3 Tor Rendezvous Specification
5 0. Overview and preliminaries
7 Read http://tor.eff.org/doc/design-paper/tor-design.html#sec:rendezvous
8 before you read this specification. It will make more sense.
10 Rendezvous points provide location-hidden services (server
11 anonymity) for the onion routing network. With rendezvous points,
12 Bob can offer a TCP service (say, a webserver) via the onion
13 routing network, without revealing the IP of that service.
15 Bob does this by anonymously advertising a public key for his
16 service, along with a list of onion routers to act as "Introduction
17 Points" for his service. He creates forward circuits to those
18 introduction points, and tells them about his public key. To
19 connect to Bob, Alice first builds a circuit to an OR to act as
20 her "Rendezvous Point." She then connects to one of Bob's chosen
21 introduction points, optionally provides authentication or
22 authorization information, and asks it to tell him about her Rendezvous
23 Point (RP). If Bob chooses to answer, he builds a circuit to her
24 RP, and tells it to connect him to Alice. The RP joins their
25 circuits together, and begins relaying cells. Alice's 'BEGIN'
26 cells are received directly by Bob's OP, which passes data to
27 and from the local server implementing Bob's service.
29 Below we describe a network-level specification of this service,
30 along with interfaces to make this process transparent to Alice
31 (so long as she is using an OP).
33 0.1. Notation, conventions and prerequisites
35 In the specifications below, we use the same notation as in
36 "tor-spec.txt". The service specified here also requires the
37 existence of an onion routing network as specified in that file.
39 H(x) is a SHA1 digest of x.
40 PKSign(SK,x) is a PKCS.1-padded RSA signature of x with SK.
41 PKEncrypt(SK,x) is a PKCS.1-padded RSA encryption of x with SK.
42 Public keys are all RSA, and encoded in ASN.1.
43 All integers are stored in network (big-endian) order.
44 All symmetric encryption uses AES in counter mode, except where
47 In all discussions, "Alice" will refer to a user connecting to a
48 location-hidden service, and "Bob" will refer to a user running a
49 location-hidden service.
53 1. Bob->Bob's OP: "Offer IP:Port as
54 public-key-name:Port". [configuration]
55 (We do not specify this step; it is left to the implementor of
58 2. Bob's OP generates keypair and rendezvous service descriptor:
59 "Meet public-key X at introduction point A, B, or C." (signed)
61 3. Bob's OP->Introduction point via Tor: [introduction setup]
64 4. Bob's OP->directory service via Tor: publishes Bob's service
65 descriptor [advertisement]
67 5. Out of band, Alice receives a [x.y.]z.onion:port address.
68 She opens a SOCKS connection to her OP, and requests
71 6. Alice's OP retrieves Bob's descriptor via Tor. [descriptor lookup.]
73 7. Alice's OP chooses a rendezvous point, opens a circuit to that
74 rendezvous point, and establishes a rendezvous circuit. [rendezvous
77 8. Alice connects to the Introduction point via Tor, and tells it about
78 her rendezvous point and optional authentication/authorization
79 information. (Encrypted to Bob.) [Introduction 1]
81 9. The Introduction point passes this on to Bob's OP via Tor, along the
82 introduction circuit. [Introduction 2]
84 10. Bob's OP decides whether to connect to Alice, and if so, creates a
85 circuit to Alice's RP via Tor. Establishes a shared circuit.
88 11. Alice's OP sends begin cells to Bob's OP. [Connection]
90 0.3. Constants and new cell types
93 32 -- RELAY_ESTABLISH_INTRO
94 33 -- RELAY_ESTABLISH_RENDEZVOUS
95 34 -- RELAY_INTRODUCE1
96 35 -- RELAY_INTRODUCE2
97 36 -- RELAY_RENDEZVOUS1
98 37 -- RELAY_RENDEZVOUS2
99 38 -- RELAY_INTRO_ESTABLISHED
100 39 -- RELAY_RENDEZVOUS_ESTABLISHED
101 40 -- RELAY_COMMAND_INTRODUCE_ACK
105 1.1. Bob configures his local OP.
107 We do not specify a format for the OP configuration file. However,
108 OPs SHOULD allow Bob to provide more than one advertised service
109 per OP, and MUST allow Bob to specify one or more virtual ports per
110 service. Bob provides a mapping from each of these virtual ports
111 to a local IP:Port pair.
113 1.2. Bob's OP generates service descriptors.
115 The first time the OP provides an advertised service, it generates
116 a public/private keypair (stored locally). Periodically, the OP
117 generates a pair of service descriptors, one "V1" and one "V0".
119 The "V1" descriptor in 0.1.1.6-alpha contains:
121 V Format byte: set to 255 [1 octet]
122 V Version byte: set to 1 [1 octet]
123 KL Key length [2 octets]
124 PK Bob's public key [KL octets]
125 TS A timestamp [4 octets]
126 PROTO Rendezvous protocol versions: bitmask [2 octets]
127 NA Number of auth mechanisms accepted [1 octet]
128 For each auth mechanism:
129 AUTHT The auth type that is supported [2 octets]
130 AUTHL Length of auth data [1 octet]
131 AUTHD Auth data [variable]
132 NI Number of introduction points [2 octets]
133 For each introduction point: (as in INTRODUCE2 cells)
134 ATYPE An address type (typically 4) [1 octet]
135 ADDR Introduction point's IP address [4 or 16 octets]
136 PORT Introduction point's OR port [2 octets]
137 AUTHT The auth type that is supported [2 octets]
138 AUTHL Length of auth data [1 octet]
139 AUTHD Auth data [variable]
140 ID Introduction point identity ID [20 octets]
141 KLEN Length of onion key [2 octets]
142 KEY Introduction point onion key [KLEN octets]
143 SIG Signature of above fields [variable]
145 The "V1" descriptor in 0.1.1.5-alpha-cvs contains:
147 V Format byte: set to 255 [1 octet]
148 V Version byte: set to 1 [1 octet]
149 KL Key length [2 octets]
150 PK Bob's public key [KL octets]
151 TS A timestamp [4 octets]
152 PROTO Protocol versions: bitmask [2 octets]
153 NI Number of introduction points [2 octets]
154 For each introduction point: (as in INTRODUCE2 cells)
155 IP Introduction point's address [4 octets]
156 PORT Introduction point's OR port [2 octets]
157 ID Introduction point identity ID [20 octets]
158 KLEN Length of onion key [2 octets]
159 KEY Introduction point onion key [KLEN octets]
160 SIG Signature of above fields [variable]
162 The "V0" descriptor contains:
164 KL Key length [2 octets]
165 PK Bob's public key [KL octets]
166 TS A timestamp [4 octets]
167 NI Number of introduction points [2 octets]
168 Ipt A list of NUL-terminated ORs [variable]
169 SIG Signature of above fields [variable]
171 KL is the length of PK, in octets.
172 TS is the number of seconds elapsed since Jan 1, 1970.
174 AUTHT specifies which authentication/authorization mechanism is
175 required by the hidden service or the introduction point. AUTHD
176 is arbitrary data that can be associated with an auth approach.
177 Currently only AUTHT of [00 00] is supported, with an AUTHL of 0.
178 See section 2 of this document for details on auth mechanisms.
180 The members of Ipt may be either (a) nicknames, or (b) identity key
181 digests, encoded in hex, and prefixed with a '$'. Clients must
182 accept both forms. Services must only generate the second form.
183 Once 0.0.9.x is obsoleted, we can drop the first form.
185 [It's ok for Bob to advertise 0 introduction points. He might want
186 to do that if he previously advertised some introduction points,
187 and now he doesn't have any. -RD]
189 [Once Tor 0.1.0.x is obsolete, we can stop generating or using V0
192 1.3. Bob's OP establishes his introduction points.
194 The OP establishes a new introduction circuit to each introduction
195 point. These circuits MUST NOT be used for anything but rendezvous
196 introduction. To establish the introduction, Bob sends a
197 RELAY_ESTABLISH_INTRO cell, containing:
199 KL Key length [2 octets]
200 PK Bob's public key [KL octets]
201 HS Hash of session info [20 octets]
202 SIG Signature of above information [variable]
204 [XXX011, need to add auth information here. -RD]
206 To prevent replay attacks, the HS field contains a SHA-1 hash based on the
207 shared secret KH between Bob's OP and the introduction point, as
209 HS = H(KH | "INTRODUCE")
211 HS = H(KH | [49 4E 54 52 4F 44 55 43 45])
212 (KH, as specified in tor-spec.txt, is H(g^xy | [00]) .)
214 Upon receiving such a cell, the OR first checks that the signature is
215 correct with the included public key. If so, it checks whether HS is
216 correct given the shared state between Bob's OP and the OR. If either
217 check fails, the OP discards the cell; otherwise, it associates the
218 circuit with Bob's public key, and dissociates any other circuits
219 currently associated with PK. On success, the OR sends Bob a
220 RELAY_INTRO_ESTABLISHED cell with an empty payload.
222 1.4. Bob's OP advertises his service descriptor(s).
224 Bob's OP opens a stream to each directory server's directory port via Tor.
225 (He may re-use old circuits for this.) Over this stream, Bob's OP makes
226 an HTTP 'POST' request, to a URL "/tor/rendezvous/publish" relative to the
227 directory server's root, containing as its body Bob's service descriptor.
229 Bob should upload a service descriptor for each version format that
230 is supported in the current Tor network.
232 Upon receiving a descriptor, the directory server checks the signature,
233 and discards the descriptor if the signature does not match the enclosed
234 public key. Next, the directory server checks the timestamp. If the
235 timestamp is more than 24 hours in the past or more than 1 hour in the
236 future, or the directory server already has a newer descriptor with the
237 same public key, the server discards the descriptor. Otherwise, the
238 server discards any older descriptors with the same public key and
239 version format, and associates the new descriptor with the public key.
240 The directory server remembers this descriptor for at least 24 hours
241 after its timestamp. At least every 18 hours, Bob's OP uploads a
244 1.5. Alice receives a x.y.z.onion address.
246 When Alice receives a pointer to a location-hidden service, it is as a
247 hostname of the form "z.onion" or "y.z.onion" or "x.y.z.onion", where
248 z is a base-32 encoding of a 10-octet hash of Bob's service's public
249 key, computed as follows:
252 2. Let H' = the first 80 bits of H, considering each octet from
253 most significant bit to least significant bit.
254 2. Generate a 16-character encoding of H', using base32 as defined
257 (We only use 80 bits instead of the 160 bits from SHA1 because we
258 don't need to worry about arbitrary collisions, and because it will
259 make handling the url's more convenient.)
261 The string "x", if present, is the base-32 encoding of the
262 authentication/authorization required by the introduction point.
263 The string "y", if present, is the base-32 encoding of the
264 authentication/authorization required by the hidden service.
265 Omitting a string is taken to mean auth type [00 00].
266 See section 2 of this document for details on auth mechanisms.
268 [Yes, numbers are allowed at the beginning. See RFC 1123. -NM]
270 1.6. Alice's OP retrieves a service descriptor.
272 Alice opens a stream to a directory server via Tor, and makes an HTTP GET
273 request for the document '/tor/rendezvous/<z>' or '/tor/rendezvous1/<z>',
274 where '<z>' is replaced with the encoding of Bob's public key as described
275 above. (She may re-use old circuits for this.) The directory replies with
276 a 404 HTTP response if it does not recognize <z>, and otherwise returns
277 Bob's most recently uploaded service descriptor. (If Alice requests
278 'rendezvous1', the directory server provides a V1 descriptor or a V0
279 descriptor if no V1 descriptor is available. If Alice requests
280 'rendezvous', the directory server returns a V0 descriptor.)
282 If Alice's OP receives a 404 response, it tries the other directory
283 servers, and only fails the lookup if none recognize the public key hash.
285 Upon receiving a service descriptor, Alice verifies with the same process
286 as the directory server uses, described above in section 1.4.
288 The directory server gives a 400 response if it cannot understand Alice's
291 Alice should cache the descriptor locally, but should not use
292 descriptors that are more than 24 hours older than their timestamp.
293 [Caching may make her partitionable, but she fetched it anonymously,
294 and we can't very well *not* cache it. -RD]
296 1.7. Alice's OP establishes a rendezvous point.
298 When Alice requests a connection to a given location-hidden service,
299 and Alice's OP does not have an established circuit to that service,
300 the OP builds a rendezvous circuit. It does this by establishing
301 a circuit to a randomly chosen OR, and sending a
302 RELAY_ESTABLISH_RENDEZVOUS cell to that OR. The body of that cell
305 RC Rendezvous cookie [20 octets]
307 [XXX011 this looks like an auth mechanism. should we generalize here? -RD]
309 The rendezvous cookie is an arbitrary 20-byte value, chosen randomly by
312 Upon receiving a RELAY_ESTABLISH_RENDEZVOUS cell, the OR associates the
313 RC with the circuit that sent it. It replies to Alice with an empty
314 RELAY_RENDEZVOUS_ESTABLISHED cell to indicate success.
316 Alice's OP MUST NOT use the circuit which sent the cell for any purpose
317 other than rendezvous with the given location-hidden service.
319 1.8. Introduction: from Alice's OP to Introduction Point
321 Alice builds a separate circuit to one of Bob's chosen introduction
322 points, and sends it a RELAY_INTRODUCE1 cell containing:
325 PK_ID Identifier for Bob's PK [20 octets]
326 [XXX011 want to put intro-level auth info here, but no version. crap. -RD]
328 Encrypted to Bob's PK:
329 RP Rendezvous point's nickname [20 octets]
330 RC Rendezvous cookie [20 octets]
331 g^x Diffie-Hellman data, part 1 [128 octets]
333 VER Version byte: set to 1. [1 octet]
334 RP Rendezvous point nick or ID [42 octets]
335 RC Rendezvous cookie [20 octets]
336 g^x Diffie-Hellman data, part 1 [128 octets]
338 VER Version byte: set to 2. [1 octet]
339 IP Rendezvous point's address [4 octets]
340 PORT Rendezvous point's OR port [2 octets]
341 ID Rendezvous point identity ID [20 octets]
342 KLEN Length of onion key [2 octets]
343 KEY Rendezvous point onion key [KLEN octets]
344 RC Rendezvous cookie [20 octets]
345 g^x Diffie-Hellman data, part 1 [128 octets]
347 VER Version byte: set to 3. [1 octet]
348 ATYPE An address type (typically 4) [1 octet]
349 ADDR Introduction point's IP address [4 or 16 octets]
350 PORT Rendezvous point's OR port [2 octets]
351 AUTHT The auth type that is supported [2 octets]
352 AUTHL Length of auth data [1 octet]
353 AUTHD Auth data [variable]
354 ID Rendezvous point identity ID [20 octets]
355 KLEN Length of onion key [2 octets]
356 KEY Rendezvous point onion key [KLEN octets]
357 RC Rendezvous cookie [20 octets]
358 g^x Diffie-Hellman data, part 1 [128 octets]
360 PK_ID is the hash of Bob's public key. RP is NUL-padded and terminated,
361 and must contain EITHER a nickname, or an identity key digest, encoded in
362 hex, and prefixed with a '$'.
364 Implementations SHOULD accept all variants, and list the variants they
365 accept in their V1 descriptor. Implementations should only generate the
366 variants listed in the service's V1 descriptor; if no V1 descriptor is
367 available, only the first variant should be generated. No version should
368 generate the second variant (version byte=1).
370 The hybrid encryption to Bob's PK works just like the hybrid
371 encryption in CREATE cells (see main spec). Thus the payload of the
372 RELAY_INTRODUCE1 cell on the wire will contain 20+42+16+20+20+128=246
373 bytes. [XXXX not really]
375 1.9. Introduction: From the Introduction Point to Bob's OP
377 If the Introduction Point recognizes PK_ID as a public key which has
378 established a circuit for introductions as in 1.3 above, it sends the body
379 of the cell in a new RELAY_INTRODUCE2 cell down the corresponding circuit.
380 (If the PK_ID is unrecognized, the RELAY_INTRODUCE1 cell is discarded.)
382 After sending the RELAY_INTRODUCE2 cell, the OR replies to Alice with an
383 empty RELAY_COMMAND_INTRODUCE_ACK cell. If no RELAY_INTRODUCE2 cell can
384 be sent, the OR replies to Alice with a non-empty cell to indicate an
385 error. (The semantics of the cell body may be determined later; the
386 current implementation sends a single '1' byte on failure.)
388 When Bob's OP receives the RELAY_INTRODUCE2 cell, it decrypts it with
389 the private key for the corresponding hidden service, and extracts the
390 rendezvous point's nickname, the rendezvous cookie, and the value of g^x
395 Bob's OP builds a new Tor circuit ending at Alice's chosen rendezvous
396 point, and sends a RELAY_RENDEZVOUS1 cell along this circuit, containing:
397 RC Rendezvous cookie [20 octets]
398 g^y Diffie-Hellman [128 octets]
399 KH Handshake digest [20 octets]
401 (Bob's OP MUST NOT use this circuit for any other purpose.)
403 If the RP recognizes RC, it relays the rest of the cell down the
404 corresponding circuit in a RELAY_RENDEZVOUS2 cell, containing:
406 g^y Diffie-Hellman [128 octets]
407 KH Handshake digest [20 octets]
409 (If the RP does not recognize the RC, it discards the cell and
410 tears down the circuit.)
412 When Alice's OP receives a RELAY_RENDEZVOUS2 cell on a circuit which
413 has sent a RELAY_ESTABLISH_RENDEZVOUS cell but which has not yet received
414 a reply, it uses g^y and H(g^xy) to complete the handshake as in the Tor
415 circuit extend process: they establish a 60-octet string as
416 K = SHA1(g^xy | [00]) | SHA1(g^xy | [01]) | SHA1(g^xy | [02])
422 Subsequently, the rendezvous point passes relay cells, unchanged, from
423 each of the two circuits to the other. When Alice's OP sends
424 RELAY cells along the circuit, it first encrypts them with the
425 Kf, then with all of the keys for the ORs in Alice's side of the circuit;
426 and when Alice's OP receives RELAY cells from the circuit, it decrypts
427 them with the keys for the ORs in Alice's side of the circuit, then
428 decrypts them with Kb. Bob's OP does the same, with Kf and Kb
431 1.11. Creating streams
433 To open TCP connections to Bob's location-hidden service, Alice's OP sends
434 a RELAY_BEGIN cell along the established circuit, using the special
435 address "", and a chosen port. Bob's OP chooses a destination IP and
436 port, based on the configuration of the service connected to the circuit,
437 and opens a TCP stream. From then on, Bob's OP treats the stream as an
438 ordinary exit connection.
439 [ Except he doesn't include addr in the connected cell or the end
442 Alice MAY send multiple RELAY_BEGIN cells along the circuit, to open
443 multiple streams to Bob. Alice SHOULD NOT send RELAY_BEGIN cells for any
444 other address along her circuit to Bob; if she does, Bob MUST reject them.
446 2.0. Authentication and authorization.