Conner Fromknecht [ARCHIVE] on Nostr: 📅 Original date posted:2018-11-13 📝 Original message: Good morning all, Taking ...
📅 Original date posted:2018-11-13
📝 Original message:
Good morning all,
Taking a step back—even if key switching can be done mathematically, it seems
dubious that we would want to introduce re-routing or rendezvous routing in this
manner. If the example provided _could_ be done, it would directly violate the
wrap-resistance property of the ideal onion routing scheme defined in [1]. This
property is proven for Sphinx in section 4.3 of [2]. Schemes like HORNET [3]
support rendezvous routing and are formally proven in this model. Seems this
would be the obvious path forward, given that we've already done a considerable
amount of work towards implementing HORNET via Sphinx.
Cheers,
Conner
[1] A Formal Treatment of Onion Routing:
https://www.iacr.org/cryptodb/archive/2005/CRYPTO/1091/1091.pdf
[2] Sphinx: https://cypherpunks.ca/~iang/pubs/Sphinx_Oakland09.pdf
[3] HORNET: https://arxiv.org/pdf/1507.05724.pdf
On Mon, Nov 12, 2018 at 8:47 PM ZmnSCPxj via Lightning-dev
<lightning-dev at lists.linuxfoundation.org> wrote:
>
> 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
> > 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 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
>
>
> _______________________________________________
> Lightning-dev mailing list
> Lightning-dev at lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/lightning-dev
📝 Original message:
Good morning all,
Taking a step back—even if key switching can be done mathematically, it seems
dubious that we would want to introduce re-routing or rendezvous routing in this
manner. If the example provided _could_ be done, it would directly violate the
wrap-resistance property of the ideal onion routing scheme defined in [1]. This
property is proven for Sphinx in section 4.3 of [2]. Schemes like HORNET [3]
support rendezvous routing and are formally proven in this model. Seems this
would be the obvious path forward, given that we've already done a considerable
amount of work towards implementing HORNET via Sphinx.
Cheers,
Conner
[1] A Formal Treatment of Onion Routing:
https://www.iacr.org/cryptodb/archive/2005/CRYPTO/1091/1091.pdf
[2] Sphinx: https://cypherpunks.ca/~iang/pubs/Sphinx_Oakland09.pdf
[3] HORNET: https://arxiv.org/pdf/1507.05724.pdf
On Mon, Nov 12, 2018 at 8:47 PM ZmnSCPxj via Lightning-dev
<lightning-dev at lists.linuxfoundation.org> wrote:
>
> 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
> > 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 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
>
>
> _______________________________________________
> Lightning-dev mailing list
> Lightning-dev at lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/lightning-dev