rend-spec.txt 17 KB

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  1. $Id$
  2. Tor Rendezvous Specification
  3. 0. Overview and preliminaries
  4. Read http://tor.eff.org/doc/design-paper/tor-design.html#sec:rendezvous
  5. before you read this specification. It will make more sense.
  6. Rendezvous points provide location-hidden services (server
  7. anonymity) for the onion routing network. With rendezvous points,
  8. Bob can offer a TCP service (say, a webserver) via the onion
  9. routing network, without revealing the IP of that service.
  10. Bob does this by anonymously advertising a public key for his
  11. service, along with a list of onion routers to act as "Introduction
  12. Points" for his service. He creates forward OR circuits to those
  13. introduction points, and tells them about his public key. To
  14. connect to Bob, Alice first builds an OR circuit to an OR to act as
  15. her "Rendezvous Point", then connects to one of Bob's chosen
  16. introduction points, and asks it to tell him about her Rendezvous
  17. Point (RP). If Bob chooses to answer, he builds an OR circuit to her
  18. RP, and tells it to connect him to Alice. The RP joints their
  19. circuits together, and begins relaying cells. Alice's 'BEGIN'
  20. cells are received directly by Bob's OP, which responds by
  21. communication with the local server implementing Bob's service.
  22. Below, we describe a network-level specification of this service,
  23. along with interfaces to make this process transparent to Alice
  24. (so long as she is using an OP).
  25. 0.1. Notation, conventions and prerequisites
  26. In the specifications below, we use the same notation as in
  27. "tor-spec.txt". The service specified here also requires the existence of
  28. an onion routing network as specified in "tor-spec.txt".
  29. H(x) is a SHA1 digest of x.
  30. PKSign(SK,x) is a PKCS.1-padded RSA signature of x with SK.
  31. PKEncrypt(SK,x) is a PKCS.1-padded RSA encryption of x with SK.
  32. Public keys are all RSA, and encoded in ASN.1.
  33. All integers are stored in network (big-endian) order.
  34. All symmetric encryption uses AES in counter mode, except where
  35. otherwise noted.
  36. In all discussions, "Alice" will refer to a user connecting to a
  37. location-hidden service, and "Bob" will refer to a user running a
  38. location-hidden service.
  39. 0.2. Protocol outline
  40. 1. Bob->Bob's OP: "Offer IP:Port as public-key-name:Port". [configuration]
  41. (We do not specify this step; it is left to the implementor of
  42. Bob's OP.)
  43. 2. Bob's OP generates keypair and rendezvous service descriptor:
  44. "Meet public-key X at introduction point A, B, or C." (signed)
  45. 3. Bob's OP->Introduction point via Tor: [introduction setup]
  46. "This pk is me."
  47. 4. Bob's OP->directory service via Tor: publishes Bob's service descriptor
  48. [advertisement]
  49. 5. Out of band, Alice receives a y.onion:port address. She opens a
  50. SOCKS connection to her OP, and requests y.onion:port.
  51. 6. Alice's OP retrieves Bob's descriptor via Tor: [descriptor lookup.]
  52. 7. Alice's OP chooses a rendezvous point, opens a circuit to that
  53. rendezvous point, and establishes a rendezvous circuit. [rendezvous
  54. setup.]
  55. 8. Alice connects to the Introduction point via Tor, and tells it about
  56. her rendezvous point. (Encrypted to Bob.) [Introduction 1]
  57. 9. The Introduction point passes this on to Bob's OP via Tor, along the
  58. introduction circuit. [Introduction 2]
  59. 10. Bob's OP decides whether to connect to Alice, and if so, creates a
  60. circuit to Alice's RP via Tor. Establishes a shared circuit.
  61. [Rendezvous.]
  62. 11. Alice's OP sends begin cells to Bob's OP. [Connection]
  63. 0.3. Constants and new cell types
  64. Relay cell types
  65. 32 -- RELAY_ESTABLISH_INTRO
  66. 33 -- RELAY_ESTABLISH_RENDEZVOUS
  67. 34 -- RELAY_INTRODUCE1
  68. 35 -- RELAY_INTRODUCE2
  69. 36 -- RELAY_RENDEZVOUS1
  70. 37 -- RELAY_RENDEZVOUS2
  71. 38 -- RELAY_INTRO_ESTABLISHED
  72. 39 -- RELAY_RENDEZVOUS_ESTABLISHED
  73. 40 -- RELAY_COMMAND_INTRODUCE_ACK
  74. 1. The Protocol
  75. 1.1. Bob configures his local OP.
  76. We do not specify a format for the OP configuration file. However,
  77. OPs SHOULD allow Bob to provide more than one advertised service
  78. per OP, and MUST allow Bob to specify one or more virtual ports per
  79. service. Bob provides a mapping from each of these virtual ports
  80. to a local IP:Port pair.
  81. 1.2. Bob's OP generates service descriptors.
  82. The first time the OP provides an advertised service, it generates
  83. a public/private keypair (stored locally). Periodically, the OP
  84. generates a pair of service descriptors, one "V1" and one "V0".
  85. The "V1" descriptor contains:
  86. V Format byte: set to 255 [1 octet]
  87. V Version byte: set to 1 [1 octet]
  88. KL Key length [2 octets]
  89. PK Bob's public key [KL octets]
  90. TS A timestamp [4 octets]
  91. PROTO Protocol versions: bitmask [2 octets]
  92. NI Number of introduction points [2 octets]
  93. For each introduction point: (as in INTRODUCE2 cells)
  94. IP Rendezvous point's address [4 octets]
  95. PORT Rendezvous point's OR port [2 octets]
  96. ID Rendezvous point identity ID [20 octets]
  97. KLEN Length of onion key [2 octets]
  98. KEY Rendezvous point onion key [KLEN octets]
  99. SIG Signature of above fields [variable]
  100. The "V0" descriptor contains:
  101. KL Key length [2 octets]
  102. PK Bob's public key [KL octets]
  103. TS A timestamp [4 octets]
  104. NI Number of introduction points [2 octets]
  105. Ipt A list of NUL-terminated ORs [variable]
  106. SIG Signature of above fields [variable]
  107. KL is the length of PK, in octets. (Currently, KL must be 128.)
  108. TS is the number of seconds elapsed since Jan 1, 1970.
  109. The members of Ipt may be either (a) nicknames, or (b) identity key
  110. digests, encoded in hex, and prefixed with a '$'. Clients must
  111. accept both forms. Services must only generate the second form.
  112. Once 0.0.9.x is obsoleted, we can drop the first form.
  113. [It's ok for Bob to advertise 0 introduction points. He might want
  114. to do that if he previously advertised some introduction points,
  115. and now he doesn't have any. -RD]
  116. [Once Tor 0.1.0.x is obsolete, we can stop generating or using V0
  117. descriptors. -NM]
  118. 1.3. Bob's OP establishes his introduction points.
  119. The OP establishes a new introduction circuit to each introduction
  120. point. These circuits MUST NOT be used for anything but rendezvous
  121. introduction. To establish the introduction, Bob sends a
  122. RELAY_ESTABLISH_INTRO cell, containing:
  123. KL Key length [2 octets]
  124. PK Bob's public key [KL octets]
  125. HS Hash of session info [20 octets]
  126. SIG Signature of above information [variable]
  127. To prevent replay attacks, the HS field contains a SHA-1 hash based on the
  128. shared secret KH between Bob's OP and the introduction point, as
  129. follows:
  130. HS = H(KH | "INTRODUCE")
  131. That is:
  132. HS = H(KH | [49 4E 54 52 4F 44 55 43 45])
  133. (KH, as specified in tor-spec.txt, is H(g^xy | [00]) .)
  134. Upon receiving such a cell, the OR first checks that the signature is
  135. correct with the included public key. If so, it checks whether HS is
  136. correct given the shared state between Bob's OP and the OR. If either
  137. check fails, the OP discards the cell; otherwise, it associates the
  138. circuit with Bob's public key, and dissociates any other circuits
  139. currently associated with PK. On success, the OR sends Bob a
  140. RELAY_INTRO_ESTABLISHED cell with an empty payload.
  141. 1.4. Bob's OP advertises his service descriptor(s)
  142. Bob's OP opens a stream to each directory server's directory port via Tor.
  143. (He may re-use old circuits for this.) Over this stream, Bob's OP makes
  144. an HTTP 'POST' request, to a URL "/tor/rendezvous/publish" relative to the
  145. directory server's root, containing as its body Bob's service descriptor.
  146. Upon receiving a descriptor, the directory server checks the signature,
  147. and discards the descriptor if the signature does not match the enclosed
  148. public key. Next, the directory server checks the timestamp. If the
  149. timestamp is more than 24 hours in the past or more than 1 hour in the
  150. future, or the directory server already has a newer descriptor with the
  151. same public key, the server discards the descriptor. Otherwise, the
  152. server discards any older descriptors with the same public key, and
  153. associates the new descriptor with the public key. The directory server
  154. remembers this descriptor for at least 24 hours after its timestamp. At
  155. least every 24 hours, Bob's OP uploads a fresh descriptor.
  156. 1.5. Alice receives a y.onion address
  157. When Alice receives a pointer to a location-hidden service, it is as a
  158. hostname of the form "y.onion", where y is a base-32 encoding of a
  159. 10-octet hash of Bob's service's public key, computed as follows:
  160. 1. Let H = H(PK).
  161. 2. Let H' = the first 80 bits of H, considering each octet from
  162. most significant bit to least significant bit.
  163. 2. Generate a 16-character encoding of H', using base32 as defined
  164. in RFC 3548.
  165. (We only use 80 bits instead of the 160 bits from SHA1 because we don't
  166. need to worry about man-in-the-middle attacks, and because it will make
  167. handling the url's more convenient.)
  168. [Yes, numbers are allowed at the beginning. See RFC1123. -NM]
  169. 1.6. Alice's OP retrieves a service descriptor
  170. Alice opens a stream to a directory server via Tor, and makes an HTTP GET
  171. request for the document '/tor/rendezvous/<y>' or '/tor/rendezvous1/<y>',
  172. where '<y> is replaced with the encoding of Bob's public key as described
  173. above. (She may re-use old circuits for this.) The directory replies with
  174. a 404 HTTP response if it does not recognize <y>, and otherwise returns
  175. Bob's most recently uploaded service descriptor. (If Alice requests
  176. 'rendezvous1', the directory server provides a V1 descriptor or a V0
  177. descriptor if no V1 descriptor is available. If Alice requests
  178. 'rendezvous', the directory server returns a V0 descriptor.)
  179. If Alice's OP receives a 404 response, it tries the other directory
  180. servers, and only fails the lookup if none recognizes the public key hash.
  181. Upon receiving a service descriptor, Alice verifies with the same process
  182. as the directory server uses, described above in section 1.4.
  183. The directory server gives a 400 response if it cannot understand Alice's
  184. request.
  185. Alice should cache the descriptor locally, but should not use
  186. descriptors that are more than 24 hours older than their timestamp.
  187. [Caching may make her partitionable, but she fetched it anonymously,
  188. and we can't very well *not* cache it. -RD]
  189. 1.7. Alice's OP establishes a rendezvous point.
  190. When Alice requests a connection to a given location-hidden service,
  191. and Alice's OP does not have an established circuit to that service,
  192. the OP builds a rendezvous circuit. It does this by establishing
  193. a circuit to a randomly chosen OR, and sending a
  194. RELAY_ESTABLISH_RENDEZVOUS cell to that OR. The body of that cell
  195. contains:
  196. RC Rendezvous cookie [20 octets]
  197. The rendezvous cookie is an arbitrary 20-byte value, chosen randomly by
  198. Alice's OP.
  199. Upon receiving a RELAY_ESTABLISH_RENDEZVOUS cell, the OR associates the
  200. RC with the circuit that sent it. It replies to Alice with an empty
  201. RELAY_RENDEZVOUS_ESTABLISHED cell to indicate success.
  202. Alice's OP MUST NOT use the circuit which sent the cell for any purpose
  203. other than rendezvous with the given location-hidden service.
  204. 1.8. Introduction: from Alice's OP to Introduction Point
  205. Alice builds a separate circuit to one of Bob's chosen introduction
  206. points, and sends it a RELAY_INTRODUCE1 cell containing:
  207. Cleartext
  208. PK_ID Identifier for Bob's PK [20 octets]
  209. Encrypted to Bob's PK:
  210. RP Rendezvous point's nickname [20 octets]
  211. RC Rendezvous cookie [20 octets]
  212. g^x Diffie-Hellman data, part 1 [128 octets]
  213. OR
  214. VER Version byte: set to 1. [1 octet]
  215. RP Rendezvous point nick or ID [42 octets]
  216. RC Rendezvous cookie [20 octets]
  217. g^x Diffie-Hellman data, part 1 [128 octets]
  218. OR
  219. VER Version byte: set to 2. [1 octet]
  220. IP Rendezvous point's address [4 octets]
  221. PORT Rendezvous point's OR port [2 octets]
  222. ID Rendezvous point identity ID [20 octets]
  223. KLEN Length of onion key [2 octets]
  224. KEY Rendezvous point onion key [KLEN octets]
  225. RC Rendezvous cookie [20 octets]
  226. g^x Diffie-Hellman data, part 1 [128 octets]
  227. PK_ID is the hash of Bob's public key. RP is NUL-padded and terminated,
  228. and must contain EITHER a nickname, or an identity key digest, encoded in
  229. hex, and prefixed with a '$'.
  230. Implementations SHOULD accept all variants, and list the variants they
  231. accept in their V1 descriptor. Implementations should only generate the
  232. variants listed in the service's V1 descriptor; if no V1 descriptor is
  233. available, only the first variant should be generated. No version should
  234. generate the second variant (version byte=1).
  235. The hybrid encryption to Bob's PK works just like the hybrid
  236. encryption in CREATE cells (see main spec). Thus the payload of the
  237. RELAY_INTRODUCE1 cell on the wire will contain 20+42+16+20+20+128=246
  238. bytes. [XXXX not really]
  239. 1.9. Introduction: From the Introduction Point to Bob's OP
  240. If the Introduction Point recognizes PK_ID as a public key which has
  241. established a circuit for introductions as in 1.3 above, it sends the body
  242. of the cell in a new RELAY_INTRODUCE2 cell down the corresponding circuit.
  243. (If the PK_ID is unrecognized, the RELAY_INTRODUCE1 cell is discarded.)
  244. After sending the RELAY_INTRODUCE2 cell, the OR replies to Alice with an
  245. empty RELAY_COMMAND_INTRODUCE_ACK cell. If no RELAY_INTRODUCE2 cell can
  246. be sent, the OR replies to Alice with a non-empty cell to indicate an
  247. error. (The semantics of the cell body may be determined later; the
  248. current implementation sends a single '1' byte on failure.)
  249. When Bob's OP receives the RELAY_INTRODUCE2 cell, it decrypts it with
  250. the private key for the corresponding hidden service, and extracts the
  251. rendezvous point's nickname, the rendezvous cookie, and the value of g^x
  252. chosen by Alice.
  253. 1.10. Rendezvous
  254. Bob's OP build a new Tor circuit ending at Alice's chosen rendezvous
  255. point, and sends a RELAY_RENDEZVOUS1 cell along this circuit, containing:
  256. RC Rendezvous cookie [20 octets]
  257. g^y Diffie-Hellman [128 octets]
  258. KH Handshake digest [20 octets]
  259. (Bob's OP MUST NOT use this circuit for any other purpose.)
  260. If the RP recognizes RC, it relays the rest of the cell down the
  261. corresponding circuit in a RELAY_RENDEZVOUS2 cell, containing:
  262. g^y Diffie-Hellman [128 octets]
  263. KH Handshake digest [20 octets]
  264. (If the RP does not recognize the RC, it discards the cell and
  265. tears down the circuit.)
  266. When Alice's OP receives a RELAY_RENDEZVOUS2 cell on a circuit which
  267. has sent a RELAY_ESTABLISH_RENDEZVOUS cell but which has not yet received
  268. a reply, it uses g^y and H(g^xy) to complete the handshake as in the Tor
  269. circuit extend process: they establish a 60-octet string as
  270. K = SHA1(g^xy | [00]) | SHA1(g^xy | [01]) | SHA1(g^xy | [02])
  271. and generate
  272. KH = K[0..15]
  273. Kf = K[16..31]
  274. Kb = K[32..47]
  275. Subsequently, the rendezvous point passes relay cells, unchanged, from
  276. each of the two circuits to the other. When Alice's OP sends
  277. RELAY cells along the circuit, it first encrypts them with the
  278. Kf, then with all of the keys for the ORs in Alice's side of the circuit;
  279. and when Alice's OP receives RELAY cells from the circuit, it decrypts
  280. them with the keys for the ORs in Alice's side of the circuit, then
  281. decrypts them with Kb. Bob's OP does the same, with Kf and Kb
  282. interchanged.
  283. 1.11. Creating streams
  284. To open TCP connections to Bob's location-hidden service, Alice's OP sends
  285. a RELAY_BEGIN cell along the established circuit, using the special
  286. address "", and a chosen port. Bob's OP chooses a destination IP and
  287. port, based on the configuration of the service connected to the circuit,
  288. and opens a TCP stream. From then on, Bob's OP treats the stream as an
  289. ordinary exit connection.
  290. [ Except he doesn't include addr in the connected cell or the end
  291. cell. -RD]
  292. Alice MAY send multiple RELAY_BEGIN cells along the circuit, to open
  293. multiple streams to Bob. Alice SHOULD NOT send RELAY_BEGIN cells for any
  294. other address along her circuit to Bob; if she does, Bob MUST reject them.