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$Id: rend-spec.txt 9490 2007-02-06 00:02:31Z arma $ Tor Rendezvous Specification 0. Overview and preliminaries Read http://tor.eff.org/doc/design-paper/tor-design.html#sec:rendezvous before you read this specification. It will make more sense. Rendezvous points provide location-hidden services (server anonymity) for the onion routing network. With rendezvous points, Bob can offer a TCP service (say, a webserver) via the onion routing network, without revealing the IP of that service. Bob does this by anonymously advertising a public key for his service, along with a list of onion routers to act as "Introduction Points" for his service. He creates forward circuits to those introduction points, and tells them about his public key. To connect to Bob, Alice first builds a circuit to an OR to act as her "Rendezvous Point." She then connects to one of Bob's chosen introduction points, optionally provides authentication or authorization information, and asks it to tell him about her Rendezvous Point (RP). If Bob chooses to answer, he builds a circuit to her RP, and tells it to connect him to Alice. The RP joins their circuits together, and begins relaying cells. Alice's 'BEGIN' cells are received directly by Bob's OP, which passes data to and from the local server implementing Bob's service. Below we describe a network-level specification of this service, along with interfaces to make this process transparent to Alice (so long as she is using an OP). 0.1. Notation, conventions and prerequisites In the specifications below, we use the same notation as in "tor-spec.txt". The service specified here also requires the existence of an onion routing network as specified in that file. H(x) is a SHA1 digest of x. PKSign(SK,x) is a PKCS.1-padded RSA signature of x with SK. PKEncrypt(SK,x) is a PKCS.1-padded RSA encryption of x with SK. Public keys are all RSA, and encoded in ASN.1. All integers are stored in network (big-endian) order. All symmetric encryption uses AES in counter mode, except where otherwise noted. In all discussions, "Alice" will refer to a user connecting to a location-hidden service, and "Bob" will refer to a user running a location-hidden service. 0.2. Protocol outline 1. Bob->Bob's OP: "Offer IP:Port as public-key-name:Port". [configuration] (We do not specify this step; it is left to the implementor of Bob's OP.) 2. Bob's OP generates keypair and rendezvous service descriptor: "Meet public-key X at introduction point A, B, or C." (signed) 3. Bob's OP->Introduction point via Tor: [introduction setup] "This pk is me." 4. Bob's OP->directory service via Tor: publishes Bob's service descriptor [advertisement] 5. Out of band, Alice receives a [x.y.]z.onion:port address. She opens a SOCKS connection to her OP, and requests x.y.z.onion:port. 6. Alice's OP retrieves Bob's descriptor via Tor. [descriptor lookup.] 7. Alice's OP chooses a rendezvous point, opens a circuit to that rendezvous point, and establishes a rendezvous circuit. [rendezvous setup.] 8. Alice connects to the Introduction point via Tor, and tells it about her rendezvous point and optional authentication/authorization information. (Encrypted to Bob.) [Introduction 1] 9. The Introduction point passes this on to Bob's OP via Tor, along the introduction circuit. [Introduction 2] 10. Bob's OP decides whether to connect to Alice, and if so, creates a circuit to Alice's RP via Tor. Establishes a shared circuit. [Rendezvous.] 11. Alice's OP sends begin cells to Bob's OP. [Connection] 0.3. Constants and new cell types Relay cell types 32 -- RELAY_ESTABLISH_INTRO 33 -- RELAY_ESTABLISH_RENDEZVOUS 34 -- RELAY_INTRODUCE1 35 -- RELAY_INTRODUCE2 36 -- RELAY_RENDEZVOUS1 37 -- RELAY_RENDEZVOUS2 38 -- RELAY_INTRO_ESTABLISHED 39 -- RELAY_RENDEZVOUS_ESTABLISHED 40 -- RELAY_COMMAND_INTRODUCE_ACK 1. The Protocol 1.1. Bob configures his local OP. We do not specify a format for the OP configuration file. However, OPs SHOULD allow Bob to provide more than one advertised service per OP, and MUST allow Bob to specify one or more virtual ports per service. Bob provides a mapping from each of these virtual ports to a local IP:Port pair. 1.2. Bob's OP generates service descriptors. The first time the OP provides an advertised service, it generates a public/private keypair (stored locally). Periodically, the OP generates a pair of service descriptors, one "V1" and one "V0". The "V1" descriptor in 0.1.1.6-alpha contains: V Format byte: set to 255 [1 octet] V Version byte: set to 1 [1 octet] KL Key length [2 octets] PK Bob's public key [KL octets] TS A timestamp [4 octets] PROTO Rendezvous protocol versions: bitmask [2 octets] NA Number of auth mechanisms accepted [1 octet] For each auth mechanism: AUTHT The auth type that is supported [2 octets] AUTHL Length of auth data [1 octet] AUTHD Auth data [variable] NI Number of introduction points [2 octets] For each introduction point: (as in INTRODUCE2 cells) ATYPE An address type (typically 4) [1 octet] ADDR Introduction point's IP address [4 or 16 octets] PORT Introduction point's OR port [2 octets] AUTHT The auth type that is supported [2 octets] AUTHL Length of auth data [1 octet] AUTHD Auth data [variable] ID Introduction point identity ID [20 octets] KLEN Length of onion key [2 octets] KEY Introduction point onion key [KLEN octets] SIG Signature of above fields [variable] The "V1" descriptor in 0.1.1.5-alpha-cvs contains: V Format byte: set to 255 [1 octet] V Version byte: set to 1 [1 octet] KL Key length [2 octets] PK Bob's public key [KL octets] TS A timestamp [4 octets] PROTO Protocol versions: bitmask [2 octets] NI Number of introduction points [2 octets] For each introduction point: (as in INTRODUCE2 cells) IP Introduction point's address [4 octets] PORT Introduction point's OR port [2 octets] ID Introduction point identity ID [20 octets] KLEN Length of onion key [2 octets] KEY Introduction point onion key [KLEN octets] SIG Signature of above fields [variable] The "V0" descriptor contains: KL Key length [2 octets] PK Bob's public key [KL octets] TS A timestamp [4 octets] NI Number of introduction points [2 octets] Ipt A list of NUL-terminated ORs [variable] SIG Signature of above fields [variable] KL is the length of PK, in octets. TS is the number of seconds elapsed since Jan 1, 1970. AUTHT specifies which authentication/authorization mechanism is required by the hidden service or the introduction point. AUTHD is arbitrary data that can be associated with an auth approach. Currently only AUTHT of [00 00] is supported, with an AUTHL of 0. See section 2 of this document for details on auth mechanisms. The members of Ipt may be either (a) nicknames, or (b) identity key digests, encoded in hex, and prefixed with a '$'. Clients must accept both forms. Services must only generate the second form. Once 0.0.9.x is obsoleted, we can drop the first form. [It's ok for Bob to advertise 0 introduction points. He might want to do that if he previously advertised some introduction points, and now he doesn't have any. -RD] [Once Tor 0.1.0.x is obsolete, we can stop generating or using V0 descriptors. -NM] 1.3. Bob's OP establishes his introduction points. The OP establishes a new introduction circuit to each introduction point. These circuits MUST NOT be used for anything but rendezvous introduction. To establish the introduction, Bob sends a RELAY_ESTABLISH_INTRO cell, containing: KL Key length [2 octets] PK Bob's public key [KL octets] HS Hash of session info [20 octets] SIG Signature of above information [variable] [XXX011, need to add auth information here. -RD] To prevent replay attacks, the HS field contains a SHA-1 hash based on the shared secret KH between Bob's OP and the introduction point, as follows: HS = H(KH | "INTRODUCE") That is: HS = H(KH | [49 4E 54 52 4F 44 55 43 45]) (KH, as specified in tor-spec.txt, is H(g^xy | [00]) .) Upon receiving such a cell, the OR first checks that the signature is correct with the included public key. If so, it checks whether HS is correct given the shared state between Bob's OP and the OR. If either check fails, the OP discards the cell; otherwise, it associates the circuit with Bob's public key, and dissociates any other circuits currently associated with PK. On success, the OR sends Bob a RELAY_INTRO_ESTABLISHED cell with an empty payload. 1.4. Bob's OP advertises his service descriptor(s). Bob's OP opens a stream to each directory server's directory port via Tor. (He may re-use old circuits for this.) Over this stream, Bob's OP makes an HTTP 'POST' request, to a URL "/tor/rendezvous/publish" relative to the directory server's root, containing as its body Bob's service descriptor. Bob should upload a service descriptor for each version format that is supported in the current Tor network. Upon receiving a descriptor, the directory server checks the signature, and discards the descriptor if the signature does not match the enclosed public key. Next, the directory server checks the timestamp. If the timestamp is more than 24 hours in the past or more than 1 hour in the future, or the directory server already has a newer descriptor with the same public key, the server discards the descriptor. Otherwise, the server discards any older descriptors with the same public key and version format, and associates the new descriptor with the public key. The directory server remembers this descriptor for at least 24 hours after its timestamp. At least every 18 hours, Bob's OP uploads a fresh descriptor. 1.5. Alice receives a x.y.z.onion address. When Alice receives a pointer to a location-hidden service, it is as a hostname of the form "z.onion" or "y.z.onion" or "x.y.z.onion", where z is a base-32 encoding of a 10-octet hash of Bob's service's public key, computed as follows: 1. Let H = H(PK). 2. Let H' = the first 80 bits of H, considering each octet from most significant bit to least significant bit. 2. Generate a 16-character encoding of H', using base32 as defined in RFC 3548. (We only use 80 bits instead of the 160 bits from SHA1 because we don't need to worry about arbitrary collisions, and because it will make handling the url's more convenient.) The string "x", if present, is the base-32 encoding of the authentication/authorization required by the introduction point. The string "y", if present, is the base-32 encoding of the authentication/authorization required by the hidden service. Omitting a string is taken to mean auth type [00 00]. See section 2 of this document for details on auth mechanisms. [Yes, numbers are allowed at the beginning. See RFC 1123. -NM] 1.6. Alice's OP retrieves a service descriptor. Alice opens a stream to a directory server via Tor, and makes an HTTP GET request for the document '/tor/rendezvous/<z>' or '/tor/rendezvous1/<z>', where '<z>' is replaced with the encoding of Bob's public key as described above. (She may re-use old circuits for this.) The directory replies with a 404 HTTP response if it does not recognize <z>, and otherwise returns Bob's most recently uploaded service descriptor. (If Alice requests 'rendezvous1', the directory server provides a V1 descriptor or a V0 descriptor if no V1 descriptor is available. If Alice requests 'rendezvous', the directory server returns a V0 descriptor.) If Alice's OP receives a 404 response, it tries the other directory servers, and only fails the lookup if none recognize the public key hash. Upon receiving a service descriptor, Alice verifies with the same process as the directory server uses, described above in section 1.4. The directory server gives a 400 response if it cannot understand Alice's request. Alice should cache the descriptor locally, but should not use descriptors that are more than 24 hours older than their timestamp. [Caching may make her partitionable, but she fetched it anonymously, and we can't very well *not* cache it. -RD] 1.7. Alice's OP establishes a rendezvous point. When Alice requests a connection to a given location-hidden service, and Alice's OP does not have an established circuit to that service, the OP builds a rendezvous circuit. It does this by establishing a circuit to a randomly chosen OR, and sending a RELAY_ESTABLISH_RENDEZVOUS cell to that OR. The body of that cell contains: RC Rendezvous cookie [20 octets] [XXX011 this looks like an auth mechanism. should we generalize here? -RD] The rendezvous cookie is an arbitrary 20-byte value, chosen randomly by Alice's OP. Upon receiving a RELAY_ESTABLISH_RENDEZVOUS cell, the OR associates the RC with the circuit that sent it. It replies to Alice with an empty RELAY_RENDEZVOUS_ESTABLISHED cell to indicate success. Alice's OP MUST NOT use the circuit which sent the cell for any purpose other than rendezvous with the given location-hidden service. 1.8. Introduction: from Alice's OP to Introduction Point Alice builds a separate circuit to one of Bob's chosen introduction points, and sends it a RELAY_INTRODUCE1 cell containing: Cleartext PK_ID Identifier for Bob's PK [20 octets] [XXX011 want to put intro-level auth info here, but no version. crap. -RD] Encrypted to Bob's PK: RP Rendezvous point's nickname [20 octets] RC Rendezvous cookie [20 octets] g^x Diffie-Hellman data, part 1 [128 octets] OR VER Version byte: set to 1. [1 octet] RP Rendezvous point nick or ID [42 octets] RC Rendezvous cookie [20 octets] g^x Diffie-Hellman data, part 1 [128 octets] OR VER Version byte: set to 2. [1 octet] IP Rendezvous point's address [4 octets] PORT Rendezvous point's OR port [2 octets] ID Rendezvous point identity ID [20 octets] KLEN Length of onion key [2 octets] KEY Rendezvous point onion key [KLEN octets] RC Rendezvous cookie [20 octets] g^x Diffie-Hellman data, part 1 [128 octets] OR VER Version byte: set to 3. [1 octet] ATYPE An address type (typically 4) [1 octet] ADDR Rendezvous point's IP address [4 or 16 octets] PORT Rendezvous point's OR port [2 octets] AUTHT The auth type that is supported [2 octets] AUTHL Length of auth data [1 octet] AUTHD Auth data [variable] ID Rendezvous point identity ID [20 octets] KLEN Length of onion key [2 octets] KEY Rendezvous point onion key [KLEN octets] RC Rendezvous cookie [20 octets] g^x Diffie-Hellman data, part 1 [128 octets] PK_ID is the hash of Bob's public key. RP is NUL-padded and terminated, and must contain EITHER a nickname, or an identity key digest, encoded in hex, and prefixed with a '$'. Implementations SHOULD accept all variants, and list the variants they accept in their V1 descriptor. Implementations should only generate the variants listed in the service's V1 descriptor; if no V1 descriptor is available, only the first variant should be generated. No version should generate the second variant (version byte=1). The hybrid encryption to Bob's PK works just like the hybrid encryption in CREATE cells (see main spec). Thus the payload of the RELAY_INTRODUCE1 cell on the wire will contain 20+42+16+20+20+128=246 bytes. [XXXX not really] 1.9. Introduction: From the Introduction Point to Bob's OP If the Introduction Point recognizes PK_ID as a public key which has established a circuit for introductions as in 1.3 above, it sends the body of the cell in a new RELAY_INTRODUCE2 cell down the corresponding circuit. (If the PK_ID is unrecognized, the RELAY_INTRODUCE1 cell is discarded.) After sending the RELAY_INTRODUCE2 cell, the OR replies to Alice with an empty RELAY_COMMAND_INTRODUCE_ACK cell. If no RELAY_INTRODUCE2 cell can be sent, the OR replies to Alice with a non-empty cell to indicate an error. (The semantics of the cell body may be determined later; the current implementation sends a single '1' byte on failure.) When Bob's OP receives the RELAY_INTRODUCE2 cell, it decrypts it with the private key for the corresponding hidden service, and extracts the rendezvous point's nickname, the rendezvous cookie, and the value of g^x chosen by Alice. 1.10. Rendezvous Bob's OP builds a new Tor circuit ending at Alice's chosen rendezvous point, and sends a RELAY_RENDEZVOUS1 cell along this circuit, containing: RC Rendezvous cookie [20 octets] g^y Diffie-Hellman [128 octets] KH Handshake digest [20 octets] (Bob's OP MUST NOT use this circuit for any other purpose.) If the RP recognizes RC, it relays the rest of the cell down the corresponding circuit in a RELAY_RENDEZVOUS2 cell, containing: g^y Diffie-Hellman [128 octets] KH Handshake digest [20 octets] (If the RP does not recognize the RC, it discards the cell and tears down the circuit.) When Alice's OP receives a RELAY_RENDEZVOUS2 cell on a circuit which has sent a RELAY_ESTABLISH_RENDEZVOUS cell but which has not yet received a reply, it uses g^y and H(g^xy) to complete the handshake as in the Tor circuit extend process: they establish a 60-octet string as K = SHA1(g^xy | [00]) | SHA1(g^xy | [01]) | SHA1(g^xy | [02]) and generate KH = K[0..15] Kf = K[16..31] Kb = K[32..47] Subsequently, the rendezvous point passes relay cells, unchanged, from each of the two circuits to the other. When Alice's OP sends RELAY cells along the circuit, it first encrypts them with the Kf, then with all of the keys for the ORs in Alice's side of the circuit; and when Alice's OP receives RELAY cells from the circuit, it decrypts them with the keys for the ORs in Alice's side of the circuit, then decrypts them with Kb. Bob's OP does the same, with Kf and Kb interchanged. 1.11. Creating streams To open TCP connections to Bob's location-hidden service, Alice's OP sends a RELAY_BEGIN cell along the established circuit, using the special address "", and a chosen port. Bob's OP chooses a destination IP and port, based on the configuration of the service connected to the circuit, and opens a TCP stream. From then on, Bob's OP treats the stream as an ordinary exit connection. [ Except he doesn't include addr in the connected cell or the end cell. -RD] Alice MAY send multiple RELAY_BEGIN cells along the circuit, to open multiple streams to Bob. Alice SHOULD NOT send RELAY_BEGIN cells for any other address along her circuit to Bob; if she does, Bob MUST reject them. 2.0. Authentication and authorization. Foo.