tor-parallel-relay-conn/doc/spec/rend-spec.txt

                    Tor Rendezvous Specification

0. Overview and preliminaries

   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 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 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 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

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. See 1.2.

   - 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 generates service descriptors.

   The first time the OP provides an advertised service, it generates
   a public/private keypair (stored locally).

   Beginning with 0.2.0.10-alpha, Bob's OP encodes "V2" 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. If
       the optional "descriptor-cookie" is used, this "descriptor-id"
       cannot be computed by anyone else. (Everyone can verify that this
       "descriptor-id" belongs to the rest of the descriptor, even without
       knowing the optional "descriptor-cookie", as described below.) 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. It can only be created by the hidden service
       and its clients, but the "signature" below can only be created by
       the service.

       "descriptor-cookie" is an optional secret password of 128 bits that
       is shared between the hidden service provider and its clients.

       "replica" denotes the number of the non-consecutive replica.

        (Each descriptor is replicated on a number of _consecutive_ nodes
         in the identifier ring by making every storing node responsible
         for the identifier intervals starting from its 3rd predecessor's
         ID to its own ID. In addition to that, every service publishes
         multiple descriptors with different descriptor IDs in order to
         distribute them to different places on the ring. Therefore,
         "replica" chooses one of the _non-consecutive_ replicas. -KL)

       The "time-period" changes periodically depending on the global time and
       as a function of "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.

     "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 NL auth-data ... reserved]

           [At start, 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.

       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 NL auth-data ... reserved]

           [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.

        (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.

1.2.1. 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.

1.3. 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 hidden service
   introduction.  To establish the introduction, Bob sends a
   RELAY_ESTABLISH_INTRO cell, containing:

        KL   Key length                             [2 octets]
        PK   Introduction 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.

   Bob's OP does not include its own public key in the RELAY_ESTABLISH_INTRO
   cell, but the public key of a freshly generated introduction key pair.
   The OP also includes these fresh public keys 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.

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.

   Bob's OP publishes v2 descriptors to a changing subset of all v2 hidden
   service directories. 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 1.2. 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 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.

   Similarly 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/<z>", where z is replaced with the encoding of the
   descriptor ID. 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'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]
       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]

   PK_ID is the hash of Bob's public key.  RP is NUL-padded and
   terminated. In version 0, it 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_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.

   If Alice has downloaded a v2 descriptor, she uses the contained public
   key ("service-key") instead of Bob's public key to create the
   RELAY_INTRODUCE1 cell as described above.

1.8.1. Other introduction formats we don't use.

    We briefly speculated about using the following format for the
    "encrypted to Bob's PK" part of the introduction, but no Tors have
    ever generated these.

          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]

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. Authentication and authorization.

Foo.

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/<z>",
   where z is replaced with the encoding of the descriptor ID.