tor/doc/spec/proposals/130-v2-conn-protocol.txt

Filename: 130-v2-conn-protocol.txt
Title: Version 2 Tor connection protocol
Version: $Revision$
Last-Modified: $Date$
Author: Nick Mathewson
Created: 2007-10-25
Status: Finished

Overview:

  This proposal describes the significant changes to be made in the v2
  Tor connection protocol.

  This proposal relates to other proposals as follows:

    It refers to and supersedes:
       Proposal 124: Blocking resistant TLS certificate usage
    It refers to aspects of:
       Proposal 105: Version negotiation for the Tor protocol


  In summary, The Tor connection protocol has been in need of a redesign
  for a while.  This proposal describes how we can add to the Tor
  protocol:

     - A new TLS handshake (to achieve blocking resistance without
       breaking backward compatibility)
     - Version negotiation (so that future connection protocol changes
       can happen without breaking compatibility)
     - The actual changes in the v2 Tor connection protocol.

Motivation:

  For motivation, see proposal 124.

Proposal:

0. Terminology

  The version of the Tor connection protocol implemented up to now is
  "version 1".  This proposal describes "version 2".

  "Old" or "Older" versions of Tor are ones not aware that version 2
  of this protocol exists;
  "New" or "Newer" versions are ones that are.

  The connection initiator is referred to below as the Client; the
  connection responder is referred to below as the Server.

1. The revised TLS handshake.

  For motivation, see proposal 124.  This is a simplified version of the
  handshake that uses TLS's renegotiation capability in order to avoid
  some of the extraneous steps in proposal 124.

  The Client connects to the Server and, as in ordinary TLS, sends a
  list of ciphers.  Older versions of Tor will send only ciphers from
  the list:
    TLS_DHE_RSA_WITH_AES_256_CBC_SHA
    TLS_DHE_RSA_WITH_AES_128_CBC_SHA
    SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA
    SSL_DHE_DSS_WITH_3DES_EDE_CBC_SHA
  Clients that support the revised handshake will send the recommended
  list of ciphers from proposal 124, in order to emulate the behavior of
  a web browser.

  If the server notices that the list of ciphers contains only ciphers
  from this list, it proceeds with Tor's version 1 TLS handshake as
  documented in tor-spec.txt.

  (The server may also notice cipher lists used by other implementations
  of the Tor protocol (in particular, the BouncyCastle default cipher
  list as used by some Java-based implementations), and whitelist them.)

  On the other hand, if the server sees a list of ciphers that could not
  have been sent from an older implementation (because it includes other
  ciphers, and does not match any known-old list), the server sends a
  reply containing a single connection certificate, constructed as for
  the link certificate in the v1 Tor protocol.  The subject names in
  this certificate SHOULD NOT have any strings to identify them as
  coming from a Tor server.  The server does not ask the client for
  certificates.

  Old Servers will (mostly) ignore the cipher list and respond as in the v1
  protocol, sending back a two-certificate chain.

  After the Client gets a response from the server, it checks for the
  number of certificates it received.  If there are two certificates,
  the client assumes a V1 connection and proceeds as in tor-spec.txt.
  But if there is only one certificate, the client assumes a V2 or later
  protocol and continues.

  At this point, the client has established a TLS connection with the
  server, but the parties have not been authenticated: the server hasn't
  sent its identity certificate, and the client hasn't sent any
  certificates at all.  To fix this, the client begins a TLS session
  renegotiation.  This time, the server continues with two certificates
  as usual, and asks for certificates so that the client will send
  certificates of its own.  Because the TLS connection has been
  established, all of this is encrypted.  (The certificate sent by the
  server in the renegotiated connection need not be the same that
  as sentin the original connection.)

  The server MUST NOT write any data until the client has renegotiated.

  Once the renegotiation is finished, the server and client check one
  another's certificates as in V1.  Now they are mutually authenticated.

1.1. Revised TLS handshake: implementation notes.

  It isn't so easy to adjust server behavior based on the client's
  ciphersuite list.  Here's how we can do it using OpenSSL.  This is a
  bit of an abuse of the OpenSSL APIs, but it's the best we can do, and
  we won't have to do it forever.

  We can use OpenSSL's SSL_set_info_callback() to register a function to
  be called when the state changes.  The type/state tuple of
     SSL_CB_ACCEPT_LOOP/SSL3_ST_SW_SRVR_HELLO_A
  happens when we have completely parsed the client hello, and are about
  to send a response.  From this callback, we can check the cipherlist
  and act accordingly:

     * If the ciphersuite list indicates a v1 protocol, we set the
       verify mode to SSL_VERIFY_NONE with a callback (so we get
       certificates).

     * If the ciphersuite list indicates a v2 protocol, we set the
       verify mode to SSL_VERIFY_NONE with no callback (so we get
       no certificates) and set the SSL_MODE_NO_AUTO_CHAIN flag (so that
       we send only 1 certificate in the response.

  Once the handshake is done, the server clears the
  SSL_MODE_NO_AUTO_CHAIN flag and sets the callback as for the V1
  protocol.  It then starts reading.

  The other problem to take care of is missing ciphers and OpenSSL's
  cipher sorting algorithms. The two main issues are a) OpenSSL doesn't
  support some of the default ciphers that Firefox advertises, and b)
  OpenSSL sorts the list of ciphers it offers in a different way than
  Firefox sorts them, so unless we fix that Tor will still look different
  than Firefox.
  [XXXX more on this.]


1.2. Compatibility for clients using libraries less hackable than OpenSSL.

  As discussed in proposal 105, servers advertise which protocol
  versions they support in their router descriptors.  Clients can simply
  behave as v1 clients when connecting to servers that do not support
  link version 2 or higher, and as v2 clients when connecting to servers
  that do support link version 2 or higher.

  (Servers can't use this strategy because we do not assume that servers
  know one another's capabilities when connecting.)

2. Version negotiation.

  Version negotiation proceeds as described in proposal 105, except as
  follows:

   * Version negotiation only happens if the TLS handshake as described
     above completes.

   * The TLS renegotiation must be finished before the client sends a
     VERSIONS cell; the server sends its VERSIONS cell in response.

   * The VERSIONS cell uses the following variable-width format:
         Circuit  [2 octets; set to 0]
         Command  [1 octet; set to 7 for VERSIONS]
         Length   [2 octets; big-endian]
         Data     [Length bytes]

     The Data in the cell is a series of big-endian two-byte integers.

   * It is not allowed to negotiate V1 conections once the v2 protocol
     has been used.  If this happens, Tor instances should close the
     connection.

3. The rest of the "v2" protocol

   Once a v2 protocol has been negotiated, NETINFO cells are exchanged
   as in proposal 105, and communications begin as per tor-spec.txt.
   Until NETINFO cells have been exchanged, the connection is not open.