[tor-dev] Proposal 249: Allow CREATE cells with >505 bytes of handshake data

[Nick Mathewson]

Filename: 249-large-create-cells.txt
Title: Allow CREATE cells with >505 bytes of handshake data
Authors: Nick Mathewson
Created: 23 July 15
Status: Draft

1. Summary

   There have been multiple proposals over the last year or so for
   adding post-quantum cryptography to Tor's circuit extension
   handshakes.  (See for example https://eprint.iacr.org/2015/008 or
   https://eprint.iacr.org/2015/287 .) These proposals share the property
   that the request and reply for a handshake message do not fit in a
   single RELAY cell.

   In this proposal I describe a new CREATE2V cell for handshakes that
   don't fit in a 505-byte CREATE2 cell's HDATA section, and a means for
   fragmenting these CREATE2V cells across multiple EXTEND2 cells.  I
   also discuss replies, migration, and DoS-mitigation strategies.

2. CREATE2V and CREATED2V

   First, we add two variable-width cell types, CREATE2V and CREATED2V.

   These cell formats are nearly the same as CREATE2 and CREATED2.  (Here
   specified using Trunnel.)

     struct create2v_cell_body {
        /* Handshake type */
        u16 htype;
        /* Length of handshake data */
        u16 hlen;
        /* Handshake data */
        u8 hdata[hlen];
        /* Padding data to be ignored */
        u8 ignored[];
     };

     struct created2v_cell_body {
        /* Handshake replay length */
        u16 hlen;
        /* Handshake reply data */
        u8 hdata[hlen];
        /* Padding data to be ignored */
        u8 ignored[];
     };

   The 'ignored' fields, which extend to the end of the variable-length
   cells, are reserved.  Initiators MAY set them to any length, but MUST
   fill them with zero-valued bytes.  Responders MUST ignore them,
   regardless of what they contain.  When a CREATE2V cell is generated
   in response to a set of EXTEND2 cells, these fields are set by the
   relay that receives the EXTEND2 cells.

   (The purpose of the 'ignored' fields here is future-proofing and
   padding.)

3. Fragmented EXTEND2 cells

   Without changing the current EXTEND2 cell format, we change its
   semantics:

   If the 'HLEN' field in an EXTEND2 cell describes a handshake data
   section that would be too long to fit in the EXTEND2 cell's payload,
   the handshake data of the EXTEND2 cell is to be continued in one or
   more subsequent EXTEND2 cells.  These subsequent cells MUST have zero
   link specifiers, handshake type 0xFFFF, and handshake data length
   field set to zero.

   Similarly, if the 'HLEN' field in an EXTENDED2 cell would be too long
   to fit into the EXTENDED2 cell's payload, the handshake reply data of
   the EXTENDED2 cell is to be continued in one or more subsequent
   EXTENDED2 cells.  These subsequent cells must have the handshake data
   length field set to zero.

   These cells must be sent on the circuit with no intervening cells.
   If any intervening cells are received, the receiver SHOULD destroy
   the circuit.

4. Example

   So for example, if we are a client, and we need to send a 2000-byte
   handshake to extend a circuit from relay X to relay Y, we might send
   cells as follows:

      EXTEND2 {
        nspec = 2;
          { node ID for Y, taking 22 bytes. }
          { node address for Y, taking 8 bytes }
        htype = {whatever the handshake type is.}
        hlen = 2000
        hdata = { the first 462 bytes of the handshake }
      }
      EXTEND2 {
        nspec = 0;
        htype = 0xffff;
        hlen = 0;
        hdata = { the next 492 bytes of the handshake }
      }
      EXTEND2 {
        nspec = 0;
        htype = 0xffff;
        hlen = 0;
        hdata = { the next 492 bytes of the handshake }
      }
      EXTEND2 {
        nspec = 0;
        htype = 0xffff;
        hlen = 0;
        hdata = { the next 492 bytes of the handshake }
      }
      EXTEND2 {
        nspec = 0;
        htype = 0xffff;
        hlen = 0;
        hdata = { the final 62 bytes of the handshake }
      }

   Upon receiving this last cell, the relay X would send a create2v cell
   to Y, containing the entire handshake.

5. Migration

   We can and should implement the EXTEND2 fragmentation feature before
   we implement anything that uses it.  If we can get it widely deployed
   before it's needed, we can use the new handshake types whenever both
   of the involved relays support this proposal.

   Clients MUST NOT sent fragmented EXTEND2 cells to relays that don't
   support them, since this would cause them to close the circuit.

   Relays MAY send CREATE2V and CREATED2V cells to relays that don't
   support them, since unrecognized cell types are ignored.

6. Resource management issues

   This feature requires relays and clients to buffer EXTEND2 cell
   bodies for incoming cells until the entire CREATE2V/CREATED2V body
   has arrived.  To avoid memory-related denial-of-service attacks,
   the buffers allocated for this data need to be counted against the
   total data usage of the circuit.



Appendix A. A rejected idea for migration

   In section 5 above, I gave up on the idea of allowing relay A to
   extend to relay B with a large CREATE cell when relay A does not
   support this proposal.

   There are other ways to do this, but they are impressively kludgey.
   For example, we could have a fake CREATE cell for new handshake types
   that always elicits a "yes, keep going!" CREATED cell.  Then the
   client could send the rest of the handshake and receive the rest of
   the CREATED cell as RELAY cells inside the circuit.

   This design would add an extra round-trip to circuit extension
   whenever it was used, however, and would violate a number of Tor's
   assumptions about circuits (e.g., by having half-created circuits,
   where authentication hasn't actually been performed).  So I'm
   guessing we shouldn't do that.