Update 2013-08-15: I have been really enthused by reactions I received to this blog post. It has been referenced from Forbes, Gawker and the Daily Mail and a number of people have been in contact about tracking the DHT for themselves. I would recommend the IEEE S&P paper, “Trawling for Tor Hidden Services: Detection, Measurement, Deanonymization” which presents the same issues allowing the DHT to be trawled. They also present some very serious attacks allowing an adversary to locate hidden services with practical resources. It is well worth checking out if you have an interest in Tor.
Tor hidden services have got more media attention lately as a result of some notorious sites like the Silk Road marketplace, an online black market. On a basic level, Tor hidden services allow you to make TCP services available while keeping your server’s physical location hidden via the Tor anonymity network.
- Tor hidden service directories (HSDir’s) receive a subset of hidden service look-ups from users, allowing them to map relative popularity/usage of hidden service.
- An adversary with minimal resources can carry out complete DoS attacks of Tor hidden services by running malicious Tor hidden service directories and positioning them in a particular part of the router list.
- Many look-ups for Tor hidden services go to the incorrect hidden service directories which negatively affects the initial time to access the site.
- Hidden services such as are popular, sites such as the Silk Road marketplace receive more than 60,000 unique user sessions a day.
For users to access a hidden service they must first retrieve a hidden service descriptor. This is a short signed message created by the hidden service approximately every hour contain a list of introduction nodes and some other identifying data such as the descriptor id (desc id). The desc ID is based on a hash of some hidden service information and it changes every 24 hours. This calculation is outlined later in the post. The hidden service then publishes its updated descriptor to a set of 6 responsible hidden service directories (HSDir’s) every hour. These responsible HSDir’s are regular node on the Tor network which have up-time longer than 24 hours and which have received the HSDir flag from the directory authorities. The set of responsible HSDir’s is based on their position of the current descriptor id in a list of all current HSDir’s ordered by their node fingerprint. This is an implementation of a simple DHT (Distributed Hash Table).