In his doctoral thesis, Dr. Sebastian Pahl investigated how secure anonymous communication on the internet really is. His focus was on the Tor network – a system that helps people protect their identity online. He discovered weaknesses, but also new ways to improve performance and anonymity. The work was a collaboration between Hof University of Applied Sciences and the University of Luxembourg and was supervised by Prof. Dr. Florian Adamsky.
How did you come to this topic?
“I was looking for a job shortly before the end of my Master’s degree. A fellow student told me that Prof. Adamsky had advertised a part-time position as an information security officer at Hof University of Applied Sciences. At the same time, there was an opportunity to do a doctorate. After the interview and the application, I actually got the job. At first I wasn’t sure whether I would actually do a doctorate. But the topic was exciting. I started work straight after my Master’s degree and was officially enrolled at the University of Luxembourg in May 2021. I was able to successfully defend my doctoral thesis in June 2025.”
What exactly did you investigate?
“My thesis is about the Tor network. The principle behind it is called onion routing. A message is packaged in several layers – similar to a letter in several envelopes. Each section along the way only knows the next step. This means that no one can trace the entire communication path. This ensures anonymity, but also makes the network slower. If more people use Tor, protection increases – because anonymity only works in the masses. It is therefore important to improve performance and security. This is exactly where my work comes in.”
What was the first focus of your research?
“I investigated so-called shared connections. These are comparable to bottlenecks in road traffic: two paths lead to a single lane, causing traffic jams. There are also bottlenecks like this in the Tor network – there is only one “two-lane road” between two relays. Earlier work was aware of the problem, but not in depth. I showed how much these bottlenecks affect performance – and how to improve it.”
Were there also security gaps?
“Yes. I was able to show that so-called side-channel attacks are possible via such shared connections. This means that indirect clues – such as time delays – can be used to find out which websites someone has visited.
A simple example: If someone has watered flowers, the soil is damp. Similarly, you can draw conclusions about Tor using measured time differences. I was able to prove these attacks in my work – and suggest protective measures.”
You have even developed your own network. What’s behind it?
“Yes, I designed an alternative system called OnionVPN. It’s somewhere between Tor and classic VPNs. VPN providers promise anonymity, but can also log which sites are visited. With Tor, you don’t have to trust anyone – that’s a big advantage. However, Tor is not ideal for large amounts of data such as downloads. This is exactly where OnionVPN comes in: better performance for downloads, but slightly less anonymity. The idea behind it: If some users switch to an alternative system, Tor will become faster for others – and thus perhaps also more attractive for new users.”
Tor is often criticized in the media. And rightly so?
“Public opinion is often distorted. Many people immediately think of crime when they think of Tor. But the network is a tool – like a knife that you can use in the kitchen or for something else. Tor helps many people around the world to protect their freedom. Journalists, activists and whistleblowers can communicate securely despite censorship – in countries such as Iran, Russia and China. Organizations such as Amnesty International and Reporters Without Borders therefore expressly recommend the use of Tor.”
