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5) #include "head.wmi" TITLE="Tor: Hidden Service Protocol" CHARSET="UTF-8"
6) <div id="content" class="clearfix">
7)   <div id="breadcrumbs">

8)     <a href="<page index>">Home &raquo; </a>

9)     <a href="<page docs/documentation>">Documentation &raquo; </a>
10)     <a href="<page docs/hidden-services>">Hidden Services</a>
11)   </div>
12)   <div id="maincol"> 
13)     <h2>Tor: Hidden Service Protocol</h2>

14)     <hr>

15)     
16)     <p>
17)     Tor makes it possible for users to hide their locations while offering
18)     various kinds of services, such as web publishing or an instant
19)     messaging server.  Using Tor "rendezvous points," other Tor users can
20)     connect to these hidden services, each without knowing the other's
21)     network identity. This page describes the technical details of how
22)     this rendezvous protocol works. For a more direct how-to, see our <a
23)     href="<page docs/tor-hidden-service>">configuring hidden services</a>
24)     page.
25)     </p>
26)     
27)     <p>
28)     A hidden service needs to advertise its existence in the Tor network before
29)     clients will be able to contact it. Therefore, the service randomly picks
30)     some relays, builds circuits to them, and asks them to act as
31)     <em>introduction points</em> by telling them its public key. Note
32)     that in the following figures the green links are circuits rather
33)     than direct connections. By using a full Tor circuit, it's hard for
34)     anyone to associate an introduction point with the hidden server's IP
35)     address. While the introduction points and others are told the hidden
36)     service's identity (public key), we don't want them to learn about the
37)     hidden server's location (IP address).
38)     </p>
39)     

40)     <img alt="Tor hidden service step one" src="$(IMGROOT)/THS-1.png">

41)     # maybe add a speech bubble containing "PK" to Bob, because that's what
42)     # Bob tells to his introduction points
43)     
44)     <p>
45)     Step two: the hidden service assembles a <em>hidden service
46)     descriptor</em>, containing its public key and a summary of each
47)     introduction point, and signs this descriptor with its private key.
48)     It uploads that descriptor to a distributed hash table. The descriptor will be
49)     found by clients requesting XYZ.onion where XYZ is a 16 character
50)     name that can be uniquely derived from the service's public key. After
51)     this step, the hidden service is set up.
52)     </p>
53)     
54)     <p>
55)     Although it might seem impractical to use an automatically-generated
56)     service name, it serves an important goal: Everyone &ndash; including
57)     the introduction points, the distributed hash table directory, and of course the
58)     clients &ndash; can verify that they are talking to the right hidden

59)     service. See also <a href="http://zooko.com/distnames.html">Zooko's

60)     conjecture</a> that out of Decentralized, Secure, and Human-Meaningful,
61)     you can achieve at most two. Perhaps one day somebody will implement a <a
62)     href="http://www.skyhunter.com/marcs/petnames/IntroPetNames.html">Petname</a>
63)     design for hidden service names?
64)     </p>
65)     

66)     <img alt="Tor hidden service step two" src="$(IMGROOT)/THS-2.png">

67)     # maybe replace "database" with "DHT"; further: how incorrect
68)     # is it to *not* add DB to the Tor cloud, now that begin dir cells are in
69)     # use?
70)     
71)     <p>
72)     Step three: A client that wants to contact a hidden service needs to
73)     learn about its
74)     onion address first. After that, the client can initiate connection
75)     establishment by downloading the descriptor from the distributed hash
76)     table. If
77)     there is a descriptor for XYZ.onion (the hidden service could also be
78)     offline or have left long ago, or there could be a typo in the onion
79)     address), the client now knows the set of introduction points and the
80)     right public key to use. Around this time, the client also creates
81)     a circuit to another randomly picked relay and asks it to act as
82)     <em>rendezvous point</em> by telling it a one-time secret.
83)     </p>
84)     

85)     <img alt="Tor hidden service step three" src="$(IMGROOT)/THS-3.png">

86)     # maybe add "cookie" to speech bubble, separated from the surrounded
87)     # "IP1-3" and "PK"
88)     
89)     <p>
90)     Step four: When the descriptor is present and the rendezvous point is
91)     ready, the client assembles an <em>introduce</em>
92)     message (encrypted to the hidden service's public key) including the
93)     address of the rendezvous point and the one-time secret. The client sends
94)     this message to one of the introduction points, requesting it be delivered
95)     to the hidden service. Again, communication takes place via a Tor circuit:
96)     nobody can relate sending the introduce message to the client's IP
97)     address, so the client remains anonymous.
98)     </p>
99)     

100)     <img alt="Tor hidden service step four" src="$(IMGROOT)/THS-4.png">

101)     
102)     <p>
103)     Step five: The hidden service decrypts the client's introduce message
104)     and finds the
105)     address of the rendezvous point and the one-time secret in it. The service
106)     creates a circuit to the rendezvous point and sends the one-time secret to
107)     it in a rendezvous message.
108)     </p>
109)     
110)     <p>
111)     At this point it is of special importance that the hidden service sticks to
112)     the same set of <a

113)     href="<wikifaq>#Whatsthisaboutentryguardformerlyknownashelpernodes">entry

114)     guards</a> when creating new circuits. Otherwise an attacker
115)     could run his own relay and force a hidden service to create an arbitrary
116)     number of circuits in the hope that the corrupt relay is picked as entry
117)     node and he learns the hidden server's IP address via timing analysis. This
118)     attack was described by &Oslash;verlier and Syverson in their paper titled
119)     <a href="http://freehaven.net/anonbib/#hs-attack06">Locating Hidden
120)     Servers</a>.
121)     </p>
122)     

123)     <img alt="Tor hidden service step five" src="$(IMGROOT)/THS-5.png">

124)     # it should say "Bob connects to Alice's ..."
125)     
126)     <p>
127)     In the last step, the rendezvous point notifies the client about successful
128)     connection establishment. After that, both client and hidden service can
129)     use their circuits to the rendezvous point for communicating with each
130)     other. The rendezvous point simply relays (end-to-end encrypted) messages
131)     from client to service and vice versa.
132)     </p>
133)     
134)     <p>
135)     One of the reasons for not using the introduction circuit
136)     for actual communication is that no single relay should
137)     appear to be responsible for a given hidden service. This is why the
138)     rendezvous point never learns about the hidden service's identity.
139)     </p>
140)     
141)     <p>
142)     In general, the complete connection between client and hidden service
143)     consists of 6 relays: 3 of them were picked by the client with the third
144)     being the rendezvous point and the other 3 were picked by the hidden
145)     service.
146)     </p>
147)     

148)     <img alt="Tor hidden service step six" src="$(IMGROOT)/THS-6.png">