducts a broad program of scientific research and advanced development....
, Tor became an Electronic Frontier Foundation
Electronic Frontier Foundation

The Electronic Frontier Foundation is an international non-profit organization advocacy and legal organization based in the United States with the stated purpose of being dedicated to preserving the right to freedom of speech, such as protected by the First Amendment to the United States Constitution, in the context of today's digital age ....
 (EFF) project in late 2004, and the EFF supported Tor financially until November 2005.






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Tor (The Onion Router) is a free software
Free software

Free Software or software libre is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with minimal restrictions only to ensure that further recipients can also do these things and to prevent consumer-facing hardware...
 implementation of second-generation onion routing
Onion routing

Onion routing is a technique for anonymity communication over a computer network. Messages are repeatedly encrypted and then sent through several network nodes called onion routers....
 – a system enabling its users to communicate anonymously
Anonymity

Anonymity is derived from the Greek word a??????a, meaning "without a name" or "namelessness". In colloquial use, the term typically refers to a person, and often means that the Identity , or personally identifiable information of that person is not known....
 on the Internet. Roger Dingledine, Nick Mathewson, and Paul Syverson presented "Tor: The Second-Generation Onion Router" at the 13th USENIX
USENIX

The USENIX Association is the Advanced Computing Technical Association. It was founded in 1975 under the name "Unix Users Group," focusing primarily on the study and development of Unix and similar systems....
 Security Symposium.

Originally sponsored by the US Naval Research Laboratory
United States Naval Research Laboratory

The United States Naval Research Laboratory is the corporate research laboratory for the United States Navy and the United States Marine Corps and conducts a broad program of scientific research and advanced development....
, Tor became an Electronic Frontier Foundation
Electronic Frontier Foundation

The Electronic Frontier Foundation is an international non-profit organization advocacy and legal organization based in the United States with the stated purpose of being dedicated to preserving the right to freedom of speech, such as protected by the First Amendment to the United States Constitution, in the context of today's digital age ....
 (EFF) project in late 2004, and the EFF supported Tor financially until November 2005. Tor software is now developed by the Tor Project, which since December 2006 is a 501(c)(3) research/education non-profit organization
Non-profit organization

A nonprofit organization is any organization that does not aim to make a profit, and which is not a public body....
 based in the United States of America that receives a diverse base of financial support.

Description

Aiming to protect its users against traffic analysis
Traffic analysis

Traffic analysis is the process of intercepting and examining messages in order to deduce information from patterns in communication. It can be performed even when the messages are encrypted and cannot be cryptanalysis....
 attacks, volunteers operate an overlay network of onion routers
Onion routing

Onion routing is a technique for anonymity communication over a computer network. Messages are repeatedly encrypted and then sent through several network nodes called onion routers....
 that enable anonymous outgoing connections and anonymous "hidden" services.

Outgoing connections


Users of Tor network run an onion proxy on their machine. This software connects out to Tor, periodically negotiating a virtual circuit through Tor network. Tor employs cryptography
Cryptography

Cryptography is the practice and study of hiding information. In modern times cryptography is considered a branch of both mathematics and computer science and is affiliated closely with information theory, computer security and engineering....
 in a layered manner (hence the onion
Onion

Onion is a term used for many plants in the genus Allium. They are known by the common name "onion" but, used without qualifiers, it usually refers to Allium cepa....
 analogy), ensuring perfect forward secrecy
Perfect forward secrecy

In an authenticated key-agreement protocol that uses public key cryptography, perfect forward secrecy is the property that ensures that a session key derived from a set of long-term public and private keys will not be compromised if one of the private keys is compromised in the future....
 between routers. At the same time, the onion proxy software presents a SOCKS
SOCKS

SOCKS is an Internet Protocol that facilitates the routing of Packet between client-server applications via a proxy server. SOCKS is an abbreviation for "Secured Over Credential-based Kerberos Services." SOCKS performs at Layer 5 of the OSI model - the Session layer ....
 interface to its clients. SOCKS-aware applications may be pointed at Tor, which then multiplexes
Multiplexing

In telecommunications and computer networks, multiplexing is a process where multiple analog message signals or digital data streams are combined into one signal over a shared medium....
 the traffic through a Tor virtual circuit.

Once inside Tor network, the traffic is sent from router to router, ultimately reaching an exit node at which point the cleartext
Cleartext

In data communications, cleartext is the form of a message or data which is in a form that is immediately comprehensible to a human being without additional processing....
 packet is available and is forwarded on to its original destination. Viewed from the destination, the traffic appears to originate at Tor exit node.

Tor's application independence sets it apart from most other anonymity networks: it works at the TCP
Transmission Control Protocol

The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is so central that the entire suite is often referred to as "TCP/IP"....
 stream level. Applications commonly anonymised using Tor include IRC
Internet Relay Chat

Internet Relay Chat is a form of real-time Internet text messaging or synchronous conferencing. It is mainly designed for Many-to-many in discussion forums, called #Channels, but also allows One-to-one via instant messaging, as well as chat and data transfers via Direct Client-to-Client....
, instant messaging
Instant messaging

Instant messaging is a form of Real-time computing communication between two or more people based on typed text. The Written language is conveyed via devices connected over a network such as the Internet....
 and Web
World Wide Web

The World Wide Web is a very large set of interlinked hypertext documents accessed via the Internet. With a Web browser, one can view Web pages that may contain writing, s, videos, and other multimedia and navigate between them using hyperlinks....
 browsing. When browsing the Web, Tor is often coupled with Privoxy
Privoxy

Privoxy is a web proxy program, frequently used in combination with Tor and Squid cache. It has filtering capabilities for protecting privacy, modifying web page data, managing [], controlling access, and selectively removing content such as ads, banners and pop-ups....
 – a filtering proxy server
Proxy server

In computer networks, a proxy server is a server that acts as a go-between for requests from client seeking resources from other servers. A client connects to the proxy server, requesting some service, such as a file, connection, web page, or other resource, available from a different server....
 – that aims to add privacy
Privacy

Privacy is the ability of an individual or group to seclude themselves or information about themselves and thereby reveal themselves selectively....
 at the application layer.

Hidden services


Tor can also provide anonymity to servers in the form of location-hidden services, which are Tor clients or relays running specially configured server software. Rather than revealing the server's IP address (and therefore its location), hidden services are accessed through Tor-specific .onion
.onion

.onion is a pseudo-top-level domain host suffix designating an Tor #Anonymous hidden services reachable via the Tor network. Such addresses are not actual DNS names, and the .onion TLD is not in the Internet DNS root, but with the appropriate proxy software installed, Internet programs such as Web browsers can access sites with .onion addr...
 pseudo top-level domain
Pseudo top-level domain

A number of pseudo-top-level domains to be used in naming computers have been defined at various times. These "pseudo-TLDs" include .bitnet, .csnet, .exit, .i2p, .local, .onion, .oz, .freenet and .uucp....
. Tor network understands this TLD
Top-level domain

A top-level domain , sometimes referred to as a top-level domain name, is the last part of an domain name, that is, the group of letters that follow the final dot of any domain name....
 and routes data anonymously both to and from the hidden service. Due to this lack of a public address, hidden services may be hosted behind firewalls or NAT
Network address translation

In computer networking, network address translation is the process of modifying network address information in datagram packet headers while in transit across a traffic router for the purpose of remapping a given address space into another....
. A Tor client is necessary in order to access a hidden service.

Hidden services have been deployed on Tor network beginning in 2004. Besides the database that stores the hidden-service descriptors, Tor is decentralized by design; there is no direct readable list of hidden services. There are a number of independent hidden services that serve this purpose.

Because location-hidden services do not use exit nodes, they are not subject to exit node eavesdropping. There are, however, a number of security issues involving Tor hidden services. For example, services that are reachable through Tor hidden services and the public Internet are susceptible to correlation attacks, and consequently are not necessarily hidden. Other pitfalls include "letting your web server reveal identifying information about you, your computer, or your location", uptime
Uptime

Uptime is a Measurement of the system time a computer system has been "up" and running. It came into use to describe the opposite of downtime, times when a system was not operational....
 and downtime
Downtime

The term downtime is used to refer to periods when a system is unavailable.Downtime or outage duration refers to a period of time that a system fails to provide or perform its primary function....
 statistics, intersection attacks and user error.

  • , a wiki focusing on Tor hidden services
  • , a popular forum
  • , a search engine and service index
  • , list of tor services and uptimes
  • , Free hidden web hosting with PHP and MySQL


Weaknesses


DNS leaks

As with many anonymous web surfing systems, direct DNS
Domain name system

The Domain Name System is a hierarchical naming system for computers, services, or any resource participating in the Internet. It associates various information with domain names assigned to such participants....
 requests are usually still performed by many applications, without using Tor proxy. Using Privoxy
Privoxy

Privoxy is a web proxy program, frequently used in combination with Tor and Squid cache. It has filtering capabilities for protecting privacy, modifying web page data, managing [], controlling access, and selectively removing content such as ads, banners and pop-ups....
 or the command "torify" included with Tor distribution is a possible solution to this problem. Additionally, applications using SOCKS5 – which supports name-based proxy requests – can route DNS requests through Tor, having lookups performed at the exit node and thus receiving the same anonymity as other Tor traffic.

As of Tor release 0.2.0.1-alpha, Tor includes its own DNS resolver which will dispatch queries over the mix network. This should close the DNS leak and can interact with Tor's address mapping facilities to provide Tor hidden service (.onion) access to non-SOCKS aware applications.

Traffic analysis

Like all current low latency
Low latency

Low latency allows human-unnoticeable Access time between an input being processed and the corresponding output providing real time characteristics....
 anonymity networks, Tor is vulnerable to traffic analysis
Traffic analysis

Traffic analysis is the process of intercepting and examining messages in order to deduce information from patterns in communication. It can be performed even when the messages are encrypted and cannot be cryptanalysis....
 from observers who can watch both ends of a user's connection.

Steven J. Murdoch and George Danezis from University of Cambridge
University of Cambridge

The University of Cambridge , located in Cambridge, England, is the List of oldest universities in continuous operation university in the Anglosphere....
 presented an article at the 2005 IEEE
Institute of Electrical and Electronics Engineers

The Institute of Electrical and Electronics Engineers or IEEE is an international non-profit, professional body for the advancement of technology related to electricity....
 Symposium
Symposium

Symposium originally referred to a drinking party but has since come to refer to any academic conference, or a style of university class characterized by an openly discursive rather than lecture and question–answer format....
 on Security and Privacy. They presented 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 and therefore greatly reduce the anonymity provided by Tor. They have also shown that otherwise unrelated streams can be linked back to the same initiator. The identity of the original user remains undiscovered by this attack, however. Murdoch has been working with and funded by Tor since 2006.

Eavesdropping by exit nodes

In September 2007, Dan Egerstad, a Swedish
Sweden

Sweden , officially the Kingdom of Sweden , is a Nordic countries on the Scandinavian Peninsula in Northern Europe. Sweden has land borders with Norway to the west and Finland to the northeast, and it is connected to Denmark by the ?resund Bridge in the south....
 security consultant, revealed that by operating and monitoring Tor exit nodes he had intercepted usernames and passwords for a large number of email accounts. As Tor does not, and by design cannot, encrypt the traffic between an exit node and the target server, any exit node is in a position to capture any traffic passing through it which does not use end-to-end encryption, e.g. SSL
Transport Layer Security

Transport Layer Security and its predecessor, Secure Sockets Layer , are cryptographic protocols that provide security and data integrity for communications over Internet Protocol Suite networks such as the Internet....
. While this does not inherently violate the anonymity of the source, it affords added opportunities for data interception by self-selected third parties, greatly increasing the risk of exposure of sensitive data by users who are careless or who mistake Tor's anonymity for security.

Etiquette


Because of its inherent anonymity, the traditional practices that network operators use to curb abuse may be insufficient with regards to connections coming from Tor network. Tor has some features intended to reduce this problem, both from the perspective of exit node operators and third party sites.

Exit nodes each maintain an exit policy of what traffic is and is not permitted to leave Tor network through that node. It is possible to prevent most major abuses of Tor network
Computer networking

Computer networking is the engineering discipline concerned with communication between computer systems or Peripheral devices. Networking, routers, routing protocols, and networking over the public Internet have their specifications defined in documents called Request for Commentss....
 using a combination of addresses and ports
TCP and UDP port

In computer networking, a port is an application-specific or process-specific software construct serving as a communications endpoint used by Transport layer protocols of the Internet protocol suite such as Transmission Control Protocol and User Datagram Protocol ....
. Potential abuses include:

Bandwidth hogging: It is considered impolite to transfer massive amounts of data across the Tor network – the onion routers are run by volunteers using their own bandwidth at their own cost. BitTorrent: Due to the high bandwidth usage caused by the use of this protocol, it is considered impolite and inappropriate to use the Tor network for BitTorrent
BitTorrent

BitTorrent is a peer-to-peer file sharing communications protocol used for distributing large amounts of data. BitTorrent is one of the most common protocols for transferring large files, and by some estimates it accounts for about 35% of all traffic on the entire Internet....
 transfers. By default, the Tor exit policy blocks the standard BitTorrent ports. Spam: The default Tor exit policy prevents connections to port 25, preventing people from sending spam directly from the Tor network. Anonymous users: The Tor project attempts to ensure that websites that wish to set different access policies for users visiting through Tor can do so.

Illegal uses


Among the large numbers of people who use the Tor network are some who have used it for illegal purposes, ranging from P2P
P2P

P2P or P-to-P may refer to:* Peer-to-peer, a type of ad-hoc computer network** Peer-to-peer file sharing, file sharing on peer-to-peer networks...
 sharing of copyrighted files to communicating child pornography
Child pornography

Child pornography refers to images or films depicting sexually explicit activities involving a child; as such, child pornography is a visual record of child sexual abuse....
. In September 2006, German authorities seized data center equipment running Tor software during a child pornography crackdown.

The Tor project's response to accusations that it is helping to facilitate illegal activity has been to emphasize that (1) many law-abiding individuals who use Tor for "good" purposes (privacy, freedom of communication) do not have readily available and practical alternatives to Tor, while (2) those who are willing to break laws already have other more effective ways to remain anonymous (such as breaking into computers, renting botnet
Botnet

Botnet is a jargon term for a collection of software robots, or Internet bots, that run autonomously and automatically. The term is often associated with malicious software but it can also refer to the network of computers using distributed computing software....
s, or stealing cell phones). The argument is that the benefit to those in group (1) is much larger than the assistance to those in group (2), who have realistic alternatives to Tor by virtue of the fact that they are willing to break the law.

Implementation

  • The main implementation of Tor is written in C and is roughly 49,000 lines of code long. The rationale for using C is that Tor requires routers to run fast.
  • Vuze
    Vuze

    Vuze is a free BitTorrent client used to transfer files via the BitTorrent protocol. Vuze is written in Java , and uses the Azureus Engine. In addition to bittorrenting, Vuze allows users to view, publish and share original DVD and High-definition video quality video content....
     (formerly known as Azureus, written in Java) includes built-in Tor support.


See also

  • Tools to use with Tor:
    • Vidalia
      Vidalia project

      Vidalia is a cross-platform controller GUI for Tor , built using the Qt . It allows the user to start, stop, and view the status of Tor; monitor bandwidth usage; view, filter, and search log messages; and configure some aspects of Tor....
       – a cross-platform
      Cross-platform

      In computing, cross-platform is a term used to refer to computer software or computing methods and concepts that are implemented and inter-operate on multiple computer platforms....
       GUI
      Graphical user interface

      A graphical user interface is a type of user interface which allows people to human-computer interaction such as computers; hand-held devices such as MP3 Players, Portable Media Players or Gaming devices; household appliances and office equipment....
       controller for Tor
    • [https://addons.mozilla.org/en-US/firefox/addon/2275 TorButton] Firefox Plug-in for Tor proxy management
    • – a caching web proxy
    • Privoxy
      Privoxy

      Privoxy is a web proxy program, frequently used in combination with Tor and Squid cache. It has filtering capabilities for protecting privacy, modifying web page data, managing [], controlling access, and selectively removing content such as ads, banners and pop-ups....
       – a filtering proxy
    • Multipurpose relay (SOcket CAT): includes socks4, and socks4a functionality (Linux / Mac)
    • Portable Tor
      Portable Tor

      PortableTor is a free repackaged version of the bundled Tor by Roger Dingledine and Nick Mathewson. The program allows the end user to connect to the Tor anonymity network from a removable media without installing anything to the hard drive....
       – Repackaged version of Tor + Vidalia + Privoxy for use on a USB flash drive
      USB flash drive

      A USB flash drive consists of a Flash memory#NAND memories-type flash memory data storage device integrated with a USB interface. USB flash drives are typically removable and rewritable, much smaller than a floppy disk , and most USB flash drives weigh less than an ounce ....
    • a KDE
      KDE

      KDE is a free software project based around its flagship product, a desktop environment for Unix-like systems. The goal of the project is to provide basic desktop functions and applications for daily needs as well as tools and documentation for developers to write stand-alone applications for the system....
       GUI
      Graphical user interface

      A graphical user interface is a type of user interface which allows people to human-computer interaction such as computers; hand-held devices such as MP3 Players, Portable Media Players or Gaming devices; household appliances and office equipment....
       controller for Tor
    • allows any network application to connect through Tor, even if the overlying application doesn't support SOCKS
      SOCKS

      SOCKS is an Internet Protocol that facilitates the routing of Packet between client-server applications via a proxy server. SOCKS is an abbreviation for "Secured Over Credential-based Kerberos Services." SOCKS performs at Layer 5 of the OSI model - the Session layer ....
      .
  • Web browsers bundled with Tor
    • [https://www.torproject.org/torbrowser/ Tor Browser Bundle] lets you use Tor on Windows without needing to install any software.
    • OperaTor
      Operator

      In mathematics, an operator is a function which operates on another function. Often, an "operator" is a function which acts on functions to produce other functions ; or it may be a generalization of such a function, as in linear algebra, where some of the terminology reflects the origin of the subject in operations on the functions which ar...
       – Preconfigured bundle with Opera (web browser)
      Opera (web browser)

      Opera is a web browser and Internet suite developed by the Opera Software company. Opera handles common Internet-related tasks such as displaying web sites, sending and receiving e-mail messages, managing contacts, IRC online chatting, downloading files via BitTorrent , and reading web feeds....
       + Tor + Privoxy
    • xB Browser – Firefox derivative with integrated Tor; can run from a USB flash drive
  • Anonymous chat
    • [https://www.torproject.org/torbrowser/ Tor IM Browser Bundle] allows instant messaging and chat via Tor on Windows without needing to install any software.
    • TorChat
      TorChat

      TorChat is a decentral anonymous Instant Messenger that uses Tor #Hidden_services as its underlying Network. It can be used for text messaging and to transfer files to other users....
       – a serverless instant messaging
      Instant messaging

      Instant messaging is a form of Real-time computing communication between two or more people based on typed text. The Written language is conveyed via devices connected over a network such as the Internet....
       application based on Tor's hidden services
  • Preinstalled environments with Tor enabled:
    • – A sandboxed
      Sandbox (computer security)

      In computer security, a sandbox is a security mechanism for safely running programs. It is often used to execute untested code, or untrusted programs from unverified third-parties, suppliers and untrusted users....
      , Gentoo
      Gentoo Linux

      Gentoo is a computer operating system built on top of the Linux Kernel and based on the Portage package management system. It is distributed as Free software....
       based Linux distribution
      Linux distribution

      A Linux distribution is a member of the family of Unix-like software distributions built on top of the Linux kernel. Such distributions consist of a large collection of software applications such as word processors, spreadsheets, media players and database applications....
      , available as a Live CD
      Live CD

      A live CD or live DVD is a CD or DVD containing a booting computer operating system. Live CDs are unique in that they have the ability to run a complete, modern operating system on a computer lacking Computer_storage , such as a hard disk drive....
       or Live USB
      Live USB

      A live USB is a USB flash drive or a USB external hard disk drive containing a full operating system which can be booting. Live USBs are closely related to live CDs, but typically have the ability to save settings and permanently install software packages back onto the USB device....
    • – A virtual OS for anonymous communication, ISO bootable and Live CD
    • – A VMware
      VMware

      VMware, Inc. is a software developer of virtualization software. The company was founded in 1998 and is based in Palo Alto, California. The Company is majority owned by EMC Corporation ....
       Virtual Machine
      Virtual machine

      In computer science, a virtual machine is a software implementation of a machine that executes programs like a real machine.Definitions...
       that among other things can provide anonymity though Tor
    • – an i686 uClibc-based micro Linux distribution whose sole purpose is to securely host a Tor server purely in RAM.
  • List of anonymous P2P networks and clients
    Anonymous P2P

    An anonymous P2P computer network is a particular type of peer-to-peer network in which the users are anonymity or pseudonymity by default. The primary difference between regular and anonymous networks is in the routing method of their respective network architectures....
  • Anonymous remailer
    Anonymous remailer

    An anonymous remailer is a Server computer which receives messages with embedded instructions on where to send them next, and which forwards them without revealing where they originally came from....
    s
  • Issues and Ideas related to online anonymity
    • Crypto-anarchism
      Crypto-anarchism

      Crypto-anarchism is an ideology that expounds the use of strong public-key cryptography to enforce privacy and individual Freedom . It was described by Vernor Vinge as a cyberspatial realization of anarcho-capitalism....
    • Hacktivism
      Hacktivism

      Hacktivism is "the nonviolent use of illegal or legally ambiguous digital tools in pursuit of political ends. These tools include web site defacements, redirects, denial-of-service attacks, information theft, web site parodies, virtual sit-ins, virtual sabotage, and software development." It is often understood as the writing of code to prom...
    • Internet privacy
      Internet privacy

      Internet privacy consists of privacy over the media of the Internet: the ability to control what information one reveals about oneself over the Internet, and to control who can access that information....
    • Internet censorship
      Internet censorship

      Internet censorship is control or suppression of the publishing or accessing of information on the Internet. The legal issues are similar to offline censorship....
    • Freedom of information
      Freedom of information

      Freedom of information refers to the protection of the right to freedom of expression with regards to the Internet and information technology ....
    • OpenNet Initiative
      OpenNet Initiative

      The OpenNet Initiative is a joint project whose goal is to monitor and report on Censorware and Computer surveillance practices by nations. The project employs a number of technical means, as well as an international network of investigators, to determine the extent and nature of government-run internet filtering programs....
    • Netsukuku
      Netsukuku

      Netsukuku is the name of an experimental peer-to-peer routing system, developed by the , born to build up a distributed network, anonymous and censorship-free, fully independent but not necessarily separated from Internet, without the support of any server, Internet Service Provider and no central authority....


Footnotes


Further reading

  • AnonWatch, Security Analysis of the Tor Network
  • Goodin, Dan "", 2007-09-10 article on The Register
    The Register

    The Register is a United Kingdom technology news and opinion website. It was founded by John Lettice and Mike Magee in 1994 as a newsletter called "Chip Connection", initially as an email service....
     news website. (Accessed 2007-09-20).
  • Krebs, Brian, 2007-08-08 (Accessed 2007-10-27)
  • 2007-09-07 (Accessed 2007-10-27)


External links

  • 18 May 2004