tor/doc/rend-spec.txt

$Id$

Tor Rendezvous Spec

0. Overview and preliminaries

   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 OR circuits to those
   introduction points, and tells them about his public key.  To
   connect to Bob, Alice first builds an OR circuit to an OR to act as
   her "Rendezvous Point", 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 an OR circuit to her
   RP, and tells it to connect him to Alice.  The RP joints their
   circuits together, and begins relaying cells.  Alice's 'BEGIN'
   cells are received directly by Bob's OP, which responds by
   communication with 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 "tor-spec.txt".  

        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 y.onion:port address.  She opens a
      SOCKS connection to her OP, and requests y.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.]

  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

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 service descriptor, containing:

         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 OR nicknames [variable]
         SIG   Signature of above fields             [variable]

   KL is the length of PK, in octets.  (Currently, KL must be 128.)
   TS is the number of seconds elapsed since Jan 1, 1970.

   [Shouldn't the nicknames be hostname:port's instead?  That way, Alice's
   directory servers don't need to know Bob's chosen introduction points.
   Not important now, but essential if we ever have a non-total-knowledge
   design. -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]

   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.

1.4. Bob's OP advertises his server descriptor

   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 the URL
   '/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 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 24 hours, Bob's OP
   uploads a fresh descriptor.

1.5. Alice receives a y.onion address

   When Alice receives a pointer to a location-hidden service, it is as a
   hostname of the form "y.onion", where y 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', taking H' 5 bits at
            a time, and mapping each 5-bit value to a character as follows:
               0..25 map to the characters 'a'...'z', respectively.
              26..31 map to the characters '0'...'5', respectively.

   (We only use 80 bits instead of the 160 bits from SHA1 because we don't
   need to worry about man-in-the-middle attacks, and because it will make
   handling the url's more convenient.)

   [Yes, numbers are allowed at the beginning.  See RFC1123. -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 '/rendezvous/<y>', where '<y> 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 <y>, 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 recognizes 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]

   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.

   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:
          RP     Rendezvous point's nickname  [variable]
          RC     Rendezvous cookie            [20 octets]
          g^x    Diffie-Hellman data, part 1 [128 octetes]

   PK_ID is the hash of Bob's public key.  RP is NUL-terminated.

   The data is encrypted to Bob's PK as follows: Suppose Bob's PK is L octets
   long.  If the data to be encrypted is shorter than L-42, then it is
   encrypted directly (with OAEP padding).  If the data is at least as long
   as L-42, then a randomly generated 16-byte symmetric key is prepended to
   the data, after which the first L-16-42 bytes of the data are encrypted with
   Bob's PK; and the rest of the data is encrypted with the symmetric key.

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.)

   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 build 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.

   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.