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.

The name "Tor" can refer to several different components.

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, it doesn't. You need to use a separate program that understands your application and protocol and knows how to clean or "scrub" the data it sends. Tor Browser tries to keep application-level data, like the user-agent string, uniform for all users. Tor Browser can't do anything about the text that you type into forms, though.

A typical proxy provider sets up a server somewhere on the Internet and allows you to use it to relay your traffic. This creates a simple, easy to maintain architecture. The users all enter and leave through the same server. The provider may charge for use of the proxy, or fund their costs through advertisements on the server. In the simplest configuration, you don't have to install anything. You just have to point your browser at their proxy server. Simple proxy providers are fine solutions if you do not want protections for your privacy and anonymity online and you trust the provider to not do bad things. Some simple proxy providers use SSL to secure your connection to them, which protects you against local eavesdroppers, such as those at a cafe with free wifi Internet.

Simple proxy providers also create a single point of failure. The provider knows both who you are and what you browse on the Internet. They can see your traffic as it passes through their server. In some cases, they can even see inside your encrypted traffic as they relay it to your banking site or to ecommerce stores. You have to trust the provider isn't watching your traffic, injecting their own advertisements into your traffic stream, or recording your personal details.

Tor passes your traffic through at least 3 different servers before sending it on to the destination. Because there's a separate layer of encryption for each of the three relays, somebody watching your Internet connection can't modify, or read, what you are sending into the Tor network. Your traffic is encrypted between the Tor client (on your computer) and where it pops out somewhere else in the world.

Doesn't the first server see who I am?

Possibly. A bad first of three servers can see encrypted Tor traffic coming from your computer. It still doesn't know who you are and what you are doing over Tor. It merely sees "This IP address is using Tor". You are still protected from this node figuring out both who you are and where you are going on the Internet.

Can't the third server see my traffic?

Possibly. A bad third of three servers can see the traffic you sent into Tor. It won't know who sent this traffic. If you're using encryption (like HTTPS), it will only know the destination. See this visualization of Tor and HTTPS to understand how Tor and HTTPS interact.

Yes.

The Tor software is free software. This means we give you the rights to redistribute the Tor software, either modified or unmodified, either for a fee or gratis. You don't have to ask us for specific permission.

However, if you want to redistribute the Tor software you must follow our LICENSE. Essentially this means that you need to include our LICENSE file along with whatever part of the Tor software you're distributing.

Most people who ask us this question don't want to distribute just the Tor software, though. They want to distribute Tor Browser. This includes Firefox Extended Support Release and the NoScript extension. You will need to follow the license for those programs as well. Both of those Firefox extensions are distributed under the GNU General Public License, while Firefox ESR is released under the Mozilla Public License. The simplest way to obey their licenses is to include the source code for these programs everywhere you include the bundles themselves.

Also, you should make sure not to confuse your readers about what Tor is, who makes it, and what properties it provides (and doesn't provide). See our trademark FAQ for details.

There are plenty of other programs you can use with Tor, but we haven't researched the application-level anonymity issues on all of them well enough to be able to recommend a safe configuration. Our wiki has a community-maintained list of instructions for Torifying specific applications. Please add to this list and help us keep it accurate!

Most people use Tor Browser, which includes everything you need to browse the web safely using Tor. Using Tor with other browsers is dangerous and not recommended.

There is absolutely no backdoor in Tor.

We know some smart lawyers who say that it is unlikely that anybody will try to make us add one in our jurisdiction (United States). If they do ask us, we will fight them, and (the lawyers say) probably win.

We will never put a backdoor in Tor. We think that putting a backdoor in Tor would be tremendously irresponsible to our users, and a bad precedent for security software in 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!

But that said, there are still plenty of subtle attacks people might try. Somebody might impersonate us, or break into our computers, or something like that. Tor is open source, and you should always check the source (or at least the diffs since the last release) for suspicious things. 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.

Also, there might be accidental bugs in Tor that could affect your anonymity. We periodically find and fix anonymity-related bugs, so make sure you keep your Tor versions up-to-date.

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 every four weeks. 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 2022 there are 8 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.