tor/doc/rendezvous.txt

                  How to make rendezvous points work
                               1-11Jun2003

0. Overview

   Rendezvous points are an implementation of server anonymity /
   location-hidden servers in the onion routing network.  There are
   three components needed for rendezvous points:

   A) A means for the client ("Alice") to tell a server ("Bob") where
      to contact her in order to establish a connection. (Introduction)
   B) A means for Bob to contact Alice to actually establish the
      connection, and for them to communicate later. (Meeting)
   C) Necessary glue code so that Alice can view webpages on a
      location-hidden webserver, and Bob can run a location-hidden
      server with minimal invasive changes. (Application)

   We'll tackle these in order.  In all cases, we'll assume that both
   Alice and Bob run local OPs.

1. Introduction service

   Bob wants to learn about client requests for communication, but
   wants to avoid responding unnecessarily to unauthorized clients.
   Bob's proxy opens a circuit, and tells some onion router on that
   circuit to expect incoming connections, and notify Bob of them.

   When establishing such an introduction point, Bob provides the onion
   router with a public "introduction" key.  The hash of this public
   key uniquely identifies Bob, and prevents anybody else from
   usurping Bob's introduction point in the future.  Additionally, Bob
   can use the same public key to establish an introduction point on
   another OR, and Alice can still be confident that Bob is the same
   server.

   (The set-up-an-introduction-point command should come via a
   RELAY_BIND_INTRODUCTION cell. This cell creates a new stream on the
   circuit from Bob to the introduction point.)

   ORs that support introduction run an introduction service on a
   separate port.  When Alice wants to notify Bob of a meeting point,
   she connects (directly or via Tor) to the introduction port, and
   sends the following:

     MEETING REQUEST
        RSA-OAEP encrypted with server's public key:
[20 bytes] Hash of Bob's public key (identifies which Bob to notify)
[ 0 bytes] Initial authentication [optional]
        RSA encrypted with Bob's public key:
[16 bytes] Symmetric key for encrypting blob past RSA
[ 6 bytes] Meeting point (IP/port)
[ 8 bytes] Meeting cookie
[ 0 bytes] End-to-end authentication [optional]
[98 bytes] g^x part 1 (inside the RSA)
[30 bytes] g^x part 2 (symmetrically encrypted)


   The meeting point and meeting cookie allow Bob to contact Alice and
   prove his identity; the end-to-end authentication enables Bob to 
   decide whether to talk to Alice; the initial authentication enables
   the meeting point to pre-screen introduction requests before
   sending them to Bob.  (See 3 for a discussion of meeting points;
   see 2.1 for a proposed authentication mechanism.)

   The authentication steps are the appropriate places for the
   introduction server or Bob to do replay prevention, if desired.

   When the introduction point receives a valid meeting request, it
   sends the portion intended for Bob along the stream
   created by Bob's RELAY_BIND_INTRODUCTION.  Bob then, at his
   discretion, connects to Alice's meeting point.

1.1. An example authentication scheme for introduction services

   Bob makes two short-term secrets SB and SN, and tells the
   introduction point about SN.  Bob gives Alice a cookie consisting
   of A,B,C such that H(A|SB)=B and H(A|SN)=C.  Alice's initial
   authentication is <A,C>; Alice's end-to-end authentication is <A,B>.

   [Maybe] Bob keeps a replay cache of A values, and doesn't allow any
   value to be used twice.  Over time, Bob rotates SB and SN.

   [Maybe] Each 'A' has an expiration time built in to it.

2. Meeting points

   For Bob to actually reply to Alice, Alice first establishes a
   circuit to an onion router R, and sends a RELAY_BIND_MEETING cell
   to that onion router.  The RELAY_BIND_MEETING cell contains a
   'Meeting cookie' (MC) that Bob can use to authenticate to R.  R
   remembers the cookie and associates it with Alice.

   Later, Bob also routes to R and sends R a RELAY_JOIN_MEETING cell with
   the meeting cookie MC.  After this point, R routes all traffic from
   Bob's circuit or Alice's circuit as if the two circuits were joined:
   any RELAY cells that are not for a recognized topic are passed down
   Alice or Bob's circuit.  Bob's first cell to Alice contains g^y.

   To prevent R from reading their traffic, Alice and Bob derive two
   end-to-end keys from g^{xy}, and they each treat R as just another
   hop on the circuit.  (These keys are used in addition to the series
   of encryption keys already in use on Alice and Bob's circuits.)

   Bob's OP accepts RELAY_BEGIN, RELAY_DATA, RELAY_END, and
   RELAY_SENDME cells from Alice.  Alice's OP accepts RELAY_DATA,
   RELAY_END, and RELAY_SENDME cells from Bob.  All RELAY_BEGIN cells
   to Bob must have target IP and port of zero; Bob's OP will redirect
   them to the actual target IP and port of Bob's server.

   Alice and Bob's OPs disallow CREATE or RELAY_EXTEND cells as usual.

3. Application interface

3.1. Application interface: server side

   Bob has a service that he wants to offer to the world but keep its
   location hidden.  He configures his local OP to know about this
   service, including the following data:

     Local IP and port of the service
     Strategy for choosing introduction points
       (for now, just randomly pick among the ORs offering it)
     Strategy for user authentication
       (for now, just accept all users)
     Public (RSA) key (one for each service Bob offers)

   Bob chooses a set of N Introduction servers on which to advertise
   his service.

   We assume the existence of a robust decentralized efficient lookup
   system (call it "DHT"). Bob publishes
     * Bob's Public Key for that service
     * Timestamp
     * Introduction server 0 ... Introduction server N
     (All signed by Bob's Public Key)
   into DHT, indexed by the hash of Bob's Public Key. Bob should
   periodically republish his introduction information with a new
   timestamp (and possibly with new/different introduction servers if
   he wants), so Alice can trust that DHT is giving her an up-to-date
   version.

3.2. Application interface: client side

   We require that the client interface remain a SOCKS proxy, and we
   require that Alice shouldn't have to modify her applications. Thus
   we encode all of the necessary information into the hostname that
   Alice uses (eg when clicking on a url in her browser, etc).

   To establish a connection to Bob, Alice needs to know an Introduction
   point, Bob's PK, and some authentication cookie. Because encoding this
   information into the hostname will be too long for a typical hostname,
   we instead use a layer of indirection. We encode a hash of Bob's PK
   (10 bytes is sufficient since we're not worrying about collisions),
   and also the authentication token (empty for now). Location-hidden
   services use the special top level domain called '.onion': thus
   hostnames take the form x.y.onion where x is the hash of PK, and y
   is the authentication cookie. If no cookie is required, the hostname
   can simply be of the form x.onion. Assuming only case insensitive
   alphanumeric and hyphen, we get a bit more than 6 bits encoded
   per character, meaning the x part of the hostname will be about
   13 characters.

   Alice's onion proxy examines hostnames and recognizes when they're
   destined for a hidden server. If so, it decodes the PK, looks it up in
   the DHT, chooses and connects to a meeting place, chooses and connects
   to one of Bob's introduction servers, and then waits to hear from Bob.