> On 9 Dec 2017, at 11:29, isis agora lovecruft <i...@torproject.org> wrote: > > Hello all, > > What: A proposal discussion meeting for prop#249 "Allow CREATE cells with >> 505 bytes of handshake data". [0] > > Who: This proposal is likely of interest to those hoping to integrate newer, > non-ECC-based, circuit-layer handshakes into the Tor protocol. > > When: Next week, on Monday or Tuesday (or Wednesday, for some timezones). > If you'd like to attend, please vote on a time here: > https://doodle.com/poll/v924cbt2at3rzvc9 > > Where: irc.oftc.net #tor-meeting > > [0]: > https://gitweb.torproject.org/torspec.git/tree/proposals/249-large-create-cells.txt > > Meeting Preparation Materials > ============================= > > The following is meant for attendees to refresh before the meeting. Please > feel free to respond with further summary and/or open questions/concerns. > > Proposal Summary > ~~~~~~~~~~~~~~~~ > The proposal outlines two new cell types, CREATE2V and CREATED2V, which are > variable-length and (like their CREATE2/CREATED2 counterparts) are sent > encapsulated in EXTEND2 cells. Due to their variable-length, however, if a > CREATE(D)2V cell's HDATA is larger than the standard allotment of 505 bytes, > these new cells are fragmented across multiple EXTEND2 cells. > > Open Questions/Concerns > ~~~~~~~~~~~~~~~~~~~~~~~ > > 1. Since CREATE2V cells are used for handshakes, and several newer, > post-quantum primitives have asymmetric payloads for the client versus > server directions, should we require that the CREATE(D)2V padding be used > to equalise the number of bytes sent in each direction?
Should we pad to a certain multiple of cells, to hide which handshake is being used? I think this matters in the onion service case, because it allows non-rendezvous points to work out the orientation of the client to service circuit using the handshake. But relays can already do this probabilistically, using cell response latency. But otherwise, I can't see how it matters unless we are trying to hide the type of handshake: circuit orientation is known by each hop, because they each know the direction they sent and received EXTENDs in. > 2. Should we randomise the bytes in the padding? (Currently, as proposed, > we require all-zero padding.) > > 3. Should we do anything more future-proofed w.r.t. the future possibility > of using an alternate (non-stateful, e.g. not TCP) transport? > (Currently, the proposal relies heavily upon the transport-layer to > provide delivery guarantee and, perhaps more important, ordering.) > > 4. Shoule we, for hybrid handshakes (handshakes which use multiple separate > primitives to derive shared secrets, e.g. ECDH+RLWE or ECDH+SIDH, by > conducting each handshake separately and composing their respective > resulting shared secrets), design some mechanism where, if a party only > supports say the ECDH portion of the hybrid handshake and not the RLWE > part, then they proceed with the portion they understand? For example, a > client sends their portion of a ECDH+RLWE handshake to a relay which only > understands ECDH, so the relay responds with only ECDH and they continue. > This is mostly a difference in subprotocol versioning for handshakes, > that is, an ECDH+RLWE handshake, rather than being "handshake type 5" or > whatever, would be "handshake type 2 AND/OR handshake type 5". > > 5. As written, the proposal doesn't specify a maximum (or minimum) size of > handshake data. However, the max is somewhat limited by the number of > allowed RELAY_EARLY cells; maximum handshake data is then limited to > 462+(7*492)=3906 bytes. More precisely, assuming all handshakes are the same size, and counting hops using the number of relays, maximum outbound handshake data is: * unlimited for the first hop * limited to 8 cells for the second hop in a two-hop circuit (bandwidth scanners use two-hop circuits) * limited to 8/2 = 4 cells for the second and subsequent hops in a three-hop circuit * limited to 8/(3,4) = 2 cells for the second and subsequent hops in a four or five-hop circuit (4 hops can happen with circuit cannibalisation, and 4 hops is the minimum for onion service client intro or service rend, and some vanguard circuits. 5-8 hops can happen with onion service client intro extends) * limited to 8/(5,6,7,8) = 1 cell for the second and subsequent hops in a six-hop (or greater) circuit (onion service client intro extends) There is no limit on inbound handshake client data. Can we invert the client/server directions in any of these handshakes? (I imagine we can't, if the server declares public key material.) Do vanguards every require 5 hops? Does this proposal mean that we have to limit the number of client intro extends on the same circuit? Should we increase the number of RELAY_EARLY cells sent on legacy circuits to hide these larger handshake sizes? T _______________________________________________ tor-dev mailing list tor-dev@lists.torproject.org https://lists.torproject.org/cgi-bin/mailman/listinfo/tor-dev
