[Lightning-dev] Link-level payment splitting via intermediary rendezvous nodes
ZmnSCPxj at protonmail.com
Tue Nov 13 04:46:53 UTC 2018
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.
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‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Tuesday, November 13, 2018 11:45 AM, Christian Decker <decker.christian at 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
> 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.
> ZmnSCPxj via Lightning-dev lightning-dev at lists.linuxfoundation.org
> > 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 at lists.linuxfoundation.org
> > https://lists.linuxfoundation.org/mailman/listinfo/lightning-dev
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