Good morning Christian, I am nowhere near a mathematician, thus, cannot countercheck your expertise here (and cannot give a counterproposal thusly).
But I want to point out the below scenarios: 1. C is the payer. He is in contact with an unknown payee (who in reality is E). E provides the onion-wrapped route D->E with ephemeral key and other data necessary, as well as informing C that D is the rendez-vous point. Then C creates a route from itself to D (via channel C->D or via C->A->D). 2. B is the payer. He knows the entire route B->C->D->E and knows that payee is C. Unfortunately the C<->D channel is low capacity or down or etc etc. At C, B has provided the onion-wrapped route D->E with ephemeral key and other data necessary, as well as informing to C that D is the next node. Then C either pays via C->D or via C->A->D. Even if there is an off-by-one error in our thinking about rendez-vous nodes, could it not be compensated also by an off-by-one in the link-level payment splitting via intermediary rendez-vous node? In short, D is the one that switches keys instead of A. The operation of processing a hop would be: 1. Unwrap the onion with current ephemeral key. 2. Dispatch based on realm byte. 2.1. If realm byte 0: 2.1.1. Normal routing behavior, extract HMAC, etc etc 2.2. If realm byte 2 "switch ephemeral keys": 2.2.1. Set current ephemeral key to bytes 1 -> 32 of packet. 2.2.2. Shift onion by one hop packet. 2.2.3. Goto 1. Would that not work? (I am being naive here, as I am not a mathist and I did not understand half what you wrote, sorry) Then at C, we have the onion from D->E, we also know the next ephemeral key to use (we can derive it since we would pass it to D anyway). It rightshifts the onion by one, storing the next ephemeral key to the new hop it just allocated. Then it encrypts the onion using a new ephemeral key that it will use to generate the D<-A<-C part of the onion. Regards, ZmnSCPxj Sent with ProtonMail Secure Email. ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐ On Tuesday, November 13, 2018 11:45 AM, Christian Decker <decker.christ...@gmail.com> wrote: > Great proposal ZmnSCPxj, but I think I need to raise a small issue with > it. While writing up the proposal for rendez-vous I came across a > problem with the mechanism I described during the spec meeting: the > padding at the rendez-vous point would usually zero-padded and then > encrypted in one go with the shared secret that was generated from the > previous ephemeral key (i.e., the one before the switch). That ephemeral > key is not known to the recipient (barring additional rounds of > communication) so the recipient would be unable to compute the correct > MACs. There are a number of solutions to this, basically setting the > padding to something that the recipient could know when generating its > half onion. > > My current favorite goes like this: > > 1. Rendez-vous RV receives an onion, performs ECDH like normal to get > the shared secret, decrypts its payload, simultaneously encrypts > the padding. > > 2. It extracts its per-hop payload and shifts the entire packet over > (shift its payload out and the newly generated padding in) > > 3. It then notices that it should perform an ephemeral key switch, now > deviating from the normal protocol (which would just be to generate > the new ephemeral key, serialize and forward) > 3.1. It zero-fills the padding that it just added (so we are in a > state that the recipient knew when generating its partial onion > 3.2 It performs ECDH with the switched in ephemeral key to get a new > shared secret that which is then used to unwrap one additional > layer of encryption, and most importantly encrypt the padding so > the next hop doesn't see the zero-filled padding. > 3.3 Only then will it generate the new ephemeral key for the next > hop, based on the switched in ephemeral key and the newly > generated shared secret, serialize the packet and forward it. > > This has the advantage of reusing all the existing machinery but > assembling it a bit differently, by adding a little detour when > generating the next onion. It involves one additional ECDH at the > rendez-vous, one ChaCha20 encryption and one scalar multiplication to > generate the next ephemeral keys. It does not need more space than the > single ephemeral key in the per-hop payload. > > And now for the reason that I write this as a reply to your post: with > this scheme it is not possible for C to find an ephemeral key that would > end up identical to the one that D would require to decrypt the onion > correctly. This would not be an issue if D is informed about this split > and would basically accept whatever it gets, but that kind of defeats > the transparency that you were going for with your proposal. > > I'm open for other proposals but I currently can't think of a way to > make sure that a) the recipient can deterministically generate the same > padding that RV will generate, and b) hide the fact that RV was indeed a > rendez-vous point (e.g., by leaving the padding be a well known > constant). > > Sorry for this problem, I had a mental off-by-one at the meeting that I > hadn't considered, the solution should work, but it makes this kind of > things a bit harder. > > Cheers, > Christian > > ZmnSCPxj via Lightning-dev lightning-dev@lists.linuxfoundation.org > > > writes: > > > Good morning list, > > As was discussed directly in summit, we accept link-lvel payment splitting > > (scid is not binding), and provisionally accept rendez-vous routing. > > It strikes me, that even if your node has only a single channel to the next > > node (c-lightning), it is possible, to still perform link-level payment > > splitting/re-routing. > > For instance, consider this below graph: > > > > E<---D--->C<---B > > ^ / > > | / > > |L > > A > > > > > > In the above, B requests a route from B->C->D->E. > > However, C cannot send to D, since the channel direction is saturated in > > favor of D. > > Alternately, C can route to D via A instead. It holds the (encrypted) route > > from D to E. It can take that sub-route and treat it as a partial > > route-to-payee under rendez-vous routing, as long as node A supports > > rendez-vous routing. > > This can allow re-routing or payment splitting over multiple hops. > > Even though C does not know the number of remaining hops between D and the > > destination, its alternative is to earn nothing anyway as its only > > alternative is to fail the routing. At least with this, there is a chance > > it can succeed to send the payment to the final destination. > > Regards, > > ZmnSCPxj > > > > Lightning-dev mailing list > > Lightning-dev@lists.linuxfoundation.org > > https://lists.linuxfoundation.org/mailman/listinfo/lightning-dev _______________________________________________ Lightning-dev mailing list Lightning-dev@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/lightning-dev
