# [Lightning-dev] Link-level payment splitting via intermediary rendezvous nodes

ZmnSCPxj 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.

Regards,
ZmnSCPxj

Sent with ProtonMail Secure Email.

‐‐‐‐‐‐‐ 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
> 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
>
> 2.  It extracts its per-hop payload and shifts the entire packet over
>
> 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
>     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 at 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 at lists.linuxfoundation.org
> > https://lists.linuxfoundation.org/mailman/listinfo/lightning-dev

```