2 Tor Rendezvous Specification
4 0. Overview and preliminaries
6 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
7 NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
8 "OPTIONAL" in this document are to be interpreted as described in
12 https://www.torproject.org/doc/design-paper/tor-design.html#sec:rendezvous
13 before you read this specification. It will make more sense.
15 Rendezvous points provide location-hidden services (server
16 anonymity) for the onion routing network. With rendezvous points,
17 Bob can offer a TCP service (say, a webserver) via the onion
18 routing network, without revealing the IP of that service.
20 Bob does this by anonymously advertising a public key for his
21 service, along with a list of onion routers to act as "Introduction
22 Points" for his service. He creates forward circuits to those
23 introduction points, and tells them about his service. To
24 connect to Bob, Alice first builds a circuit to an OR to act as
25 her "Rendezvous Point." She then connects to one of Bob's chosen
26 introduction points, and asks it to tell him about her Rendezvous
27 Point (RP). If Bob chooses to answer, he builds a circuit to her
28 RP, and tells it to connect him to Alice. The RP joins their
29 circuits together, and begins relaying cells. Alice's 'BEGIN'
30 cells are received directly by Bob's OP, which passes data to
31 and from the local server implementing Bob's service.
33 Below we describe a network-level specification of this service,
34 along with interfaces to make this process transparent to Alice
35 (so long as she is using an OP).
37 0.1. Notation, conventions and prerequisites
39 In the specifications below, we use the same notation and terminology
40 as in "tor-spec.txt". The service specified here also requires the
41 existence of an onion routing network as specified in that file.
43 H(x) is a SHA1 digest of x.
44 PKSign(SK,x) is a PKCS.1-padded RSA signature of x with SK.
45 PKEncrypt(SK,x) is a PKCS.1-padded RSA encryption of x with SK.
46 Public keys are all RSA, and encoded in ASN.1.
47 All integers are stored in network (big-endian) order.
48 All symmetric encryption uses AES in counter mode, except where
51 In all discussions, "Alice" will refer to a user connecting to a
52 location-hidden service, and "Bob" will refer to a user running a
53 location-hidden service.
55 An OP is (as defined elsewhere) an "Onion Proxy" or Tor client.
57 An OR is (as defined elsewhere) an "Onion Router" or Tor server.
59 An "Introduction point" is a Tor server chosen to be Bob's medium-term
60 'meeting place'. A "Rendezvous point" is a Tor server chosen by Alice to
61 be a short-term communication relay between her and Bob. All Tor servers
62 potentially act as introduction and rendezvous points.
66 1. Bob->Bob's OP: "Offer IP:Port as public-key-name:Port". [configuration]
67 (We do not specify this step; it is left to the implementor of
70 2. Bob's OP generates a long-term keypair.
72 3. Bob's OP->Introduction point via Tor: [introduction setup]
73 "This public key is (currently) associated to me."
75 4. Bob's OP->directory service via Tor: publishes Bob's service descriptor
77 "Meet public-key X at introduction point A, B, or C." (signed)
79 5. Out of band, Alice receives a z.onion:port address.
80 She opens a SOCKS connection to her OP, and requests z.onion:port.
82 6. Alice's OP retrieves Bob's descriptor via Tor. [descriptor lookup.]
84 7. Alice's OP chooses a rendezvous point, opens a circuit to that
85 rendezvous point, and establishes a rendezvous circuit. [rendezvous
88 8. Alice connects to the Introduction point via Tor, and tells it about
89 her rendezvous point. (Encrypted to Bob.) [Introduction 1]
91 9. The Introduction point passes this on to Bob's OP via Tor, along the
92 introduction circuit. [Introduction 2]
94 10. Bob's OP decides whether to connect to Alice, and if so, creates a
95 circuit to Alice's RP via Tor. Establishes a shared circuit.
98 11. The Rendezvous point forwards Bob's confirmation to Alice's OP.
101 12. Alice's OP sends begin cells to Bob's OP. [Connection]
103 0.3. Constants and new cell types
106 32 -- RELAY_COMMAND_ESTABLISH_INTRO
107 33 -- RELAY_COMMAND_ESTABLISH_RENDEZVOUS
108 34 -- RELAY_COMMAND_INTRODUCE1
109 35 -- RELAY_COMMAND_INTRODUCE2
110 36 -- RELAY_COMMAND_RENDEZVOUS1
111 37 -- RELAY_COMMAND_RENDEZVOUS2
112 38 -- RELAY_COMMAND_INTRO_ESTABLISHED
113 39 -- RELAY_COMMAND_RENDEZVOUS_ESTABLISHED
114 40 -- RELAY_COMMAND_INTRODUCE_ACK
116 0.4. Version overview
118 There are several parts in the hidden service protocol that have
119 changed over time, each of them having its own version number, whereas
120 other parts remained the same. The following list of potentially
121 versioned protocol parts should help reduce some confusion:
123 - Hidden service descriptor: the binary-based v0 was the default for a
124 long time, and an ASCII-based v2 has been added by proposal 114. The
125 v0 descriptor format has been deprecated in 0.2.2.1-alpha. See 1.3.
127 - Hidden service descriptor propagation mechanism: currently related to
128 the hidden service descriptor version -- v0 publishes to the original
129 hs directory authorities, whereas v2 publishes to a rotating subset
130 of relays with the "HSDir" flag; see 1.4 and 1.6.
132 - Introduction protocol for how to generate an introduction cell:
133 v0 specified a nickname for the rendezvous point and assumed the
134 relay would know about it, whereas v2 now specifies IP address,
135 port, and onion key so the relay doesn't need to already recognize
140 1.1. Bob configures his local OP.
142 We do not specify a format for the OP configuration file. However,
143 OPs SHOULD allow Bob to provide more than one advertised service
144 per OP, and MUST allow Bob to specify one or more virtual ports per
145 service. Bob provides a mapping from each of these virtual ports
146 to a local IP:Port pair.
148 1.2. Bob's OP establishes his introduction points.
150 The first time the OP provides an advertised service, it generates
151 a public/private keypair (stored locally).
153 The OP choses a small number of Tor servers as introduction points.
154 The OP establishes a new introduction circuit to each introduction
155 point. These circuits MUST NOT be used for anything but hidden service
156 introduction. To establish the introduction, Bob sends a
157 RELAY_COMMAND_ESTABLISH_INTRO cell, containing:
159 KL Key length [2 octets]
160 PK Bob's public key or service key [KL octets]
161 HS Hash of session info [20 octets]
162 SIG Signature of above information [variable]
164 KL is the length of PK, in octets.
166 To prevent replay attacks, the HS field contains a SHA-1 hash based on the
167 shared secret KH between Bob's OP and the introduction point, as
169 HS = H(KH | "INTRODUCE")
171 HS = H(KH | [49 4E 54 52 4F 44 55 43 45])
172 (KH, as specified in tor-spec.txt, is H(g^xy | [00]) .)
174 Upon receiving such a cell, the OR first checks that the signature is
175 correct with the included public key. If so, it checks whether HS is
176 correct given the shared state between Bob's OP and the OR. If either
177 check fails, the OP discards the cell; otherwise, it associates the
178 circuit with Bob's public key, and dissociates any other circuits
179 currently associated with PK. On success, the OR sends Bob a
180 RELAY_COMMAND_INTRO_ESTABLISHED cell with an empty payload.
182 Bob's OP uses either Bob's public key or a freshly generated, single-use
183 service key in the RELAY_COMMAND_ESTABLISH_INTRO cell, depending on the
184 configured hidden service descriptor version. The public key is used for
185 v0 descriptors, the service key for v2 descriptors. In the latter case, the
186 service keys of all introduction points are included in the v2 hidden
187 service descriptor together with the other introduction point information.
188 The reason is that the introduction point does not need to and therefore
189 should not know for which hidden service it works, so as to prevent it from
190 tracking the hidden service's activity. If the hidden service is configured
191 to publish both v0 and v2 descriptors, two separate sets of introduction
192 points are established.
194 1.3. Bob's OP generates service descriptors.
196 For versions before 0.2.2.1-alpha, Bob's OP periodically generates and
197 publishes a descriptor of type "V0".
199 The "V0" descriptor contains:
201 KL Key length [2 octets]
202 PK Bob's public key [KL octets]
203 TS A timestamp [4 octets]
204 NI Number of introduction points [2 octets]
205 Ipt A list of NUL-terminated ORs [variable]
206 SIG Signature of above fields [variable]
208 TS is the number of seconds elapsed since Jan 1, 1970.
210 The members of Ipt may be either (a) nicknames, or (b) identity key
211 digests, encoded in hex, and prefixed with a '$'. Clients must
212 accept both forms. Services must only generate the second form.
213 Once 0.0.9.x is obsoleted, we can drop the first form.
215 [It's ok for Bob to advertise 0 introduction points. He might want
216 to do that if he previously advertised some introduction points,
217 and now he doesn't have any. -RD]
219 Beginning with 0.2.0.10-alpha, Bob's OP encodes "V2" descriptors in
220 addition to (or instead of) "V0" descriptors. The format of a "V2"
221 descriptor is as follows:
223 "rendezvous-service-descriptor" descriptor-id NL
225 [At start, exactly once]
227 Indicates the beginning of the descriptor. "descriptor-id" is a
228 periodically changing identifier of 160 bits formatted as 32 base32
229 chars that is calculated by the hidden service and its clients. The
230 "descriptor-id" is calculated by performing the following operation:
233 H(permanent-id | H(time-period | descriptor-cookie | replica))
235 "permanent-id" is the permanent identifier of the hidden service,
236 consisting of 80 bits. It can be calculated by computing the hash value
237 of the public hidden service key and truncating after the first 80 bits:
239 permanent-id = H(public-key)[:10]
241 "H(time-period | descriptor-cookie | replica)" is the (possibly
242 secret) id part that is necessary to verify that the hidden service is
243 the true originator of this descriptor and that is therefore contained
244 in the descriptor, too. The descriptor ID can only be created by the
245 hidden service and its clients, but the "signature" below can only be
246 created by the service.
248 "time-period" changes periodically as a function of time and
250 "permanent-id". The current value for "time-period" can be calculated
251 using the following formula:
253 time-period = (current-time + permanent-id-byte * 86400 / 256)
256 "current-time" contains the current system time in seconds since
257 1970-01-01 00:00, e.g. 1188241957. "permanent-id-byte" is the first
258 (unsigned) byte of the permanent identifier (which is in network
259 order), e.g. 143. Adding the product of "permanent-id-byte" and
260 86400 (seconds per day), divided by 256, prevents "time-period" from
261 changing for all descriptors at the same time of the day. The result
262 of the overall operation is a (network-ordered) 32-bit integer, e.g.
263 13753 or 0x000035B9 with the example values given above.
265 "descriptor-cookie" is an optional secret password of 128 bits that
266 is shared between the hidden service provider and its clients. If the
267 descriptor-cookie is left out, the input to the hash function is 128
270 "replica" denotes the number of the replica. A service publishes
271 multiple descriptors with different descriptor IDs in order to
272 distribute them to different places on the ring.
274 "version" version-number NL
278 The version number of this descriptor's format. In this case: 2.
280 "permanent-key" NL a public key in PEM format
284 The public key of the hidden service which is required to verify the
285 "descriptor-id" and the "signature".
287 "secret-id-part" secret-id-part NL
291 The result of the following operation as explained above, formatted as
292 32 base32 chars. Using this secret id part, everyone can verify that
293 the signed descriptor belongs to "descriptor-id".
295 secret-id-part = H(time-period | descriptor-cookie | replica)
297 "publication-time" YYYY-MM-DD HH:MM:SS NL
301 A timestamp when this descriptor has been created.
303 "protocol-versions" version-string NL
307 A comma-separated list of recognized and permitted version numbers
308 for use in INTRODUCE cells; these versions are described in section
311 "introduction-points" NL encrypted-string
315 A list of introduction points. If the optional "descriptor-cookie" is
316 used, this list is encrypted with AES in CTR mode with a random
317 initialization vector of 128 bits that is written to
318 the beginning of the encrypted string, and the "descriptor-cookie" as
319 secret key of 128 bits length.
321 The string containing the introduction point data (either encrypted
322 or not) is encoded in base64, and surrounded with
323 "-----BEGIN MESSAGE-----" and "-----END MESSAGE-----".
325 The unencrypted string may begin with:
327 "service-authentication" auth-type auth-data NL
331 The service-specific authentication data can be used to perform
332 client authentication. This data is independent of the selected
333 introduction point as opposed to "intro-authentication" below. The
334 format of auth-data (base64-encoded or PEM format) depends on
335 auth-type. See section 2 of this document for details on auth
338 Subsequently, an arbitrary number of introduction point entries may
339 follow, each containing the following data:
341 "introduction-point" identifier NL
343 [At start, exactly once]
345 The identifier of this introduction point: the base-32 encoded
346 hash of this introduction point's identity key.
348 "ip-address" ip-address NL
352 The IP address of this introduction point.
358 The TCP port on which the introduction point is listening for
359 incoming onion requests.
361 "onion-key" NL a public key in PEM format
365 The public key that can be used to encrypt messages to this
368 "service-key" NL a public key in PEM format
372 The public key that can be used to encrypt messages to the hidden
375 "intro-authentication" auth-type auth-data NL
379 The introduction-point-specific authentication data can be used
380 to perform client authentication. This data depends on the
381 selected introduction point as opposed to "service-authentication"
382 above. The format of auth-data (base64-encoded or PEM format)
383 depends on auth-type. See section 2 of this document for details
386 (This ends the fields in the encrypted portion of the descriptor.)
388 [It's ok for Bob to advertise 0 introduction points. He might want
389 to do that if he previously advertised some introduction points,
390 and now he doesn't have any. -RD]
392 "signature" NL signature-string
394 [At end, exactly once]
396 A signature of all fields above with the private key of the hidden
399 1.3.1. Other descriptor formats we don't use.
401 Support for the V0 descriptor format was dropped in 0.2.2.0-alpha-dev:
403 KL Key length [2 octets]
404 PK Bob's public key [KL octets]
405 TS A timestamp [4 octets]
406 NI Number of introduction points [2 octets]
407 Ipt A list of NUL-terminated ORs [variable]
408 SIG Signature of above fields [variable]
410 KL is the length of PK, in octets.
411 TS is the number of seconds elapsed since Jan 1, 1970.
413 The members of Ipt may be either (a) nicknames, or (b) identity key
414 digests, encoded in hex, and prefixed with a '$'.
416 The V1 descriptor format was understood and accepted from
417 0.1.1.5-alpha-cvs to 0.2.0.6-alpha-dev, but no Tors generated it and
420 V Format byte: set to 255 [1 octet]
421 V Version byte: set to 1 [1 octet]
422 KL Key length [2 octets]
423 PK Bob's public key [KL octets]
424 TS A timestamp [4 octets]
425 PROTO Protocol versions: bitmask [2 octets]
426 NI Number of introduction points [2 octets]
427 For each introduction point: (as in INTRODUCE2 cells)
428 IP Introduction point's address [4 octets]
429 PORT Introduction point's OR port [2 octets]
430 ID Introduction point identity ID [20 octets]
431 KLEN Length of onion key [2 octets]
432 KEY Introduction point onion key [KLEN octets]
433 SIG Signature of above fields [variable]
435 A hypothetical "V1" descriptor, that has never been used but might
436 be useful for historical reasons, contains:
438 V Format byte: set to 255 [1 octet]
439 V Version byte: set to 1 [1 octet]
440 KL Key length [2 octets]
441 PK Bob's public key [KL octets]
442 TS A timestamp [4 octets]
443 PROTO Rendezvous protocol versions: bitmask [2 octets]
444 NA Number of auth mechanisms accepted [1 octet]
445 For each auth mechanism:
446 AUTHT The auth type that is supported [2 octets]
447 AUTHL Length of auth data [1 octet]
448 AUTHD Auth data [variable]
449 NI Number of introduction points [2 octets]
450 For each introduction point: (as in INTRODUCE2 cells)
451 ATYPE An address type (typically 4) [1 octet]
452 ADDR Introduction point's IP address [4 or 16 octets]
453 PORT Introduction point's OR port [2 octets]
454 AUTHT The auth type that is supported [2 octets]
455 AUTHL Length of auth data [1 octet]
456 AUTHD Auth data [variable]
457 ID Introduction point identity ID [20 octets]
458 KLEN Length of onion key [2 octets]
459 KEY Introduction point onion key [KLEN octets]
460 SIG Signature of above fields [variable]
462 AUTHT specifies which authentication/authorization mechanism is
463 required by the hidden service or the introduction point. AUTHD
464 is arbitrary data that can be associated with an auth approach.
465 Currently only AUTHT of [00 00] is supported, with an AUTHL of 0.
466 See section 2 of this document for details on auth mechanisms.
468 1.4. Bob's OP advertises his service descriptor(s).
470 Bob's OP advertises his service descriptor to a fixed set of v0 hidden
471 service directory servers and/or a changing subset of all v2 hidden service
474 For versions before 0.2.2.1-alpha, Bob's OP opens a stream to each v0
475 directory server's directory port via Tor. (He may re-use old circuits for
476 this.) Over this stream, Bob's OP makes an HTTP 'POST' request, to a URL
477 "/tor/rendezvous/publish" relative to the directory server's root,
478 containing as its body Bob's service descriptor.
480 Upon receiving a descriptor, the directory server checks the signature,
481 and discards the descriptor if the signature does not match the enclosed
482 public key. Next, the directory server checks the timestamp. If the
483 timestamp is more than 24 hours in the past or more than 1 hour in the
484 future, or the directory server already has a newer descriptor with the
485 same public key, the server discards the descriptor. Otherwise, the
486 server discards any older descriptors with the same public key and
487 version format, and associates the new descriptor with the public key.
488 The directory server remembers this descriptor for at least 24 hours
489 after its timestamp. At least every 18 hours, Bob's OP uploads a
492 If Bob's OP is configured to publish v2 descriptors, it does so to a
493 changing subset of all v2 hidden service directories instead of the
494 authoritative directory servers. Therefore, Bob's OP opens a stream via
495 Tor to each responsible hidden service directory. (He may re-use old
496 circuits for this.) Over this stream, Bob's OP makes an HTTP 'POST'
497 request to a URL "/tor/rendezvous2/publish" relative to the hidden service
498 directory's root, containing as its body Bob's service descriptor.
500 At any time, there are 6 hidden service directories responsible for
501 keeping replicas of a descriptor; they consist of 2 sets of 3 hidden
502 service directories with consecutive onion IDs. Bob's OP learns about
503 the complete list of hidden service directories by filtering the
504 consensus status document received from the directory authorities. A
505 hidden service directory is deemed responsible for all descriptor IDs in
506 the interval from its direct predecessor, exclusive, to its own ID,
507 inclusive; it further holds replicas for its 2 predecessors. A
508 participant only trusts its own routing list and never learns about
509 routing information from other parties.
511 Bob's OP publishes a new v2 descriptor once an hour or whenever its
512 content changes. V2 descriptors can be found by clients within a given
513 time period of 24 hours, after which they change their ID as described
514 under 1.3. If a published descriptor would be valid for less than 60
515 minutes (= 2 x 30 minutes to allow the server to be 30 minutes behind
516 and the client 30 minutes ahead), Bob's OP publishes the descriptor
517 under the ID of both, the current and the next publication period.
519 1.5. Alice receives a z.onion address.
521 When Alice receives a pointer to a location-hidden service, it is as a
522 hostname of the form "z.onion", where z is a base-32 encoding of a
523 10-octet hash of Bob's service's public key, computed as follows:
526 2. Let H' = the first 80 bits of H, considering each octet from
527 most significant bit to least significant bit.
528 3. Generate a 16-character encoding of H', using base32 as defined
531 (We only use 80 bits instead of the 160 bits from SHA1 because we
532 don't need to worry about arbitrary collisions, and because it will
533 make handling the url's more convenient.)
535 [Yes, numbers are allowed at the beginning. See RFC 1123. -NM]
537 1.6. Alice's OP retrieves a service descriptor.
539 Alice's OP fetches the service descriptor from the fixed set of v0 hidden
540 service directory servers and/or a changing subset of all v2 hidden service
543 For versions before 0.2.2.1-alpha, Alice's OP opens a stream to a directory
544 server via Tor, and makes an HTTP GET request for the document
545 '/tor/rendezvous/<z>', where '<z>' is replaced with the encoding of Bob's
546 public key as described above. (She may re-use old circuits for this.) The
547 directory replies with a 404 HTTP response if it does not recognize <z>,
548 and otherwise returns Bob's most recently uploaded service descriptor.
550 If Alice's OP receives a 404 response, it tries the other directory
551 servers, and only fails the lookup if none recognize the public key hash.
553 Upon receiving a service descriptor, Alice verifies with the same process
554 as the directory server uses, described above in section 1.4.
556 The directory server gives a 400 response if it cannot understand Alice's
559 Alice should cache the descriptor locally, but should not use
560 descriptors that are more than 24 hours older than their timestamp.
561 [Caching may make her partitionable, but she fetched it anonymously,
562 and we can't very well *not* cache it. -RD]
564 If Alice's OP is running 0.2.1.10-alpha or higher, it fetches v2 hidden
565 service descriptors. Versions before 0.2.2.1-alpha are fetching both v0 and
566 v2 descriptors in parallel. Similar to the description in section 1.4,
567 Alice's OP fetches a v2 descriptor from a randomly chosen hidden service
568 directory out of the changing subset of 6 nodes. If the request is
569 unsuccessful, Alice retries the other remaining responsible hidden service
570 directories in a random order. Alice relies on Bob to care about a potential
571 clock skew between the two by possibly storing two sets of descriptors (see
574 Alice's OP opens a stream via Tor to the chosen v2 hidden service
575 directory. (She may re-use old circuits for this.) Over this stream,
576 Alice's OP makes an HTTP 'GET' request for the document
577 "/tor/rendezvous2/<z>", where z is replaced with the encoding of the
578 descriptor ID. The directory replies with a 404 HTTP response if it does
579 not recognize <z>, and otherwise returns Bob's most recently uploaded
582 1.7. Alice's OP establishes a rendezvous point.
584 When Alice requests a connection to a given location-hidden service,
585 and Alice's OP does not have an established circuit to that service,
586 the OP builds a rendezvous circuit. It does this by establishing
587 a circuit to a randomly chosen OR, and sending a
588 RELAY_COMMAND_ESTABLISH_RENDEZVOUS cell to that OR. The body of that cell
591 RC Rendezvous cookie [20 octets]
593 The rendezvous cookie is an arbitrary 20-byte value, chosen randomly by
594 Alice's OP. Alice SHOULD choose a new rendezvous cookie for each new
597 Upon receiving a RELAY_COMMAND_ESTABLISH_RENDEZVOUS cell, the OR associates
598 the RC with the circuit that sent it. It replies to Alice with an empty
599 RELAY_COMMAND_RENDEZVOUS_ESTABLISHED cell to indicate success.
601 Alice's OP MUST NOT use the circuit which sent the cell for any purpose
602 other than rendezvous with the given location-hidden service.
604 1.8. Introduction: from Alice's OP to Introduction Point
606 Alice builds a separate circuit to one of Bob's chosen introduction
607 points, and sends it a RELAY_COMMAND_INTRODUCE1 cell containing:
610 PK_ID Identifier for Bob's PK [20 octets]
611 Encrypted to Bob's PK: (in the v0 intro protocol)
612 RP Rendezvous point's nickname [20 octets]
613 RC Rendezvous cookie [20 octets]
614 g^x Diffie-Hellman data, part 1 [128 octets]
615 OR (in the v1 intro protocol)
616 VER Version byte: set to 1. [1 octet]
617 RP Rendezvous point nick or ID [42 octets]
618 RC Rendezvous cookie [20 octets]
619 g^x Diffie-Hellman data, part 1 [128 octets]
620 OR (in the v2 intro protocol)
621 VER Version byte: set to 2. [1 octet]
622 IP Rendezvous point's address [4 octets]
623 PORT Rendezvous point's OR port [2 octets]
624 ID Rendezvous point identity ID [20 octets]
625 KLEN Length of onion key [2 octets]
626 KEY Rendezvous point onion key [KLEN octets]
627 RC Rendezvous cookie [20 octets]
628 g^x Diffie-Hellman data, part 1 [128 octets]
629 OR (in the v3 intro protocol)
630 VER Version byte: set to 3. [1 octet]
631 AUTHT The auth type that is used [1 octet]
632 AUTHL Length of auth data [2 octets]
633 AUTHD Auth data [variable]
634 TS A timestamp [4 octets]
635 IP Rendezvous point's address [4 octets]
636 PORT Rendezvous point's OR port [2 octets]
637 ID Rendezvous point identity ID [20 octets]
638 KLEN Length of onion key [2 octets]
639 KEY Rendezvous point onion key [KLEN octets]
640 RC Rendezvous cookie [20 octets]
641 g^x Diffie-Hellman data, part 1 [128 octets]
643 PK_ID is the hash of Bob's public key or the service key, depending on the
644 hidden service descriptor version. In case of a v0 descriptor, Alice's OP
645 uses Bob's public key. If Alice has downloaded a v2 descriptor, she uses
646 the contained public key ("service-key").
648 RP is NUL-padded and terminated. In version 0 of the intro protocol, RP
649 must contain a nickname. In version 1, it must contain EITHER a nickname or
650 an identity key digest that is encoded in hex and prefixed with a '$'.
652 The hybrid encryption to Bob's PK works just like the hybrid
653 encryption in CREATE cells (see tor-spec). Thus the payload of the
654 version 0 RELAY_COMMAND_INTRODUCE1 cell on the wire will contain
655 20+42+16+20+20+128=246 bytes, and the version 1 and version 2
656 introduction formats have other sizes.
658 Through Tor 0.2.0.6-alpha, clients only generated the v0 introduction
659 format, whereas hidden services have understood and accepted v0,
660 v1, and v2 since 0.1.1.x. As of Tor 0.2.0.7-alpha and 0.1.2.18,
661 clients switched to using the v2 intro format.
663 1.9. Introduction: From the Introduction Point to Bob's OP
665 If the Introduction Point recognizes PK_ID as a public key which has
666 established a circuit for introductions as in 1.2 above, it sends the body
667 of the cell in a new RELAY_COMMAND_INTRODUCE2 cell down the corresponding
668 circuit. (If the PK_ID is unrecognized, the RELAY_COMMAND_INTRODUCE1 cell is
671 After sending the RELAY_COMMAND_INTRODUCE2 cell, the OR replies to Alice
672 with an empty RELAY_COMMAND_INTRODUCE_ACK cell. If no
673 RELAY_COMMAND_INTRODUCE2 cell can be sent, the OR replies to Alice with a
674 non-empty cell to indicate an error. (The semantics of the cell body may be
675 determined later; the current implementation sends a single '1' byte on
678 When Bob's OP receives the RELAY_COMMAND_INTRODUCE2 cell, it decrypts it
679 with the private key for the corresponding hidden service, and extracts the
680 rendezvous point's nickname, the rendezvous cookie, and the value of g^x
685 Bob's OP builds a new Tor circuit ending at Alice's chosen rendezvous
686 point, and sends a RELAY_COMMAND_RENDEZVOUS1 cell along this circuit,
688 RC Rendezvous cookie [20 octets]
689 g^y Diffie-Hellman [128 octets]
690 KH Handshake digest [20 octets]
692 (Bob's OP MUST NOT use this circuit for any other purpose.)
694 If the RP recognizes RC, it relays the rest of the cell down the
695 corresponding circuit in a RELAY_COMMAND_RENDEZVOUS2 cell, containing:
697 g^y Diffie-Hellman [128 octets]
698 KH Handshake digest [20 octets]
700 (If the RP does not recognize the RC, it discards the cell and
701 tears down the circuit.)
703 When Alice's OP receives a RELAY_COMMAND_RENDEZVOUS2 cell on a circuit which
704 has sent a RELAY_COMMAND_ESTABLISH_RENDEZVOUS cell but which has not yet
705 received a reply, it uses g^y and H(g^xy) to complete the handshake as in
706 the Tor circuit extend process: they establish a 60-octet string as
707 K = SHA1(g^xy | [00]) | SHA1(g^xy | [01]) | SHA1(g^xy | [02])
713 Subsequently, the rendezvous point passes relay cells, unchanged, from
714 each of the two circuits to the other. When Alice's OP sends
715 RELAY cells along the circuit, it first encrypts them with the
716 Kf, then with all of the keys for the ORs in Alice's side of the circuit;
717 and when Alice's OP receives RELAY cells from the circuit, it decrypts
718 them with the keys for the ORs in Alice's side of the circuit, then
719 decrypts them with Kb. Bob's OP does the same, with Kf and Kb
722 1.11. Creating streams
724 To open TCP connections to Bob's location-hidden service, Alice's OP sends
725 a RELAY_COMMAND_BEGIN cell along the established circuit, using the special
726 address "", and a chosen port. Bob's OP chooses a destination IP and
727 port, based on the configuration of the service connected to the circuit,
728 and opens a TCP stream. From then on, Bob's OP treats the stream as an
729 ordinary exit connection.
730 [ Except he doesn't include addr in the connected cell or the end
733 Alice MAY send multiple RELAY_COMMAND_BEGIN cells along the circuit, to open
734 multiple streams to Bob. Alice SHOULD NOT send RELAY_COMMAND_BEGIN cells
735 for any other address along her circuit to Bob; if she does, Bob MUST reject
738 2. Authentication and authorization.
740 The rendezvous protocol as described in Section 1 provides a few options
741 for implementing client-side authorization. There are two steps in the
742 rendezvous protocol that can be used for performing client authorization:
743 when downloading and decrypting parts of the hidden service descriptor and
744 at Bob's Tor client before contacting the rendezvous point. A service
745 provider can restrict access to his service at these two points to
746 authorized clients only.
748 There are currently two authorization protocols specified that are
749 described in more detail below:
751 1. The first protocol allows a service provider to restrict access
752 to clients with a previously received secret key only, but does not
753 attempt to hide service activity from others.
755 2. The second protocol, albeit being feasible for a limited set of about
756 16 clients, performs client authorization and hides service activity
757 from everyone but the authorized clients.
759 2.1. Service with large-scale client authorization
761 The first client authorization protocol aims at performing access control
762 while consuming as few additional resources as possible. A service
763 provider should be able to permit access to a large number of clients
764 while denying access for everyone else. However, the price for
765 scalability is that the service won't be able to hide its activity from
766 unauthorized or formerly authorized clients.
768 The main idea of this protocol is to encrypt the introduction-point part
769 in hidden service descriptors to authorized clients using symmetric keys.
770 This ensures that nobody else but authorized clients can learn which
771 introduction points a service currently uses, nor can someone send a
772 valid INTRODUCE1 message without knowing the introduction key. Therefore,
773 a subsequent authorization at the introduction point is not required.
775 A service provider generates symmetric "descriptor cookies" for his
776 clients and distributes them outside of Tor. The suggested key size is
777 128 bits, so that descriptor cookies can be encoded in 22 base64 chars
778 (which can hold up to 22 * 5 = 132 bits, leaving 4 bits to encode the
779 authorization type (here: "0") and allow a client to distinguish this
780 authorization protocol from others like the one proposed below).
781 Typically, the contact information for a hidden service using this
782 authorization protocol looks like this:
784 v2cbb2l4lsnpio4q.onion Ll3X7Xgz9eHGKCCnlFH0uz
786 When generating a hidden service descriptor, the service encrypts the
787 introduction-point part with a single randomly generated symmetric
788 128-bit session key using AES-CTR as described for v2 hidden service
789 descriptors in rend-spec. Afterwards, the service encrypts the session
790 key to all descriptor cookies using AES. Authorized client should be able
791 to efficiently find the session key that is encrypted for him/her, so
792 that 4 octet long client ID are generated consisting of descriptor cookie
793 and initialization vector. Descriptors always contain a number of
794 encrypted session keys that is a multiple of 16 by adding fake entries.
795 Encrypted session keys are ordered by client IDs in order to conceal
796 addition or removal of authorized clients by the service provider.
798 ATYPE Authorization type: set to 1. [1 octet]
799 ALEN Number of clients := 1 + ((clients - 1) div 16) [1 octet]
800 for each symmetric descriptor cookie:
801 ID Client ID: H(descriptor cookie | IV)[:4] [4 octets]
802 SKEY Session key encrypted with descriptor cookie [16 octets]
803 (end of client-specific part)
804 RND Random data [(15 - ((clients - 1) mod 16)) * 20 octets]
805 IV AES initialization vector [16 octets]
806 IPOS Intro points, encrypted with session key [remaining octets]
808 An authorized client needs to configure Tor to use the descriptor cookie
809 when accessing the hidden service. Therefore, a user adds the contact
810 information that she received from the service provider to her torrc
811 file. Upon downloading a hidden service descriptor, Tor finds the
812 encrypted introduction-point part and attempts to decrypt it using the
813 configured descriptor cookie. (In the rare event of two or more client
814 IDs being equal a client tries to decrypt all of them.)
816 Upon sending the introduction, the client includes her descriptor cookie
817 as auth type "1" in the INTRODUCE2 cell that she sends to the service.
818 The hidden service checks whether the included descriptor cookie is
819 authorized to access the service and either responds to the introduction
822 2.2. Authorization for limited number of clients
824 A second, more sophisticated client authorization protocol goes the extra
825 mile of hiding service activity from unauthorized clients. With all else
826 being equal to the preceding authorization protocol, the second protocol
827 publishes hidden service descriptors for each user separately and gets
828 along with encrypting the introduction-point part of descriptors to a
829 single client. This allows the service to stop publishing descriptors for
830 removed clients. As long as a removed client cannot link descriptors
831 issued for other clients to the service, it cannot derive service
832 activity any more. The downside of this approach is limited scalability.
833 Even though the distributed storage of descriptors (cf. proposal 114)
834 tackles the problem of limited scalability to a certain extent, this
835 protocol should not be used for services with more than 16 clients. (In
836 fact, Tor should refuse to advertise services for more than this number
839 A hidden service generates an asymmetric "client key" and a symmetric
840 "descriptor cookie" for each client. The client key is used as
841 replacement for the service's permanent key, so that the service uses a
842 different identity for each of his clients. The descriptor cookie is used
843 to store descriptors at changing directory nodes that are unpredictable
844 for anyone but service and client, to encrypt the introduction-point
845 part, and to be included in INTRODUCE2 cells. Once the service has
846 created client key and descriptor cookie, he tells them to the client
847 outside of Tor. The contact information string looks similar to the one
848 used by the preceding authorization protocol (with the only difference
849 that it has "1" encoded as auth-type in the remaining 4 of 132 bits
850 instead of "0" as before).
852 When creating a hidden service descriptor for an authorized client, the
853 hidden service uses the client key and descriptor cookie to compute
854 secret ID part and descriptor ID:
856 secret-id-part = H(time-period | descriptor-cookie | replica)
858 descriptor-id = H(client-key[:10] | secret-id-part)
860 The hidden service also replaces permanent-key in the descriptor with
861 client-key and encrypts introduction-points with the descriptor cookie.
863 ATYPE Authorization type: set to 2. [1 octet]
864 IV AES initialization vector [16 octets]
865 IPOS Intro points, encr. with descriptor cookie [remaining octets]
867 When uploading descriptors, the hidden service needs to make sure that
868 descriptors for different clients are not uploaded at the same time (cf.
869 Section 1.1) which is also a limiting factor for the number of clients.
871 When a client is requested to establish a connection to a hidden service
872 it looks up whether it has any authorization data configured for that
873 service. If the user has configured authorization data for authorization
874 protocol "2", the descriptor ID is determined as described in the last
875 paragraph. Upon receiving a descriptor, the client decrypts the
876 introduction-point part using its descriptor cookie. Further, the client
877 includes its descriptor cookie as auth-type "2" in INTRODUCE2 cells that
878 it sends to the service.
880 2.3. Hidden service configuration
882 A hidden service that is meant to perform client authorization adds a
883 new option HiddenServiceAuthorizeClient to its hidden service
884 configuration. This option contains the authorization type which is
885 either "1" for the protocol described in 2.1 or "2" for the protocol in
886 2.2 and a comma-separated list of human-readable client names, so that
887 Tor can create authorization data for these clients:
889 HiddenServiceAuthorizeClient auth-type client-name,client-name,...
891 If this option is configured, HiddenServiceVersion is automatically
892 reconfigured to contain only version numbers of 2 or higher.
894 Tor stores all generated authorization data for the authorization
895 protocols described in Sections 2.1 and 2.2 in a new file using the
896 following file format:
898 "client-name" human-readable client identifier NL
899 "descriptor-cookie" 128-bit key ^= 22 base64 chars NL
901 If the authorization protocol of Section 2.2 is used, Tor also generates
902 and stores the following data:
904 "client-key" NL a public key in PEM format
906 2.4. Client configuration
908 Clients need to make their authorization data known to Tor using another
909 configuration option that contains a service name (mainly for the sake of
910 convenience), the service address, and the descriptor cookie that is
911 required to access a hidden service (the authorization protocol number is
912 encoded in the descriptor cookie):
914 HidServAuth service-name service-address descriptor-cookie
916 3. Hidden service directory operation
918 This section has been introduced with the v2 hidden service descriptor
919 format. It describes all operations of the v2 hidden service descriptor
920 fetching and propagation mechanism that are required for the protocol
921 described in section 1 to succeed with v2 hidden service descriptors.
923 3.1. Configuring as hidden service directory
925 Every onion router that has its directory port open can decide whether it
926 wants to store and serve hidden service descriptors. An onion router which
927 is configured as such includes the "hidden-service-dir" flag in its router
928 descriptors that it sends to directory authorities.
930 The directory authorities include a new flag "HSDir" for routers that
931 decided to provide storage for hidden service descriptors and that
932 have been running for at least 24 hours.
934 3.2. Accepting publish requests
936 Hidden service directory nodes accept publish requests for v2 hidden service
937 descriptors and store them to their local memory. (It is not necessary to
938 make descriptors persistent, because after restarting, the onion router
939 would not be accepted as a storing node anyway, because it has not been
940 running for at least 24 hours.) All requests and replies are formatted as
941 HTTP messages. Requests are initiated via BEGIN_DIR cells directed to
942 the router's directory port, and formatted as HTTP POST requests to the URL
943 "/tor/rendezvous2/publish" relative to the hidden service directory's root,
944 containing as its body a v2 service descriptor.
946 A hidden service directory node parses every received descriptor and only
947 stores it when it thinks that it is responsible for storing that descriptor
948 based on its own routing table. See section 1.4 for more information on how
949 to determine responsibility for a certain descriptor ID.
951 3.3. Processing fetch requests
953 Hidden service directory nodes process fetch requests for hidden service
954 descriptors by looking them up in their local memory. (They do not need to
955 determine if they are responsible for the passed ID, because it does no harm
956 if they deliver a descriptor for which they are not (any more) responsible.)
957 All requests and replies are formatted as HTTP messages. Requests are
958 initiated via BEGIN_DIR cells directed to the router's directory port,
959 and formatted as HTTP GET requests for the document "/tor/rendezvous2/<z>",
960 where z is replaced with the encoding of the descriptor ID.