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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
73)     to learn about its onion address first. After that, the client can
74)     initiate connection establishment by downloading the descriptor from
75)     the distributed hash table. If there is a descriptor for XYZ.onion
76)     (the hidden service could also be offline or have left long ago,
77)     or there could be a typo in the onion address), the client now
78)     knows the set of introduction points and the right public key to
79)     use. Around this time, the client also creates a circuit to another
80)     randomly picked relay and asks it to act as <em>rendezvous point</em>
81)     by telling it a one-time secret.

82)     </p>
83)     

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

85)     # maybe add "cookie" to speech bubble, separated from the surrounded
86)     # "IP1-3" and "PK"
87)     
88)     <p>

89)     Step four: When the descriptor is present and the rendezvous
90)     point is ready, the client assembles an <em>introduce</em> message
91)     (encrypted to the hidden service's public key) including the address
92)     of the rendezvous point and the one-time secret. The client sends
93)     this message to one of the introduction points, requesting it be
94)     delivered to the hidden service. Again, communication takes place
95)     via a Tor circuit: nobody can relate sending the introduce message
96)     to the client's IP address, so the client remains anonymous.

97)     </p>
98)     

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

100)     
101)     <p>
102)     Step five: The hidden service decrypts the client's introduce message

103)     and finds the address of the rendezvous point and the one-time secret
104)     in it. The service creates a circuit to the rendezvous point and
105)     sends the one-time secret to it in a rendezvous message.

106)     </p>
107)     
108)     <p>
109)     At this point it is of special importance that the hidden service sticks to
110)     the same set of <a

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

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

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

122)     # it should say "Bob connects to Alice's ..."
123)     
124)     <p>
125)     In the last step, the rendezvous point notifies the client about successful
126)     connection establishment. After that, both client and hidden service can
127)     use their circuits to the rendezvous point for communicating with each
128)     other. The rendezvous point simply relays (end-to-end encrypted) messages
129)     from client to service and vice versa.
130)     </p>
131)     
132)     <p>
133)     One of the reasons for not using the introduction circuit
134)     for actual communication is that no single relay should
135)     appear to be responsible for a given hidden service. This is why the
136)     rendezvous point never learns about the hidden service's identity.
137)     </p>
138)     
139)     <p>
140)     In general, the complete connection between client and hidden service
141)     consists of 6 relays: 3 of them were picked by the client with the third
142)     being the rendezvous point and the other 3 were picked by the hidden
143)     service.
144)     </p>
145)     

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

147)     
148)     <p>
149)     There are more detailed descriptions about the hidden service protocol than
150)     this one. See the
151)     <a href="<svnprojects>design-paper/tor-design.pdf">Tor design paper</a>
152)     for an in-depth design description and the

153)     <a href="<specblob>rend-spec.txt">rendezvous specification</a>