Anonymity and the Tor Network

As the name implies, Alcoholics Anonymous meetings are anonymous. You don’t have to sign anything, show ID or even reveal your real name. But the meetings are not private. Anyone is free to attend. And anyone is free to recognize you: by your face, by your voice, by the stories you tell. Anonymity is not the same as privacy.

That’s obvious and uninteresting, but many of us seem to forget it when we’re on a computer. We think “it’s secure,” and forget that secure can mean many different things.

Tor is a free tool that allows people to use the internet anonymously. Basically, by joining Tor you join a network of computers around the world that pass internet traffic randomly amongst each other before sending it out to wherever it is going. Imagine a tight huddle of people passing letters around. Once in a while a letter leaves the huddle, sent off to some destination. If you can’t see what’s going on inside the huddle, you can’t tell who sent what letter based on watching letters leave the huddle.

I’ve left out a lot of details, but that’s basically how Tor works. It’s called “onion routing,” and it was first developed at the Naval Research Laboratory. The communications between Tor nodes are encrypted in a layered protocol — hence the onion analogy — but the traffic that leaves the Tor network is in the clear. It has to be.

If you want your Tor traffic to be private, you need to encrypt it. If you want it to be authenticated, you need to sign it as well. The Tor website even says:

Yes, the guy running the exit node can read the bytes that come in and out there. Tor anonymizes the origin of your traffic, and it makes sure to encrypt everything inside the Tor network, but it does not magically encrypt all traffic throughout the internet.

Tor anonymizes, nothing more.

Dan Egerstad is a Swedish security researcher; he ran five Tor nodes. Last month, he posted a list of 100 e-mail credentials — server IP addresses, e-mail accounts and the corresponding passwords — for

embassies and government ministries around the globe, all obtained by sniffing exit traffic for usernames and passwords of e-mail servers.

The list contains mostly third-world embassies: Kazakhstan, Uzbekistan, Tajikistan, India, Iran, Mongolia — but there’s a Japanese embassy on the list, as well as the UK Visa Application Center in Nepal, the Russian Embassy in Sweden, the Office of the Dalai Lama and several Hong Kong Human Rights Groups. And this is just the tip of the iceberg; Egerstad sniffed more than 1,000 corporate accounts this way, too. Scary stuff, indeed.

Presumably, most of these organizations are using Tor to hide their network traffic from their host countries’ spies. But because anyone can join the Tor network, Tor users necessarily pass their traffic to organizations they might not trust: various intelligence agencies, hacker groups, criminal organizations and so on.

It’s simply inconceivable that Egerstad is the first person to do this sort of eavesdropping; Len Sassaman published a paper on this attack earlier this year. The price you pay for anonymity is exposing your traffic to shady people.

We don’t really know whether the Tor users were the accounts’ legitimate owners, or if they were hackers who had broken into the accounts by other means and were now using Tor to avoid being caught. But certainly most of these users didn’t realize that anonymity doesn’t mean privacy. The fact that most of the accounts listed by Egerstad were from small nations is no surprise; that’s where you’d expect weaker security practices.

True anonymity is hard. Just as you could be recognized at an AA meeting, you can be recognized on the internet as well. There’s a lot of research on breaking anonymity in general — and Tor specifically — but sometimes it doesn’t even take much. Last year, AOL made 20,000 anonymous search queries public as a research tool. It wasn’t very hard to identify people from the data.

A research project called Dark Web, funded by the National Science Foundation, even tried to identify anonymous writers by their style:

One of the tools developed by Dark Web is a technique called Writeprint, which automatically extracts thousands of multilingual, structural, and semantic features to determine who is creating “anonymous” content online. Writeprint can look at a posting on an online bulletin board, for example, and compare it with writings found elsewhere on the Internet. By analyzing these certain features, it can determine with more than 95 percent accuracy if the author has produced other content in the past.

And if your name or other identifying information is in just one of those writings, you can be identified.

Like all security tools, Tor is used by both good guys and bad guys. And perversely, the very fact that something is on the Tor network means that someone — for some reason — wants to hide the fact he’s doing it.

As long as Tor is a magnet for “interesting” traffic, Tor will also be a magnet for those who want to eavesdrop on that traffic — especially because more than 90 percent of Tor users don’t encrypt.

This essay previously appeared on Wired.com.

Posted on September 20, 2007 at 5:38 AM • 67 Comments