rend-spec.txt 20 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 circuits to those
  13. introduction points, and tells them about his public key. To
  14. connect to Bob, Alice first builds a circuit to an OR to act as
  15. her "Rendezvous Point." She then connects to one of Bob's chosen
  16. introduction points, optionally provides authentication or
  17. authorization information, and asks it to tell him about her Rendezvous
  18. Point (RP). If Bob chooses to answer, he builds a circuit to her
  19. RP, and tells it to connect him to Alice. The RP joins their
  20. circuits together, and begins relaying cells. Alice's 'BEGIN'
  21. cells are received directly by Bob's OP, which passes data to
  22. and from the local server implementing Bob's service.
  23. Below we describe a network-level specification of this service,
  24. along with interfaces to make this process transparent to Alice
  25. (so long as she is using an OP).
  26. 0.1. Notation, conventions and prerequisites
  27. In the specifications below, we use the same notation as in
  28. "tor-spec.txt". The service specified here also requires the
  29. existence of an onion routing network as specified in that file.
  30. H(x) is a SHA1 digest of x.
  31. PKSign(SK,x) is a PKCS.1-padded RSA signature of x with SK.
  32. PKEncrypt(SK,x) is a PKCS.1-padded RSA encryption of x with SK.
  33. Public keys are all RSA, and encoded in ASN.1.
  34. All integers are stored in network (big-endian) order.
  35. All symmetric encryption uses AES in counter mode, except where
  36. otherwise noted.
  37. In all discussions, "Alice" will refer to a user connecting to a
  38. location-hidden service, and "Bob" will refer to a user running a
  39. location-hidden service.
  40. 0.2. Protocol outline
  41. 1. Bob->Bob's OP: "Offer IP:Port as
  42. public-key-name:Port". [configuration]
  43. (We do not specify this step; it is left to the implementor of
  44. Bob's OP.)
  45. 2. Bob's OP generates keypair and rendezvous service descriptor:
  46. "Meet public-key X at introduction point A, B, or C." (signed)
  47. 3. Bob's OP->Introduction point via Tor: [introduction setup]
  48. "This pk is me."
  49. 4. Bob's OP->directory service via Tor: publishes Bob's service
  50. descriptor [advertisement]
  51. 5. Out of band, Alice receives a [x.y.]z.onion:port address.
  52. She opens a SOCKS connection to her OP, and requests
  53. x.y.z.onion:port.
  54. 6. Alice's OP retrieves Bob's descriptor via Tor. [descriptor lookup.]
  55. 7. Alice's OP chooses a rendezvous point, opens a circuit to that
  56. rendezvous point, and establishes a rendezvous circuit. [rendezvous
  57. setup.]
  58. 8. Alice connects to the Introduction point via Tor, and tells it about
  59. her rendezvous point and optional authentication/authorization
  60. information. (Encrypted to Bob.) [Introduction 1]
  61. 9. The Introduction point passes this on to Bob's OP via Tor, along the
  62. introduction circuit. [Introduction 2]
  63. 10. Bob's OP decides whether to connect to Alice, and if so, creates a
  64. circuit to Alice's RP via Tor. Establishes a shared circuit.
  65. [Rendezvous.]
  66. 11. Alice's OP sends begin cells to Bob's OP. [Connection]
  67. 0.3. Constants and new cell types
  68. Relay cell types
  69. 32 -- RELAY_ESTABLISH_INTRO
  70. 33 -- RELAY_ESTABLISH_RENDEZVOUS
  71. 34 -- RELAY_INTRODUCE1
  72. 35 -- RELAY_INTRODUCE2
  73. 36 -- RELAY_RENDEZVOUS1
  74. 37 -- RELAY_RENDEZVOUS2
  75. 38 -- RELAY_INTRO_ESTABLISHED
  76. 39 -- RELAY_RENDEZVOUS_ESTABLISHED
  77. 40 -- RELAY_COMMAND_INTRODUCE_ACK
  78. 1. The Protocol
  79. 1.1. Bob configures his local OP.
  80. We do not specify a format for the OP configuration file. However,
  81. OPs SHOULD allow Bob to provide more than one advertised service
  82. per OP, and MUST allow Bob to specify one or more virtual ports per
  83. service. Bob provides a mapping from each of these virtual ports
  84. to a local IP:Port pair.
  85. 1.2. Bob's OP generates service descriptors.
  86. The first time the OP provides an advertised service, it generates
  87. a public/private keypair (stored locally). Periodically, the OP
  88. generates a pair of service descriptors, one "V1" and one "V0".
  89. The "V1" descriptor in 0.1.1.6-alpha contains:
  90. V Format byte: set to 255 [1 octet]
  91. V Version byte: set to 1 [1 octet]
  92. KL Key length [2 octets]
  93. PK Bob's public key [KL octets]
  94. TS A timestamp [4 octets]
  95. PROTO Rendezvous protocol versions: bitmask [2 octets]
  96. NA Number of auth mechanisms accepted [1 octet]
  97. For each auth mechanism:
  98. AUTHT The auth type that is supported [2 octets]
  99. AUTHL Length of auth data [1 octet]
  100. AUTHD Auth data [variable]
  101. NI Number of introduction points [2 octets]
  102. For each introduction point: (as in INTRODUCE2 cells)
  103. ATYPE An address type (typically 4) [1 octet]
  104. ADDR Introduction point's IP address [4 or 16 octets]
  105. PORT Introduction point's OR port [2 octets]
  106. AUTHT The auth type that is supported [2 octets]
  107. AUTHL Length of auth data [1 octet]
  108. AUTHD Auth data [variable]
  109. ID Introduction point identity ID [20 octets]
  110. KLEN Length of onion key [2 octets]
  111. KEY Introduction point onion key [KLEN octets]
  112. SIG Signature of above fields [variable]
  113. The "V1" descriptor in 0.1.1.5-alpha-cvs contains:
  114. V Format byte: set to 255 [1 octet]
  115. V Version byte: set to 1 [1 octet]
  116. KL Key length [2 octets]
  117. PK Bob's public key [KL octets]
  118. TS A timestamp [4 octets]
  119. PROTO Protocol versions: bitmask [2 octets]
  120. NI Number of introduction points [2 octets]
  121. For each introduction point: (as in INTRODUCE2 cells)
  122. IP Introduction point's address [4 octets]
  123. PORT Introduction point's OR port [2 octets]
  124. ID Introduction point identity ID [20 octets]
  125. KLEN Length of onion key [2 octets]
  126. KEY Introduction point onion key [KLEN octets]
  127. SIG Signature of above fields [variable]
  128. The "V0" descriptor contains:
  129. KL Key length [2 octets]
  130. PK Bob's public key [KL octets]
  131. TS A timestamp [4 octets]
  132. NI Number of introduction points [2 octets]
  133. Ipt A list of NUL-terminated ORs [variable]
  134. SIG Signature of above fields [variable]
  135. KL is the length of PK, in octets. (Currently, KL must be 128.)
  136. TS is the number of seconds elapsed since Jan 1, 1970.
  137. AUTHT specifies which authentication/authorization mechanism is
  138. required by the hidden service or the introduction point. AUTHD
  139. is arbitrary data that can be associated with an auth approach.
  140. Currently only AUTHT of [00 00] is supported, with an AUTHL of 0.
  141. See section 2 of this document for details on auth mechanisms.
  142. The members of Ipt may be either (a) nicknames, or (b) identity key
  143. digests, encoded in hex, and prefixed with a '$'. Clients must
  144. accept both forms. Services must only generate the second form.
  145. Once 0.0.9.x is obsoleted, we can drop the first form.
  146. [It's ok for Bob to advertise 0 introduction points. He might want
  147. to do that if he previously advertised some introduction points,
  148. and now he doesn't have any. -RD]
  149. [Once Tor 0.1.0.x is obsolete, we can stop generating or using V0
  150. descriptors. -NM]
  151. 1.3. Bob's OP establishes his introduction points.
  152. The OP establishes a new introduction circuit to each introduction
  153. point. These circuits MUST NOT be used for anything but rendezvous
  154. introduction. To establish the introduction, Bob sends a
  155. RELAY_ESTABLISH_INTRO cell, containing:
  156. KL Key length [2 octets]
  157. PK Bob's public key [KL octets]
  158. HS Hash of session info [20 octets]
  159. SIG Signature of above information [variable]
  160. [XXX011, need to add auth information here. -RD]
  161. To prevent replay attacks, the HS field contains a SHA-1 hash based on the
  162. shared secret KH between Bob's OP and the introduction point, as
  163. follows:
  164. HS = H(KH | "INTRODUCE")
  165. That is:
  166. HS = H(KH | [49 4E 54 52 4F 44 55 43 45])
  167. (KH, as specified in tor-spec.txt, is H(g^xy | [00]) .)
  168. Upon receiving such a cell, the OR first checks that the signature is
  169. correct with the included public key. If so, it checks whether HS is
  170. correct given the shared state between Bob's OP and the OR. If either
  171. check fails, the OP discards the cell; otherwise, it associates the
  172. circuit with Bob's public key, and dissociates any other circuits
  173. currently associated with PK. On success, the OR sends Bob a
  174. RELAY_INTRO_ESTABLISHED cell with an empty payload.
  175. 1.4. Bob's OP advertises his service descriptor(s).
  176. Bob's OP opens a stream to each directory server's directory port via Tor.
  177. (He may re-use old circuits for this.) Over this stream, Bob's OP makes
  178. an HTTP 'POST' request, to a URL "/tor/rendezvous/publish" relative to the
  179. directory server's root, containing as its body Bob's service descriptor.
  180. Bob should upload a service descriptor for each version format that
  181. is supported in the current Tor network.
  182. Upon receiving a descriptor, the directory server checks the signature,
  183. and discards the descriptor if the signature does not match the enclosed
  184. public key. Next, the directory server checks the timestamp. If the
  185. timestamp is more than 24 hours in the past or more than 1 hour in the
  186. future, or the directory server already has a newer descriptor with the
  187. same public key, the server discards the descriptor. Otherwise, the
  188. server discards any older descriptors with the same public key and
  189. version format, and associates the new descriptor with the public key.
  190. The directory server remembers this descriptor for at least 24 hours
  191. after its timestamp. At least every 18 hours, Bob's OP uploads a
  192. fresh descriptor.
  193. 1.5. Alice receives a x.y.z.onion address.
  194. When Alice receives a pointer to a location-hidden service, it is as a
  195. hostname of the form "z.onion" or "y.z.onion" or "x.y.z.onion", where
  196. z is a base-32 encoding of a 10-octet hash of Bob's service's public
  197. key, computed as follows:
  198. 1. Let H = H(PK).
  199. 2. Let H' = the first 80 bits of H, considering each octet from
  200. most significant bit to least significant bit.
  201. 2. Generate a 16-character encoding of H', using base32 as defined
  202. in RFC 3548.
  203. (We only use 80 bits instead of the 160 bits from SHA1 because we don't
  204. need to worry about man-in-the-middle attacks, and because it will make
  205. handling the url's more convenient.)
  206. The string "x", if present, is the base-32 encoding of the
  207. authentication/authorization required by the introduction point.
  208. The string "y", if present, is the base-32 encoding of the
  209. authentication/authorization required by the hidden service.
  210. Omitting a string is taken to mean auth type [00 00].
  211. See section 2 of this document for details on auth mechanisms.
  212. [Yes, numbers are allowed at the beginning. See RFC1123. -NM]
  213. 1.6. Alice's OP retrieves a service descriptor.
  214. Alice opens a stream to a directory server via Tor, and makes an HTTP GET
  215. request for the document '/tor/rendezvous/<z>' or '/tor/rendezvous1/<z>',
  216. where '<z>' is replaced with the encoding of Bob's public key as described
  217. above. (She may re-use old circuits for this.) The directory replies with
  218. a 404 HTTP response if it does not recognize <z>, and otherwise returns
  219. Bob's most recently uploaded service descriptor. (If Alice requests
  220. 'rendezvous1', the directory server provides a V1 descriptor or a V0
  221. descriptor if no V1 descriptor is available. If Alice requests
  222. 'rendezvous', the directory server returns a V0 descriptor.)
  223. If Alice's OP receives a 404 response, it tries the other directory
  224. servers, and only fails the lookup if none recognize the public key hash.
  225. Upon receiving a service descriptor, Alice verifies with the same process
  226. as the directory server uses, described above in section 1.4.
  227. The directory server gives a 400 response if it cannot understand Alice's
  228. request.
  229. Alice should cache the descriptor locally, but should not use
  230. descriptors that are more than 24 hours older than their timestamp.
  231. [Caching may make her partitionable, but she fetched it anonymously,
  232. and we can't very well *not* cache it. -RD]
  233. 1.7. Alice's OP establishes a rendezvous point.
  234. When Alice requests a connection to a given location-hidden service,
  235. and Alice's OP does not have an established circuit to that service,
  236. the OP builds a rendezvous circuit. It does this by establishing
  237. a circuit to a randomly chosen OR, and sending a
  238. RELAY_ESTABLISH_RENDEZVOUS cell to that OR. The body of that cell
  239. contains:
  240. RC Rendezvous cookie [20 octets]
  241. [XXX011 this looks like an auth mechanism. should we generalize here? -RD]
  242. The rendezvous cookie is an arbitrary 20-byte value, chosen randomly by
  243. Alice's OP.
  244. Upon receiving a RELAY_ESTABLISH_RENDEZVOUS cell, the OR associates the
  245. RC with the circuit that sent it. It replies to Alice with an empty
  246. RELAY_RENDEZVOUS_ESTABLISHED cell to indicate success.
  247. Alice's OP MUST NOT use the circuit which sent the cell for any purpose
  248. other than rendezvous with the given location-hidden service.
  249. 1.8. Introduction: from Alice's OP to Introduction Point
  250. Alice builds a separate circuit to one of Bob's chosen introduction
  251. points, and sends it a RELAY_INTRODUCE1 cell containing:
  252. Cleartext
  253. PK_ID Identifier for Bob's PK [20 octets]
  254. [XXX011 want to put intro-level auth info here, but no version. crap. -RD]
  255. Encrypted to Bob's PK:
  256. RP Rendezvous point's nickname [20 octets]
  257. RC Rendezvous cookie [20 octets]
  258. g^x Diffie-Hellman data, part 1 [128 octets]
  259. OR
  260. VER Version byte: set to 1. [1 octet]
  261. RP Rendezvous point nick or ID [42 octets]
  262. RC Rendezvous cookie [20 octets]
  263. g^x Diffie-Hellman data, part 1 [128 octets]
  264. OR
  265. VER Version byte: set to 2. [1 octet]
  266. IP Rendezvous point's address [4 octets]
  267. PORT Rendezvous point's OR port [2 octets]
  268. ID Rendezvous point identity ID [20 octets]
  269. KLEN Length of onion key [2 octets]
  270. KEY Rendezvous point onion key [KLEN octets]
  271. RC Rendezvous cookie [20 octets]
  272. g^x Diffie-Hellman data, part 1 [128 octets]
  273. OR
  274. VER Version byte: set to 3. [1 octet]
  275. ATYPE An address type (typically 4) [1 octet]
  276. ADDR Introduction point's IP address [4 or 16 octets]
  277. PORT Rendezvous point's OR port [2 octets]
  278. AUTHT The auth type that is supported [2 octets]
  279. AUTHL Length of auth data [1 octet]
  280. AUTHD Auth data [variable]
  281. ID Rendezvous point identity ID [20 octets]
  282. KLEN Length of onion key [2 octets]
  283. KEY Rendezvous point onion key [KLEN octets]
  284. RC Rendezvous cookie [20 octets]
  285. g^x Diffie-Hellman data, part 1 [128 octets]
  286. PK_ID is the hash of Bob's public key. RP is NUL-padded and terminated,
  287. and must contain EITHER a nickname, or an identity key digest, encoded in
  288. hex, and prefixed with a '$'.
  289. Implementations SHOULD accept all variants, and list the variants they
  290. accept in their V1 descriptor. Implementations should only generate the
  291. variants listed in the service's V1 descriptor; if no V1 descriptor is
  292. available, only the first variant should be generated. No version should
  293. generate the second variant (version byte=1).
  294. The hybrid encryption to Bob's PK works just like the hybrid
  295. encryption in CREATE cells (see main spec). Thus the payload of the
  296. RELAY_INTRODUCE1 cell on the wire will contain 20+42+16+20+20+128=246
  297. bytes. [XXXX not really]
  298. 1.9. Introduction: From the Introduction Point to Bob's OP
  299. If the Introduction Point recognizes PK_ID as a public key which has
  300. established a circuit for introductions as in 1.3 above, it sends the body
  301. of the cell in a new RELAY_INTRODUCE2 cell down the corresponding circuit.
  302. (If the PK_ID is unrecognized, the RELAY_INTRODUCE1 cell is discarded.)
  303. After sending the RELAY_INTRODUCE2 cell, the OR replies to Alice with an
  304. empty RELAY_COMMAND_INTRODUCE_ACK cell. If no RELAY_INTRODUCE2 cell can
  305. be sent, the OR replies to Alice with a non-empty cell to indicate an
  306. error. (The semantics of the cell body may be determined later; the
  307. current implementation sends a single '1' byte on failure.)
  308. When Bob's OP receives the RELAY_INTRODUCE2 cell, it decrypts it with
  309. the private key for the corresponding hidden service, and extracts the
  310. rendezvous point's nickname, the rendezvous cookie, and the value of g^x
  311. chosen by Alice.
  312. 1.10. Rendezvous
  313. Bob's OP builds a new Tor circuit ending at Alice's chosen rendezvous
  314. point, and sends a RELAY_RENDEZVOUS1 cell along this circuit, containing:
  315. RC Rendezvous cookie [20 octets]
  316. g^y Diffie-Hellman [128 octets]
  317. KH Handshake digest [20 octets]
  318. (Bob's OP MUST NOT use this circuit for any other purpose.)
  319. If the RP recognizes RC, it relays the rest of the cell down the
  320. corresponding circuit in a RELAY_RENDEZVOUS2 cell, containing:
  321. g^y Diffie-Hellman [128 octets]
  322. KH Handshake digest [20 octets]
  323. (If the RP does not recognize the RC, it discards the cell and
  324. tears down the circuit.)
  325. When Alice's OP receives a RELAY_RENDEZVOUS2 cell on a circuit which
  326. has sent a RELAY_ESTABLISH_RENDEZVOUS cell but which has not yet received
  327. a reply, it uses g^y and H(g^xy) to complete the handshake as in the Tor
  328. circuit extend process: they establish a 60-octet string as
  329. K = SHA1(g^xy | [00]) | SHA1(g^xy | [01]) | SHA1(g^xy | [02])
  330. and generate
  331. KH = K[0..15]
  332. Kf = K[16..31]
  333. Kb = K[32..47]
  334. Subsequently, the rendezvous point passes relay cells, unchanged, from
  335. each of the two circuits to the other. When Alice's OP sends
  336. RELAY cells along the circuit, it first encrypts them with the
  337. Kf, then with all of the keys for the ORs in Alice's side of the circuit;
  338. and when Alice's OP receives RELAY cells from the circuit, it decrypts
  339. them with the keys for the ORs in Alice's side of the circuit, then
  340. decrypts them with Kb. Bob's OP does the same, with Kf and Kb
  341. interchanged.
  342. 1.11. Creating streams
  343. To open TCP connections to Bob's location-hidden service, Alice's OP sends
  344. a RELAY_BEGIN cell along the established circuit, using the special
  345. address "", and a chosen port. Bob's OP chooses a destination IP and
  346. port, based on the configuration of the service connected to the circuit,
  347. and opens a TCP stream. From then on, Bob's OP treats the stream as an
  348. ordinary exit connection.
  349. [ Except he doesn't include addr in the connected cell or the end
  350. cell. -RD]
  351. Alice MAY send multiple RELAY_BEGIN cells along the circuit, to open
  352. multiple streams to Bob. Alice SHOULD NOT send RELAY_BEGIN cells for any
  353. other address along her circuit to Bob; if she does, Bob MUST reject them.
  354. 2.0. Authentication and authorization.
  355. Foo.