1

My name is Alice and I would like to send a package to my beloved Bob. In order to avoid getting recognized by Bob I decide to send my message through several nodes. At each step, a node decrypts the message he received reading:

  • the address of next node
  • an encrypted message.

He will then proceed to send it to the address he found in the uncrypted message.

In order to securly encrypt my message I need to know the address of every node and their public keys. How does Tor collects such information?

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Your Tor software comes with a list of predefined directory authorities. These authorities maintain signed lists of relays from which your client can choose to build the connection. A new relay publishes a server descriptor to the authorities to advertise itself.

Since not every Tor client acts as a relay, the number is smaller than you might assume and you don't need to somehow aggregate the node list yourself.

The FAQ explains this pretty well:

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 2012 there are 8 directory authorities), he can't trick the Tor client into using other Tor relays.

You can explore the bridges and relays at Atlas or Globe.

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