Tor Rendezvous Specification 0. Overview and preliminaries The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119. Read https://www.torproject.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 service. 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, 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 and terminology 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. An OP is (as defined elsewhere) an "Onion Proxy" or Tor client. An OR is (as defined elsewhere) an "Onion Router" or Tor server. An "Introduction point" is a Tor server chosen to be Bob's medium-term 'meeting place'. A "Rendezvous point" is a Tor server chosen by Alice to be a short-term communication relay between her and Bob. All Tor servers potentially act as introduction and rendezvous points. 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 a long-term keypair. 3. Bob's OP->Introduction point via Tor: [introduction setup] "This public key is (currently) associated to me." 4. Bob's OP->directory service via Tor: publishes Bob's service descriptor [advertisement] "Meet public-key X at introduction point A, B, or C." (signed) 5. Out of band, Alice receives a z.onion:port address. She opens a SOCKS connection to her OP, and requests 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. (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 1] 11. The Rendezvous point forwards Bob's confirmation to Alice's OP. [Rendezvous 2] 12. Alice's OP sends begin cells to Bob's OP. [Connection] 0.3. Constants and new cell types Relay cell types 32 -- RELAY_COMMAND_ESTABLISH_INTRO 33 -- RELAY_COMMAND_ESTABLISH_RENDEZVOUS 34 -- RELAY_COMMAND_INTRODUCE1 35 -- RELAY_COMMAND_INTRODUCE2 36 -- RELAY_COMMAND_RENDEZVOUS1 37 -- RELAY_COMMAND_RENDEZVOUS2 38 -- RELAY_COMMAND_INTRO_ESTABLISHED 39 -- RELAY_COMMAND_RENDEZVOUS_ESTABLISHED 40 -- RELAY_COMMAND_INTRODUCE_ACK 0.4. Version overview There are several parts in the hidden service protocol that have changed over time, each of them having its own version number, whereas other parts remained the same. The following list of potentially versioned protocol parts should help reduce some confusion: - Hidden service descriptor: the binary-based v0 was the default for a long time, and an ASCII-based v2 has been added by proposal 114. The v0 descriptor format has been deprecated in 0.2.2.1-alpha. See 1.3. - Hidden service descriptor propagation mechanism: currently related to the hidden service descriptor version -- v0 publishes to the original hs directory authorities, whereas v2 publishes to a rotating subset of relays with the "HSDir" flag; see 1.4 and 1.6. - Introduction protocol for how to generate an introduction cell: v0 specified a nickname for the rendezvous point and assumed the relay would know about it, whereas v2 now specifies IP address, port, and onion key so the relay doesn't need to already recognize it. See 1.8. 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 establishes his introduction points. The first time the OP provides an advertised service, it generates a public/private keypair (stored locally). The OP choses a small number of Tor servers as introduction points. The OP establishes a new introduction circuit to each introduction point. These circuits MUST NOT be used for anything but hidden service introduction. To establish the introduction, Bob sends a RELAY_COMMAND_ESTABLISH_INTRO cell, containing: KL Key length [2 octets] PK Bob's public key or service key [KL octets] HS Hash of session info [20 octets] SIG Signature of above information [variable] KL is the length of PK, in octets. 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_COMMAND_INTRO_ESTABLISHED cell with an empty payload. Bob's OP uses either Bob's public key or a freshly generated, single-use service key in the RELAY_COMMAND_ESTABLISH_INTRO cell, depending on the configured hidden service descriptor version. The public key is used for v0 descriptors, the service key for v2 descriptors. In the latter case, the service keys of all introduction points are included in the v2 hidden service descriptor together with the other introduction point information. The reason is that the introduction point does not need to and therefore should not know for which hidden service it works, so as to prevent it from tracking the hidden service's activity. If the hidden service is configured to publish both v0 and v2 descriptors, two separate sets of introduction points are established. 1.3. Bob's OP generates service descriptors. For versions before 0.2.2.1-alpha, Bob's OP periodically generates and publishes a descriptor of type "V0". 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] TS is the number of seconds elapsed since Jan 1, 1970. 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] Beginning with 0.2.0.10-alpha, Bob's OP encodes "V2" descriptors in addition to (or instead of) "V0" descriptors. The format of a "V2" descriptor is as follows: "rendezvous-service-descriptor" descriptor-id NL [At start, exactly once] Indicates the beginning of the descriptor. "descriptor-id" is a periodically changing identifier of 160 bits formatted as 32 base32 chars that is calculated by the hidden service and its clients. The "descriptor-id" is calculated by performing the following operation: descriptor-id = H(permanent-id | H(time-period | descriptor-cookie | replica)) "permanent-id" is the permanent identifier of the hidden service, consisting of 80 bits. It can be calculated by computing the hash value of the public hidden service key and truncating after the first 80 bits: permanent-id = H(public-key)[:10] "H(time-period | descriptor-cookie | replica)" is the (possibly secret) id part that is necessary to verify that the hidden service is the true originator of this descriptor and that is therefore contained in the descriptor, too. The descriptor ID can only be created by the hidden service and its clients, but the "signature" below can only be created by the service. "time-period" changes periodically as a function of time and "permanent-id". The current value for "time-period" can be calculated using the following formula: time-period = (current-time + permanent-id-byte * 86400 / 256) / 86400 "current-time" contains the current system time in seconds since 1970-01-01 00:00, e.g. 1188241957. "permanent-id-byte" is the first (unsigned) byte of the permanent identifier (which is in network order), e.g. 143. Adding the product of "permanent-id-byte" and 86400 (seconds per day), divided by 256, prevents "time-period" from changing for all descriptors at the same time of the day. The result of the overall operation is a (network-ordered) 32-bit integer, e.g. 13753 or 0x000035B9 with the example values given above. "descriptor-cookie" is an optional secret password of 128 bits that is shared between the hidden service provider and its clients. If the descriptor-cookie is left out, the input to the hash function is 128 bits shorter. "replica" denotes the number of the replica. A service publishes multiple descriptors with different descriptor IDs in order to distribute them to different places on the ring. "version" version-number NL [Exactly once] The version number of this descriptor's format. In this case: 2. "permanent-key" NL a public key in PEM format [Exactly once] The public key of the hidden service which is required to verify the "descriptor-id" and the "signature". "secret-id-part" secret-id-part NL [Exactly once] The result of the following operation as explained above, formatted as 32 base32 chars. Using this secret id part, everyone can verify that the signed descriptor belongs to "descriptor-id". secret-id-part = H(time-period | descriptor-cookie | replica) "publication-time" YYYY-MM-DD HH:MM:SS NL [Exactly once] A timestamp when this descriptor has been created. "protocol-versions" version-string NL [Exactly once] A comma-separated list of recognized and permitted version numbers for use in INTRODUCE cells; these versions are described in section 1.8 below. "introduction-points" NL encrypted-string [At most once] A list of introduction points. If the optional "descriptor-cookie" is used, this list is encrypted with AES in CTR mode with a random initialization vector of 128 bits that is written to the beginning of the encrypted string, and the "descriptor-cookie" as secret key of 128 bits length. The string containing the introduction point data (either encrypted or not) is encoded in base64, and surrounded with "-----BEGIN MESSAGE-----" and "-----END MESSAGE-----". The unencrypted string may begin with: "service-authentication" auth-type auth-data NL [Any number] The service-specific authentication data can be used to perform client authentication. This data is independent of the selected introduction point as opposed to "intro-authentication" below. The format of auth-data (base64-encoded or PEM format) depends on auth-type. See section 2 of this document for details on auth mechanisms. Subsequently, an arbitrary number of introduction point entries may follow, each containing the following data: "introduction-point" identifier NL [At start, exactly once] The identifier of this introduction point: the base-32 encoded hash of this introduction point's identity key. "ip-address" ip-address NL [Exactly once] The IP address of this introduction point. "onion-port" port NL [Exactly once] The TCP port on which the introduction point is listening for incoming onion requests. "onion-key" NL a public key in PEM format [Exactly once] The public key that can be used to encrypt messages to this introduction point. "service-key" NL a public key in PEM format [Exactly once] The public key that can be used to encrypt messages to the hidden service. "intro-authentication" auth-type auth-data NL [Any number] The introduction-point-specific authentication data can be used to perform client authentication. This data depends on the selected introduction point as opposed to "service-authentication" above. The format of auth-data (base64-encoded or PEM format) depends on auth-type. See section 2 of this document for details on auth mechanisms. (This ends the fields in the encrypted portion of the descriptor.) [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] "signature" NL signature-string [At end, exactly once] A signature of all fields above with the private key of the hidden service. 1.3.1. Other descriptor formats we don't use. Support for the V0 descriptor format was dropped in 0.2.2.0-alpha-dev: 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. The members of Ipt may be either (a) nicknames, or (b) identity key digests, encoded in hex, and prefixed with a '$'. The V1 descriptor format was understood and accepted from 0.1.1.5-alpha-cvs to 0.2.0.6-alpha-dev, but no Tors generated it and it was removed: 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] A hypothetical "V1" descriptor, that has never been used but might be useful for historical reasons, 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] 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. 1.4. Bob's OP advertises his service descriptor(s). Bob's OP advertises his service descriptor to a fixed set of v0 hidden service directory servers and/or a changing subset of all v2 hidden service directories. For versions before 0.2.2.1-alpha, Bob's OP opens a stream to each v0 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. 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. If Bob's OP is configured to publish v2 descriptors, it does so to a changing subset of all v2 hidden service directories instead of the authoritative directory servers. Therefore, Bob's OP opens a stream via Tor to each responsible hidden service directory. (He may re-use old circuits for this.) Over this stream, Bob's OP makes an HTTP 'POST' request to a URL "/tor/rendezvous2/publish" relative to the hidden service directory's root, containing as its body Bob's service descriptor. At any time, there are 6 hidden service directories responsible for keeping replicas of a descriptor; they consist of 2 sets of 3 hidden service directories with consecutive onion IDs. Bob's OP learns about the complete list of hidden service directories by filtering the consensus status document received from the directory authorities. A hidden service directory is deemed responsible for all descriptor IDs in the interval from its direct predecessor, exclusive, to its own ID, inclusive; it further holds replicas for its 2 predecessors. A participant only trusts its own routing list and never learns about routing information from other parties. Bob's OP publishes a new v2 descriptor once an hour or whenever its content changes. V2 descriptors can be found by clients within a given time period of 24 hours, after which they change their ID as described under 1.3. If a published descriptor would be valid for less than 60 minutes (= 2 x 30 minutes to allow the server to be 30 minutes behind and the client 30 minutes ahead), Bob's OP publishes the descriptor under the ID of both, the current and the next publication period. 1.5. Alice receives a z.onion address. When Alice receives a pointer to a location-hidden service, it is as a hostname of the form "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. 3. 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.) [Yes, numbers are allowed at the beginning. See RFC 1123. -NM] 1.6. Alice's OP retrieves a service descriptor. Alice's OP fetches the service descriptor from the fixed set of v0 hidden service directory servers and/or a changing subset of all v2 hidden service directories. For versions before 0.2.2.1-alpha, Alice's OP opens a stream to a directory server via Tor, and makes an HTTP GET request for the document '/tor/rendezvous/', where '' 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 , and otherwise returns Bob's most recently uploaded service 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] If Alice's OP is running 0.2.1.10-alpha or higher, it fetches v2 hidden service descriptors. Versions before 0.2.2.1-alpha are fetching both v0 and v2 descriptors in parallel. Similar to the description in section 1.4, Alice's OP fetches a v2 descriptor from a randomly chosen hidden service directory out of the changing subset of 6 nodes. If the request is unsuccessful, Alice retries the other remaining responsible hidden service directories in a random order. Alice relies on Bob to care about a potential clock skew between the two by possibly storing two sets of descriptors (see end of section 1.4). Alice's OP opens a stream via Tor to the chosen v2 hidden service directory. (She may re-use old circuits for this.) Over this stream, Alice's OP makes an HTTP 'GET' request for the document "/tor/rendezvous2/", where z is replaced with the encoding of the descriptor ID. The directory replies with a 404 HTTP response if it does not recognize , and otherwise returns Bob's most recently uploaded service descriptor. 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_COMMAND_ESTABLISH_RENDEZVOUS cell to that OR. The body of that cell contains: RC Rendezvous cookie [20 octets] The rendezvous cookie is an arbitrary 20-byte value, chosen randomly by Alice's OP. Upon receiving a RELAY_COMMAND_ESTABLISH_RENDEZVOUS cell, the OR associates the RC with the circuit that sent it. It replies to Alice with an empty RELAY_COMMAND_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_COMMAND_INTRODUCE1 cell containing: Cleartext PK_ID Identifier for Bob's PK [20 octets] Encrypted to Bob's PK: (in the v0 intro protocol) RP Rendezvous point's nickname [20 octets] RC Rendezvous cookie [20 octets] g^x Diffie-Hellman data, part 1 [128 octets] OR (in the v1 intro protocol) 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 (in the v2 intro protocol) 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 (in the v3 intro protocol) VER Version byte: set to 3. [1 octet] AUTHT The auth type that is used [1 octet] AUTHL Length of auth data [2 octets] AUTHD Auth data [variable] TS A timestamp [4 octets] 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] PK_ID is the hash of Bob's public key or the service key, depending on the hidden service descriptor version. In case of a v0 descriptor, Alice's OP uses Bob's public key. If Alice has downloaded a v2 descriptor, she uses the contained public key ("service-key"). RP is NUL-padded and terminated. In version 0 of the intro protocol, RP must contain a nickname. In version 1, it must contain EITHER a nickname or an identity key digest that is encoded in hex and prefixed with a '$'. The hybrid encryption to Bob's PK works just like the hybrid encryption in CREATE cells (see tor-spec). Thus the payload of the version 0 RELAY_COMMAND_INTRODUCE1 cell on the wire will contain 20+42+16+20+20+128=246 bytes, and the version 1 and version 2 introduction formats have other sizes. Through Tor 0.2.0.6-alpha, clients only generated the v0 introduction format, whereas hidden services have understood and accepted v0, v1, and v2 since 0.1.1.x. As of Tor 0.2.0.7-alpha and 0.1.2.18, clients switched to using the v2 intro format. 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 1.2 above, it sends the body of the cell in a new RELAY_COMMAND_INTRODUCE2 cell down the corresponding circuit. (If the PK_ID is unrecognized, the RELAY_COMMAND_INTRODUCE1 cell is discarded.) After sending the RELAY_COMMAND_INTRODUCE2 cell, the OR replies to Alice with an empty RELAY_COMMAND_INTRODUCE_ACK cell. If no RELAY_COMMAND_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_COMMAND_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_COMMAND_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_COMMAND_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_COMMAND_RENDEZVOUS2 cell on a circuit which has sent a RELAY_COMMAND_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_COMMAND_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_COMMAND_BEGIN cells along the circuit, to open multiple streams to Bob. Alice SHOULD NOT send RELAY_COMMAND_BEGIN cells for any other address along her circuit to Bob; if she does, Bob MUST reject them. 2. Authentication and authorization. The rendezvous protocol as described in Section 1 provides a few options for implementing client-side authorization. There are two steps in the rendezvous protocol that can be used for performing client authorization: when downloading and decrypting parts of the hidden service descriptor and at Bob's Tor client before contacting the rendezvous point. A service provider can restrict access to his service at these two points to authorized clients only. There are currently two authorization protocols specified that are described in more detail below: 1. The first protocol allows a service provider to restrict access to clients with a previously received secret key only, but does not attempt to hide service activity from others. 2. The second protocol, albeit being feasible for a limited set of about 16 clients, performs client authorization and hides service activity from everyone but the authorized clients. 2.1. Service with large-scale client authorization The first client authorization protocol aims at performing access control while consuming as few additional resources as possible. A service provider should be able to permit access to a large number of clients while denying access for everyone else. However, the price for scalability is that the service won't be able to hide its activity from unauthorized or formerly authorized clients. The main idea of this protocol is to encrypt the introduction-point part in hidden service descriptors to authorized clients using symmetric keys. This ensures that nobody else but authorized clients can learn which introduction points a service currently uses, nor can someone send a valid INTRODUCE1 message without knowing the introduction key. Therefore, a subsequent authorization at the introduction point is not required. A service provider generates symmetric "descriptor cookies" for his clients and distributes them outside of Tor. The suggested key size is 128 bits, so that descriptor cookies can be encoded in 22 base64 chars (which can hold up to 22 * 5 = 132 bits, leaving 4 bits to encode the authorization type (here: "0") and allow a client to distinguish this authorization protocol from others like the one proposed below). Typically, the contact information for a hidden service using this authorization protocol looks like this: v2cbb2l4lsnpio4q.onion Ll3X7Xgz9eHGKCCnlFH0uz When generating a hidden service descriptor, the service encrypts the introduction-point part with a single randomly generated symmetric 128-bit session key using AES-CTR as described for v2 hidden service descriptors in rend-spec. Afterwards, the service encrypts the session key to all descriptor cookies using AES. Authorized client should be able to efficiently find the session key that is encrypted for him/her, so that 4 octet long client ID are generated consisting of descriptor cookie and initialization vector. Descriptors always contain a number of encrypted session keys that is a multiple of 16 by adding fake entries. Encrypted session keys are ordered by client IDs in order to conceal addition or removal of authorized clients by the service provider. ATYPE Authorization type: set to 1. [1 octet] ALEN Number of clients := 1 + ((clients - 1) div 16) [1 octet] for each symmetric descriptor cookie: ID Client ID: H(descriptor cookie | IV)[:4] [4 octets] SKEY Session key encrypted with descriptor cookie [16 octets] (end of client-specific part) RND Random data [(15 - ((clients - 1) mod 16)) * 20 octets] IV AES initialization vector [16 octets] IPOS Intro points, encrypted with session key [remaining octets] An authorized client needs to configure Tor to use the descriptor cookie when accessing the hidden service. Therefore, a user adds the contact information that she received from the service provider to her torrc file. Upon downloading a hidden service descriptor, Tor finds the encrypted introduction-point part and attempts to decrypt it using the configured descriptor cookie. (In the rare event of two or more client IDs being equal a client tries to decrypt all of them.) Upon sending the introduction, the client includes her descriptor cookie as auth type "1" in the INTRODUCE2 cell that she sends to the service. The hidden service checks whether the included descriptor cookie is authorized to access the service and either responds to the introduction request, or not. 2.2. Authorization for limited number of clients A second, more sophisticated client authorization protocol goes the extra mile of hiding service activity from unauthorized clients. With all else being equal to the preceding authorization protocol, the second protocol publishes hidden service descriptors for each user separately and gets along with encrypting the introduction-point part of descriptors to a single client. This allows the service to stop publishing descriptors for removed clients. As long as a removed client cannot link descriptors issued for other clients to the service, it cannot derive service activity any more. The downside of this approach is limited scalability. Even though the distributed storage of descriptors (cf. proposal 114) tackles the problem of limited scalability to a certain extent, this protocol should not be used for services with more than 16 clients. (In fact, Tor should refuse to advertise services for more than this number of clients.) A hidden service generates an asymmetric "client key" and a symmetric "descriptor cookie" for each client. The client key is used as replacement for the service's permanent key, so that the service uses a different identity for each of his clients. The descriptor cookie is used to store descriptors at changing directory nodes that are unpredictable for anyone but service and client, to encrypt the introduction-point part, and to be included in INTRODUCE2 cells. Once the service has created client key and descriptor cookie, he tells them to the client outside of Tor. The contact information string looks similar to the one used by the preceding authorization protocol (with the only difference that it has "1" encoded as auth-type in the remaining 4 of 132 bits instead of "0" as before). When creating a hidden service descriptor for an authorized client, the hidden service uses the client key and descriptor cookie to compute secret ID part and descriptor ID: secret-id-part = H(time-period | descriptor-cookie | replica) descriptor-id = H(client-key[:10] | secret-id-part) The hidden service also replaces permanent-key in the descriptor with client-key and encrypts introduction-points with the descriptor cookie. ATYPE Authorization type: set to 2. [1 octet] IV AES initialization vector [16 octets] IPOS Intro points, encr. with descriptor cookie [remaining octets] When uploading descriptors, the hidden service needs to make sure that descriptors for different clients are not uploaded at the same time (cf. Section 1.1) which is also a limiting factor for the number of clients. When a client is requested to establish a connection to a hidden service it looks up whether it has any authorization data configured for that service. If the user has configured authorization data for authorization protocol "2", the descriptor ID is determined as described in the last paragraph. Upon receiving a descriptor, the client decrypts the introduction-point part using its descriptor cookie. Further, the client includes its descriptor cookie as auth-type "2" in INTRODUCE2 cells that it sends to the service. 2.3. Hidden service configuration A hidden service that is meant to perform client authorization adds a new option HiddenServiceAuthorizeClient to its hidden service configuration. This option contains the authorization type which is either "1" for the protocol described in 2.1 or "2" for the protocol in 2.2 and a comma-separated list of human-readable client names, so that Tor can create authorization data for these clients: HiddenServiceAuthorizeClient auth-type client-name,client-name,... If this option is configured, HiddenServiceVersion is automatically reconfigured to contain only version numbers of 2 or higher. Tor stores all generated authorization data for the authorization protocols described in Sections 2.1 and 2.2 in a new file using the following file format: "client-name" human-readable client identifier NL "descriptor-cookie" 128-bit key ^= 22 base64 chars NL If the authorization protocol of Section 2.2 is used, Tor also generates and stores the following data: "client-key" NL a public key in PEM format 2.4. Client configuration Clients need to make their authorization data known to Tor using another configuration option that contains a service name (mainly for the sake of convenience), the service address, and the descriptor cookie that is required to access a hidden service (the authorization protocol number is encoded in the descriptor cookie): HidServAuth service-name service-address descriptor-cookie 3. Hidden service directory operation This section has been introduced with the v2 hidden service descriptor format. It describes all operations of the v2 hidden service descriptor fetching and propagation mechanism that are required for the protocol described in section 1 to succeed with v2 hidden service descriptors. 3.1. Configuring as hidden service directory Every onion router that has its directory port open can decide whether it wants to store and serve hidden service descriptors. An onion router which is configured as such includes the "hidden-service-dir" flag in its router descriptors that it sends to directory authorities. The directory authorities include a new flag "HSDir" for routers that decided to provide storage for hidden service descriptors and that have been running for at least 24 hours. 3.2. Accepting publish requests Hidden service directory nodes accept publish requests for v2 hidden service descriptors and store them to their local memory. (It is not necessary to make descriptors persistent, because after restarting, the onion router would not be accepted as a storing node anyway, because it has not been running for at least 24 hours.) All requests and replies are formatted as HTTP messages. Requests are initiated via BEGIN_DIR cells directed to the router's directory port, and formatted as HTTP POST requests to the URL "/tor/rendezvous2/publish" relative to the hidden service directory's root, containing as its body a v2 service descriptor. A hidden service directory node parses every received descriptor and only stores it when it thinks that it is responsible for storing that descriptor based on its own routing table. See section 1.4 for more information on how to determine responsibility for a certain descriptor ID. 3.3. Processing fetch requests Hidden service directory nodes process fetch requests for hidden service descriptors by looking them up in their local memory. (They do not need to determine if they are responsible for the passed ID, because it does no harm if they deliver a descriptor for which they are not (any more) responsible.) All requests and replies are formatted as HTTP messages. Requests are initiated via BEGIN_DIR cells directed to the router's directory port, and formatted as HTTP GET requests for the document "/tor/rendezvous2/", where z is replaced with the encoding of the descriptor ID.