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From reading this and this, I understand that a hidden service builds a virtual network to an introduction point and provides it with its public key.

What I'm wondering is, how does the introduction point then communicate with the hidden service using this information?

(as the second link says we do not want these nodes to know the hidden service's IP address)

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The hidden service builds a circuit just like a normal tor client does, passing through 3 different nodes to connect to it's introduction point.

It is nowhere explicitly mentioned in the tor protocols how comes that an introduction point can still contact the hidden service if time has elapsed since the service has chosen them, but I think it is implied here that the hidden service keeps it's connections to introduction points open.

That means also that if the service goes down temporarily, when going back up their tor client will reconnect to the introduction points and if it doesn't there shall be no way and no need for the IP to find the service back.

  • Are you not referring to the rendezvous point? What I'm wondering is how once the client has contacted the introduction point (that holds the public key of the hidden serivce), does it then go on to communicate with the hidden service, just before it is made aware of the identification of the rendezvous point? – JackWoot Apr 20 '14 at 9:13
  • @JackWoot I updated my answer to be more clear. – user517 Apr 20 '14 at 10:17
  • Much more clear, thank you. So the hidden service keeps up a connection through the virtual circuit, therefore the introduction point doesn't know the hidden service's IP address (and the other nodes in the virtual circuit don't know which position they are in). – JackWoot Apr 21 '14 at 20:52
  • In principle nodes in the tor network have a pretty good idea about their position. As there are only three, you're either first or last or neither, that's hard to hide. In this case, the first node would be a normal guard node as for any other circuit, unless the hidden service was a tor relay as well, and the second knows he is neither first or last, so only the first node would possibly think he is a second in case the service is a relay. That's what I think, but I haven't looked it all up in detail. – user517 Apr 21 '14 at 22:15

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