en/hidden-services.wml (3bbf34795) - tor-webwml.git

hidden-services.wml

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# Revision: $Revision$
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#include "head.wmi" TITLE="Tor: Hidden Service Protocol"

<h2>Tor: Hidden Service Protocol</h2>
<hr />

A hidden service needs to advertise its existence in the Tor network before
clients will be able to contact it. Therefore, the service randomly picks
some relays, builds circuits to them, and asks them to act as introduction
points telling them its public key. Note that in the following figures the
green links are circuits rather than direct connections. This makes it
impossible for anyone to associate the introduction points with the hidden
server's IP address. This is important, because although the introduction
points and others are told the hidden service's identity (public key), they
must not learn about the hidden server's location (IP address).

<img alt="Tor hidden service step one" src="$(IMGROOT)/THS-1.png" />
# maybe add a speech bubble containing "PK" to Bob, because that's what
# Bob tells to his introduction points

In a second step, the hidden service assembles a hidden service descriptor
containing the introduction points' addresses and its public key and signs
it with its private key. It stores that descriptor on a set of directory
servers, again using a circuit that hides the link between the directory
server storing the
descriptor with the hidden server's IP address. The descriptor will be
found by clients requesting XYZ.onion where XYZ is a 16 characters long
name that can be uniquely derived from the service's public key. Although
it might seem impractical to use an automatically-generated service name,
it serves an important goal: Everyone &ndash; including the introduction points,
the directory servers, and of course the clients &ndash; can verify that they
are talking to the hidden service. After this step, the hidden service is
set up.

<img alt="Tor hidden service step two" src="$(IMGROOT)/THS-2.png" />
# maybe replace "database" with "directory servers"; further: how incorrect
# is it to *not* add DB to the Tor cloud, now that begin dir cells are in
# use?

A client that wants to contact a hidden service needs to learn about its
onion address first. After that, the client can initiate connection
establishment by downloading the descriptor from the directory servers. If
there is a descriptor for XYZ.onion (the hidden service could also be
offline or have left long ago, or there could be a typo in the onion
address), the client creates a circuit to another randomly picked relay and
asks it to act as rendezvous point, telling it a one-time secret.

<img alt="Tor hidden service step three" src="$(IMGROOT)/THS-3.png" />
# maybe add "cookie" to speech bubble, separated from the surrounded
# "IP1-3" and "PK"

Upon setting up the rendezvous point, the client assembles an introduce
message (encrypted to the hidden service's public key) including the
address of the rendezvous point and the one-time secret. The client sends
this message to one of the introduction points, requesting it to deliver it
to the hidden service. Again, communication takes place via a circuit, so
that nobody can relate sending the introduce message to the client's IP
address, ensuring the client's anonymity.

The hidden service decrypts the client's introduce message and finds the
address of the rendezvous point and the one-time secret in it. The service
creates a circuit to the rendezvous point and sends the one-time secret to
it in a rendezvous message.

At this point it is of special importance that the hidden service sticks to
the same set of guard nodes for creating new circuits. Otherwise an attacker
could run an own relay and force a hidden service to create an arbitrary
number of circuits in the hope of the corrupt relay to be picked as entry
node and learn the hidden server's IP address via timing analysis. This
attack was described by &Oslash;verlier and Syverson in their paper titled
Locating Hidden Servers.

<img alt="Tor hidden service step five" src="$(IMGROOT)/THS-5.png" />
# it should say "Bob connects to Alice's ..."

In the last step, the rendezvous point notifies the client about successful
connection establishment. After that, both, client and hidden service can
use their circuits to the rendezvous point for communicating with each
other. The rendezvous point simply relays (end-to-end encrypted) messages
from client to service and vice versa.

One of the reasons for not using the earlier created connection via the
introduction point for actual communication is that no single relay should
appear to be responsible for a given hidden service. This is why the
rendezvous point never learns about the hidden service's identity.

In general, the complete connection between client and hidden service
consists of 6 relays: 3 of them were picked by the client with the third
being the rendezvous point and the other 3 were picked by the hidden
service.

There are more detailed descriptions about the hidden service protocol than
this one. See the
<a href="<svnsandbox>doc/design-paper/tor-design.pdf">Tor design paper</a>
for an in-depth design description and the
<a href="<svnsandbox>doc/spec/rend-spec.txt">rendezvous specification</a>
for the message formats.

</div>

#include <foot.wmi>

tor-webwml.git

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