Quant al Tor

As mentioned above, it is possible for an observer who can view both you and either the destination website or your Tor exit node to correlate timings of your traffic as it enters the Tor network and also as it exits. Tor does not defend against such a threat model.

In a more limited sense, note that if a censor or law enforcement agency has the ability to obtain specific observation of parts of the network, it is possible for them to verify a suspicion that you talk regularly to your friend by observing traffic at both ends and correlating the timing of only that traffic. Again, this is only useful to verify that parties already suspected of communicating with one another are doing so. In most countries, the suspicion required to obtain a warrant already carries more weight than timing correlation would provide.

Furthermore, since Tor reuses circuits for multiple TCP connections, it is possible to associate non anonymous and anonymous traffic at a given exit node, so be careful about what applications you run concurrently over Tor. Perhaps even run separate Tor clients for these applications.

Internet communication is based on a store-and-forward model that can be understood in analogy to postal mail: Data is transmitted in blocks called IP datagrams or packets. Every packet includes a source IP address (of the sender) and a destination IP address (of the receiver), just as ordinary letters contain postal addresses of sender and receiver. The way from sender to receiver involves multiple hops of routers, where each router inspects the destination IP address and forwards the packet closer to its destination. Thus, every router between sender and receiver learns that the sender is communicating with the receiver. In particular, your local ISP is in the position to build a complete profile of your Internet usage. In addition, every server in the Internet that can see any of the packets can profile your behavior.

The aim of Tor is to improve your privacy by sending your traffic through a series of proxies. Your communication is encrypted in multiple layers and routed via multiple hops through the Tor network to the final receiver. More details on this process can be found in this visualization. Note that all your local ISP can observe now is that you are communicating with Tor nodes. Similarly, servers in the Internet just see that they are being contacted by Tor nodes.

Generally speaking, Tor aims to solve three privacy problems:

First, Tor prevents websites and other services from learning your location, which they can use to build databases about your habits and interests. With Tor, your Internet connections don't give you away by default -- now you can have the ability to choose, for each connection, how much information to reveal.

Second, Tor prevents people watching your traffic locally (such as your ISP or someone with access to your home wifi or router) from learning what information you're fetching and where you're fetching it from. It also stops them from deciding what you're allowed to learn and publish -- if you can get to any part of the Tor network, you can reach any site on the Internet.

Third, Tor routes your connection through more than one Tor relay so no single relay can learn what you're up to. Because these relays are run by different individuals or organizations, distributing trust provides more security than the old one hop proxy approach.

Note, however, that there are situations where Tor fails to solve these privacy problems entirely: see the entry below on remaining attacks.

El nom «Tor» pot fer referència a uns quants components diferents.

Tor is a program you can run on your computer that helps keep you safe on the Internet. It protects you by bouncing your communications around a distributed network of relays run by volunteers all around the world: it prevents somebody watching your Internet connection from learning what sites you visit, and it prevents the sites you visit from learning your physical location. This set of volunteer relays is called the Tor network.

The way most people use Tor is with Tor Browser, which is a version of Firefox that fixes many privacy issues. You can read more about Tor on our about page.

The Tor Project is a non-profit (charity) organization that maintains and develops the Tor software.

Tor is the onion routing network. When we were starting the new next-generation design and implementation of onion routing in 2001-2002, we would tell people we were working on onion routing, and they would say "Neat. Which one?" Even if onion routing has become a standard household term, Tor was born out of the actual onion routing project run by the Naval Research Lab.

(It's also got a fine meaning in German and Turkish.)

Note: even though it originally came from an acronym, Tor is not spelled "TOR". Only the first letter is capitalized. In fact, we can usually spot people who haven't read any of our website (and have instead learned everything they know about Tor from news articles) by the fact that they spell it wrong.

No, no ho fa. Cal que utilitzeu un altre programa que comprengui eixa aplicació i el seu protocol, i que conegui com netejar o «agranar» les dades que hi envia. El Navegador Tor prova de mantenir les dades a escala d'aplicació, com ara la cadena d'agent d'usuari, uniforme per a tots els usuaris. Tor Browser can't do anything about the text that you type into forms, though.

Un servidor intermediari típic configura un servidor arreu a internet i us permet utilitzar-lo per a reconduir el vostre trànsit. Això crea una arquitectura simple i fàcil de mantenir. Tots els usuaris entren i se'n van a través del mateix servidor. El proveïdor podria cobrar per utilitzar el servidor, o finançar els seus costs mitjançant publicitat al servidor. Amb la configuració més simple, no heu d'instal·lar res. Només cal que dirigiu el navegador al servidor intermediari. Els servidors intermediaris simples són solucions adequades si no voleu protegir la vostra privadesa i anonimat en línia, i confieu que el proveïdor no faci res dolent. Alguns servidors simples utilitzen SSL per a fer segura la vostra connexió, i això us protegeix contra tafaners locals, com els que podríeu trobar en un cafè amb WiFi gratis.

Els servidors intermediaris simples també creen un punt de fallada únic. El proveïdor coneix qui sou i on navegueu en internet. Poden veure el vostre transit mentre passa a través del seu servidor. De vegades, inclús poden veure el vostre trànsit xifrat mentre l'envien al lloc web del vostre banc o comerç electrònic. Heu de confiar que el proveïdor no està vigilant el vostre trànsit, injectant els seus anuncis en el flux de dades, o enregistrant els vostres detalls personals.

El Tor passa el vostre trànsit a través de, com a mínim, 3 servidors diferents abans d'enviar-lo al seu destí. Ningú que vigili la vostra connexió a internet pot modificar ni llegir el que esteu enviat a la xarxa Tor, perquè hi ha una capa separada de xifratge per a cadascun dels tres relés. El vostre trànsit es xifra entre el client Tor (en el vostre equip) i on surt, en algun altre lloc del planeta.

I el primer servidor no veu on sóc?

Possiblement. Un primer servidor dolent pot veure que des del vostre equip arriba trànsit Tor xifrat. Tot i això, no sap qui sou i què esteu fent en la xarxa Tor. Gairebé no veu «Aquesta IP està utilitzant el Tor». De tota manera, la protecció fa que aquest servidor no pugui esbrinar ni qui sou ni on aneu en la internet.

Pot veure el meu trànsit el tercer servidor?

Possiblement. Un tercer servidor dolent pot veure el trànsit que envieu al Tor. Però no sabrà qui l'ha enviat. Si utilitzeu algun xifratge (com ara HTTPS), només sabrà quin és el seu destí. See this visualization of Tor and HTTPS to understand how Tor and HTTPS interact.

Sí.

El Tor és programari lliure. Això significa que us donem els drets per a distribuir el programari Tor, tant modificat com sense modificar, tant de pagament com gratis. No cal que ens demaneu cap permís en concret.

Tot i això, si voleu redistribuir el Tor, haureu de seguir la nostra LLICÈNCIA. En resum, això significa que cal que inclogueu el nostre fitxer de LLICÈNCIA junt amb qualsevol part del programari Tor que distribuïu.

De tota manera, la majoria dels qui ens pregunten sobre això no volen distribuir només el Tor. Volen distribuir el Navegador Tor. Això inclou el Firefox Extended Support Release, i els complements NoScript i HTTPS-Everywhere. També cal que seguiu la llicència d'aquests programes. Ambdós complements del Firefox es distribueixen sota la GNU General Public License; en canvi, el Firefox ESR s'allibera sota la Mozilla Public License. La forma més senzilla d'obeir les seves llicències és incloure el codi font d'eixos programes a tots els llocs on inclogueu els mateixos programes.

També haureu d'assegurar-vos de no confondre els vostres lectors sobre què és el Tor, qui el fa, i quines propietats proporciona (i quines no). Mireu les nostres PMF sobre la marca per a més detalls.

Hi ha una gran quantitat d'altres programes que podeu utilitzar amb el Tor, però no hem investigat prou, a un nivell detallat, els problemes d'anonimat de tots ells per a poder recomanar una configuració segura. La nostra wiki té una llista d'instruccions mantinguda per la comunitat per a Torificar aplicacions específiques. Please add to this list and help us keep it accurate!

La majoria de gent utilitza el Navegador Tor, que inclou tot el que necessiteu per a navegar per la web amb seguretat utilitzant el Tor. Using Tor with other browsers is dangerous and not recommended.

No hi ha cap porta del darrere en el Tor.

Coneixem uns quants advocats molt intel·ligents que diuen que és molt poc probable que ningú no ens faci afegir-ne cap en la nostra jurisdicció (EUA). Si ho intenten, els plantarem cara, i (els advocats diuen que) probablement guanyarem.

Mai no posarem cap porta del darrere en el Tor. Pensem que posar-li una porta del darrere al Tor seria tremendament irresponsable de cara als nostres usuaris, i un precedent dolent per a la seguretat del programari en general. If we ever put a deliberate backdoor in our security software, it would ruin our professional reputation. Nobody would trust our software ever again - for excellent reasons!

Però, dit això, hi ha multitud d'atacs subtils que la gent podria intentar. Algú podria fer-se passar per nosaltres, o colar-se en els nostres ordinadors, o alguna cosa similar. El Tor és de codi obert, i sempre hauríeu de comprovar el codi (o almenys les diferències amb la versió anterior) si hi noteu alguna cosa sospitosa. If we (or the distributors that gave you Tor) don't give you access to the source code, that's a sure sign something funny might be going on. You should also check the PGP signatures on the releases, to make sure nobody messed with the distribution sites.

A més d'això, podrien haver-hi errors accidentals en el Tor que podrien afectar el vostre anonimat. Trobem i corregim periòdicament errors relacionats amb l'anonimat, així que assegureu-vos que teniu actualitzada la vostra versió del Tor.

Tor (like all current practical low-latency anonymity designs) fails when the attacker can see both ends of the communications channel. For example, suppose the attacker controls or watches the Tor relay you choose to enter the network, and also controls or watches the website you visit. In this case, the research community knows no practical low-latency design that can reliably stop the attacker from correlating volume and timing information on the two sides.

So, what should we do? Suppose the attacker controls, or can observe, C relays. Suppose there are N relays total. If you select new entry and exit relays each time you use the network, the attacker will be able to correlate all traffic you send with probability around (c/n)2. But profiling is, for most users, as bad as being traced all the time: they want to do something often without an attacker noticing, and the attacker noticing once is as bad as the attacker noticing more often. Thus, choosing many random entries and exits gives the user no chance of escaping profiling by this kind of attacker.

The solution is "entry guards": each Tor client selects a few relays at random to use as entry points, and uses only those relays for their first hop. If those relays are not controlled or observed, the attacker can't win, ever, and the user is secure. If those relays are observed or controlled by the attacker, the attacker sees a larger fraction of the user's traffic - but still the user is no more profiled than before. Thus, the user has some chance (on the order of (n-c)/n) of avoiding profiling, whereas they had none before.

You can read more at An Analysis of the Degradation of Anonymous Protocols, Defending Anonymous Communication Against Passive Logging Attacks, and especially Locating Hidden Servers.

Restricting your entry nodes may also help against attackers who want to run a few Tor nodes and easily enumerate all of the Tor user IP addresses. (Even though they can't learn what destinations the users are talking to, they still might be able to do bad things with just a list of users.) However, that feature won't really become useful until we move to a "directory guard" design as well.

Tor uses a variety of different keys, with three goals in mind: 1) encryption to ensure privacy of data within the Tor network, 2) authentication so clients know they're talking to the relays they meant to talk to, and 3) signatures to make sure all clients know the same set of relays.

Encryption: first, all connections in Tor use TLS link encryption, so observers can't look inside to see which circuit a given cell is intended for. Further, the Tor client establishes an ephemeral encryption key with each relay in the circuit; these extra layers of encryption mean that only the exit relay can read the cells. Both sides discard the circuit key when the circuit ends, so logging traffic and then breaking into the relay to discover the key won't work.

Authentication: Every Tor relay has a public decryption key called the "onion key". Each relay rotates its onion key once a week. When the Tor client establishes circuits, at each step it demands that the Tor relay prove knowledge of its onion key. That way the first node in the path can't just spoof the rest of the path. Because the Tor client chooses the path, it can make sure to get Tor's "distributed trust" property: no single relay in the path can know about both the client and what the client is doing.

Coordination: How do clients know what the relays are, and how do they know that they have the right keys for them? Each relay has a long-term public signing key called the "identity key". Each directory authority additionally has a "directory signing key". The directory authorities provide a signed list of all the known relays, and in that list are a set of certificates from each relay (self-signed by their identity key) specifying their keys, locations, exit policies, and so on. So unless the adversary can control a majority of the directory authorities (as of 2021 there are 10 directory authorities), they can't trick the Tor client into using other Tor relays.

How do clients know what the directory authorities are?

The Tor software comes with a built-in list of location and public key for each directory authority. So the only way to trick users into using a fake Tor network is to give them a specially modified version of the software.

How do users know they've got the right software?

When we distribute the source code or a package, we digitally sign it with GNU Privacy Guard. See the instructions on how to check Tor Browser's signature.

In order to be certain that it's really signed by us, you need to have met us in person and gotten a copy of our GPG key fingerprint, or you need to know somebody who has. If you're concerned about an attack on this level, we recommend you get involved with the security community and start meeting people.

Tor will reuse the same circuit for new TCP streams for 10 minutes, as long as the circuit is working fine. (If the circuit fails, Tor will switch to a new circuit immediately.)

But note that a single TCP stream (e.g. a long IRC connection) will stay on the same circuit forever. We don't rotate individual streams from one circuit to the next. Otherwise, an adversary with a partial view of the network would be given many chances over time to link you to your destination, rather than just one chance.