Tor for Technologists

15 June 2015

Tor is a technology that is cropping up in news articles quite often nowadays. However, there exists a lot of misunderstanding about it. Even many technologists don't see past its use for negative purposes, but Tor is much more than that. It is an important tool for democracy and freedom of speech - but it's also something that is very useful in the day-to-day life of a technologist. Tor is also an interesting case study in how to design a system that has very specific security requirements.

The Internet is currently a quite hostile place. There are threats of all kinds, ranging from script kiddies and drive-by phishing attacks to pervasive dragnet surveillance by many of the major intelligence services in the world. The extent of these problems have only recently become clear to us. In this context, a tool like Tor fills a very important niche. You could argue that it's a sign of the times that even a company like Facebook encourages the use of Tor to access their services. The time is right to add Tor to your tool belt.

How does Tor work?

The goal of Tor is to enable anonymous network traffic. The word Tor originally stood for The Onion Router (although Tor should not be capitalized like an acronym). Onion routing is the method that Tor uses to hide the IP address of the client. This also has the added benefit of making it extremely hard for anyone in between to see who is asking for what information on the Internet. In practice, this means that if someone is intercepting your traffic close to your computer, Tor will hide what servers you are talking to. If someone is intercepting traffic close to the server, they will not be able to see who is visiting that server.

Tor works by having thousands of volunteers hosting something called Tor Relays. These are fundamentally servers that spend all their time forwarding traffic. So when you want to access a service, the Tor client program that is running on your computer will choose three different relays. It will then take the packets of information and wrap them in encryption using public key cryptography. It will encrypt the packet three times over, to each one of the public keys of the relays that were chosen for this communication session. It will then send the triple-encrypted package to the first relay on the list. That relay will unwrap the encryption meant for itself - and what is left is a packet with two layers of encryption. The first relay will send the packet to the second relay, which will unwrap another layer of encryption. It sends this to the final relay which will unwrap the last piece of encryption. Since there is no encryption left, the final relay knows the real destination of the packet - so it contacts the original service requested. It then receives the answer and sends the information back, using the same process in reverse.

The final relay is usually called an exit relay. The first relay is usually called an entry relay. The way this works only the entry relay knows the real IP address of your machine - and only the exit relay knows which service you actually wanted to contact. And the service will only see the IP address of the exit relay. All of these things come together to protect the anonymity of the client quite well.

One important point is that you don't have to trust all the Tor relays in order to be able to trust the Tor network. Since anyone can set up their own relay, there is nothing that stops bad actors from setting up relays. Thus, Tor is designed to not be compromised even if this happens.

Tor is explicitly designed to be a low-latency network - that means it is possible to use it for things like Instant Messaging, audio conferencing or even video conferencing under good circumstances.

The Tor client

If you want to use Tor, there are two options: the first one is to run the client program, and the second is to download and run something called the Tor Browser Bundle. If you want to use Tor for other kinds of traffic than web, it's useful to run the Tor client software directly. Installing it is usually done through the regular package management systems for your operating system - and once installed it should be running on your machine in the background, without you having to do anything at all. Since Tor works as a proxy, you don't interact with Tor directly. Instead you ask your regular programs to tunnel traffic through Tor.

The Tor client program by default will listen to port 9050 using a protocol called SOCKS - this is a standard proxy protocol that many applications can be configured to use. For example, I use a program called Gajim for instant messaging. In this program I have configured all my accounts to connect over Tor by specifying the correct SOCKS proxy settings.

If you are using command line tools, it is usually possible to use a program called usewithtor or torsocks in order to transparently make the program use Tor. There are other ways of using Tor as well - Thunderbird can be configured to run all its traffic over Tor if you want to make your email harder to track and intercept - however, there are some risks associated with the manual configuration of Tor in Thunderbird. There are many possible types of information leakage, such as DNS lookups, geo-location information in headers and even IP addresses in mail headers in some cases. Fortunately, there exists a plugin called TorBirdy that automates many of the steps and tries to stop such unsafe information leakage from happening.

The Tor Browser Bundle

By far the easiest way of using Tor is to download the Tor Browser Bundle which doesn't even have to be installed - it can be run directly after download (and you can have it on a USB stick or a DVD and run it from there as well). The Tor Browser Bundle is basically the Tor client software combined with a very customized browser. Since browsers have many ways they can leak your anonymity, and there are many other things that can threaten your security, the Tor Browser Bundle is much safer than just configuring your regular browser to use Tor.

The Tor Browser Bundle comes with several plugins that will improve your privacy, and it has also been configured to not leak information through side channels such as font configuration, Flash plugins and other features.

Hidden Services

Tor also has another interesting feature that has gotten a lot of attention lately. Tor hidden services give a service the ability to hide its IP address, just as the regular Tor operation allows a client to hide its IP address. It works by using something called a rendezvous protocol, which is a bit complicated. Basically, you can imagine that both the client and the server establishes their own full circuits of three relays. They then meet in the middle, somewhere in the Tor network - and from that point they can establish a connection. In general, a hidden service is found and contacted using a .onion-URL, which consists of 16 alphanumeric characters followed by .onion. The 16 characters is a representation of the public key of the hidden service, which means that routing of the connection is self-verifying. No-one can fake being a hidden service without stealing their private key.

Hidden services are extremely powerful for several reasons. First, they allow the service provider to be anonymous. This has of course been used for a lot of good and bad reasons. Secondly, if you only expose a service as a hidden service, you can force your clients to use Tor - which makes it much harder for them to accidentally expose their information. Third, hidden services are implemented in such a way that the traffic will never leave the Tor network. There are no exit relays for hidden services. What this means is that communication to a hidden service will always be encrypted all the way to the service. This isn't necessarily true for regular Tor connections. Finally, you can expose a hidden service without having a publicly routable IP address. A very practical example of how to use this is to expose SSH as a hidden service. This allows you to SSH to your own server wherever it may be, without having to open firewalls or having a public IP. I run several servers at home that are not publicly reachable, and being able to SSH into them and manage them anyway is extremely powerful.

Facebook recently made a splash by exposing their servers as a hidden service. This is a great step forward. So you might ask whether this is something you should do for the projects you work on. In general, I would say yes - although it does require some extra work and testing. Making this available to your users is something that in general is good for everyone.

Bridges and obfuscated protocols

In some regions there is enough censorship going on that using Tor in the regular manner doesn't work. Most of the time this is because they simply block traffic to all the Tor relays. And since the Tor relays have to be public, it's easy for a censor to simple download the list of IPs to be blocked.

Because of this situation, the Tor software supports something called bridges. These are entry relays that are not publicly known - if you need one you can request one on the fly. The other solution that is sometimes necessary is to use obfuscated protocols. Since the most advanced censors use deep packet inspection it's important that Tor traffic can masquerade as other kinds of traffic. There are several different plugins implemented in Tor right now for solving this problem - but it's an ongoing arms race to beat censorship.

Anonymity is hard - what doesn't Tor protect against?

Tor is a tool, and just like any tool it has things it does well, and things it doesn't do so well. It is important to know that Tor can't protect against every threat out there. Even when you use Tor correctly, there are things to keep in mind.

The biggest day-to-day problem when it comes to anonymity and security with Tor is that if the traffic you are sending to a server isn't encrypted, then Tor will not help with that. Specifically, the traffic between you and the first two relays will be encrypted and hard to attack. However, the exit relay will be able to see all traffic that passes through. The way to protect against this is to either use a protocol that encrypt your information all the way to the endpoint (such as HTTPS or SMTP with STARTTLS). The other way is to use services that expose a hidden service - which ensures you get end-to-end encryption of all traffic.

Tor is usually classified as a low-latency network. This has a consequence for anonymity. Basically, Tor is designed in such a way that if an attacker can observe a large percentage of the network traffic between relays, it is possible for them to de-anonymize much of that traffic. The reason is that since Tor traffic is supposed to go out and come back in realtime, it can be possible to do correlation of packet times. This could lead to a privacy break. However, this is the way Tor was designed. Current research seems to imply that it is basically impossible to get total anonymity if you want something that is close to real time traffic. If you are fine with waiting a few hours for each packet to arrive, there are other kinds of techniques that can be used, but Tor is not designed for those use cases.

Tor can also not protect you if you make a mistake and expose your IP address in any way. A typical example of something that is a bad idea is to use Tor for peer-to-peer file sharing - not only does it destroy the Tor network - it also doesn't work to hide your address, since most file-sharing protocols expose your real IP address. If you want to be sure to not expose your IP address you also need to avoid running plugins that can expose it. This is one of the reasons why the Tor Browser Bundle doesn't ship with a Flash plugin - it is just too dangerous from a privacy-standpoint.

The same caveat is true if you expose your server as a hidden service. If you really want to keep it hidden you need to make sure that none of your server components expose your IP address in any way. This can be easier said than done, as the next section will talk about.

Is Tor broken?

When I talk to technologists about Tor, one of the more common questions I will get is whether I think Tor is broken or whether we really can trust it, whether the government has put a backdoor in it, etc. There are many rumors and theories about Tor out there, and in general they are just that - rumors and theories. That said, Tor doesn't protect against everything, as I mentioned above. However, I would be extremely surprised if Tor contains a backdoor. The code is open source and many, many experts have looked at it over the years. The protocol is also open and has been implemented more than once in other open source projects. It is of course not impossible that there exists a backdoor that no-one has noticed, but personally I wouldn't bet on it. I do recommend any technologist that is worried about this eventuality to spend a few hours and go through the source code. The project can always do with more eyeballs.


All that said, Tor has been broken many times. The main reason for this is that anonymity is extremely hard, and Tor is pushing the boundaries of what's been done before. Thus, Tor is a favorite subject for researchers to poke at and see if they can figure out ways of getting it to do unexpected things. It's also the main way we have of getting an idea of where the hard limits of anonymity lie. That Tor is the subject of lots of research is ultimately good for the community, since every time a new paper comes out that talks about a weakness in Tor, the Tor developers fix it. Nothing is perfect at the start, so this way of iterating and fixing is how we get to a state where we can trust a tool.

Events in 2014

One reason that people are worried about the degree of security of Tor, is because there has been a string of suspicious events over the last few years. Specifically, both users of Tor and people that have used hidden services have been arrested under various circumstances. However, we have at this point no indication that any of these take downs were actually because of flaws in the Tor software or protocol. In the cases we know of, it seems that the reasons can be chalked down to mistakes of various kinds. In one case, there was a bug in the Firefox version that the Tor Browser Bundle was based on, so after the police took over a server, they could attack this browser and de-anonymize the users in question (but only those that visited that specific hidden service). In the case of the first Silk Road, it seems clear that investigators managed to hack in to the administrator interface and convinced it to send back its real IP address.

And then we have Operation Onymous. This operation was a collaboration between several different law enforcement agencies with the aim to take down a large number of illegal online marketplaces. What is interesting about this operation is all the propaganda that surrounds it. Initially they were claiming to have taken down over 400 hidden services. But after the dust settled it seems only 27 sites were taken down, and only 17 arrests were made. You can still argue that this is a lot, since they were supposed to have been protected by Tor and hidden services. But the truth is that apparently the person hosting Silk Road 2 had his name written down somewhere - the law enforcement agencies took him in and started questioning him. Apparently he named 16 other people that were promptly arrested and had their computers seized. So no Tor break was involved in this - even though one UK law enforcement agency actually tweeted a taunt about Tor being broken. It was all propaganda. More info can be found in this video.

Further, looking at the documents leaked by Edward Snowden, it is still clear that the strongest attackers out there don't seem to have a good way of breaking Tor. I find this to be very encouraging.

Everyday usage of Tor

Now that you have a better idea of what Tor is, it is time for us to quickly talk about how you can use Tor in your day-to-day job. As you can probably guess from the information above, Tor can be used for a lot of different purposes. But for the kind of work a developer is doing, the first and easiest thing you can do using Tor is to test your system in different ways. In many cases it's hard to see what a system looks like without cookies and personalization. Tor makes it very easy.

The other kind of testing you can do is to make it possible to very easily try a system in such a way that it looks like it comes from many different countries. It is possible to control which relays Tor will use in such a way that you can control which country your traffic will come out from.

As I mentioned before, putting SSH behind a Tor hidden service is a very useful way of getting access to your systems from anywhere on the planet, in a way that puts at least two layers of encryption on your connection. However, even for a setup like this, it's important to have good passwords (or only use public-key logins for SSH) and to make it impossible to login to the root account remotely.


If you happen to be in the kind of situation where Tor is absolutely necessary for you and you expect to be attacked by strong adversaries, you are in a tricky situation. Just installing Tor on your regular machine isn't necessarily going to be enough. Thus, there is a project called Tails, which is a Linux distribution that runs from a CD/DVD or USB drive. It is preconfigured to only send network traffic over Tor and also has many other privacy and anonymity features. It comes bundled with several different tools that will allow you to easily communicate privately, send encrypted email and also working with documents in a safe way.

The interesting thing with Tails is that it's completely amnesiac - it doesn't leave a trace on your computer after you have run it. It is possible to have it save a small amount of data, but by default no traces will be left.

Tails is not something you will need in your regular day-job, but there are certain cases where something like Tails can be extremely powerful. Every journalist should know how to use it - and so should every lawyer. It's a powerful tool and it's a good idea to be prepared to use it before it becomes necessary.


Tor is a fantastic tool that can do a lot of amazing things. It is one of the strongest examples of useful cryptography deployed in a safe way we have right now. As we have seen though, no tool is perfect. And in order to use Tor correctly, it's a good idea to know how it works. As a next step I would recommend you to download Tor and try it out. One of the interesting things about anonymity is that it loves company - even if you don't need the anonymity you are protecting others who do need it by using Tor. One day you might go forward and run your own Tor relay as well - something that improves the Tor network for everyone.

Significant Revisions

15 June 2015: First published