[Lightning-dev] Sphinx Rendezvous Update

Bastien TEINTURIER bastien at acinq.fr
Tue Feb 25 16:24:29 UTC 2020


Hi Christian,

This is great, thanks for sharing!
What's really nice with your proposal is that there is effectively no onion
payload for `RV` in the partial onion, which frees up more space than mine.

I believe this makes it quite usable in Bolt 11 invoices, without blowing up
the size of the QR code (but more experimentation is needed on that).

As an example such an onion, with 5 legacy hops (65 byte each) results
> in a 325 + 66 bytes onion, and we save 975 bytes.


While having flexibility when choosing the length of the prefill stream
feels nice,
wouldn't it be safer to impose a fixed size to avoid any kind of heuristic
at `RV`
to try to guess how many hops there are between him and the recipient?

Compute a shared secret using a random ephemeral private key and
> `RV`s public key, and then generate a prefill-key


While implementing, I felt that the part about the shared secret used to
generate
the prefill stream is a bit blurry (your proposal on Github doesn't phrase
it the
same way). I think it's important to stress that this secret is derived
from both
`ephkey` and `RV`'s private key, so that `RV+1` can't compute the same
stream.

Another thing that may be worth mentioning is error forwarding. Since the
recipient generated the onion, `RV` won't have the shared secrets (that's by
design). So it's expected that payment errors won't be readable by `RV`, but
it's probably a good idea if `RV` returns an indication to the sender that
the
payment failed *after* the rendezvous point.

An important side-note is that your proposal is really quick and simple to
implement
from the existing Sphinx code. I have made ASCII diagrams of the scheme
(see [1]).
This may help readers visualize it more easily.

It still has the issue that each hop's amount/cltv is fixed at invoice
generation
time by the recipient. That means MPP cannot be used, and if any channel
along the
path updates their fee the partial onion becomes invalid (unless you
overpay the fees).

Trampoline should be able to address that since it provides more freedom to
each
trampoline node to find an efficient way to forward to the next trampoline.
It's not yet obvious to me how I can mix these two proposals to make it
work though.
I'll spend more time experimenting with that.

Thanks,
Bastien

[1]
https://gist.github.com/t-bast/ab42a7f52eb2e73105557957c8359601#Christian

Le lun. 24 févr. 2020 à 19:22, Christian Decker <decker.christian at gmail.com>
a écrit :

> Hi Bastien,
>
> seems you were a bit quicker than I was with my writeup of my
> proposal. I came up with a scheme that allows us to drop a large part of
> the partial onion, so that it can indeed fit into an outer onion, and
> the rendez-vous node RV can re-construct the original packet from the
> included data [1].
>
> The construction comes down to initializing the part of the routing info
> string that is not going to be used, in such a way that the incremental
> unwrappings at the nodes in the partial onion cancels out. Like you
> mentioned in your mail it comes down extending the filler generation to
> also cover the unused part and then applying all the encryption streams
> xored to the unused space. By doing this we get the middle part of the
> onion consisting of only 0x00 bytes.
>
> I then decided to apply an additional ChaCha20 stream to this prefill,
> such that the onion will not consist of mostly 0x00 bytes which would be
> a dead giveaway to `RV+1` that `RV` was a rendez-vous node.
>
> The process for the partial onion creator boils down to:
>
>  - Compute a path from `RV` of its choice to recipient `R`.
>  - Compute a shared secret using a random ephemeral private key and
>   `RV`s public key, and then generate a prefill-key
>  - Compute the prefill by combining the correct substrings of the
>    encryption streams for the nodes along the path, then add the
>    ChaCha20 stream keyed with the prefill-key.
>  - Wrap the onion, including payloads for each of the nodes along path
>    `RV` to `R`
>  - Trim out the unused space, which now will match the obfuscation
>    stream generated with the prefill-key
>
> As an example such an onion, with 5 legacy hops (65 byte each) results
> in a 325 + 66 bytes onion, and we save 975 bytes. See [2] for an example
> of how this looks like.
>
> The sender `S` then just does the following:
>
>  - Compute a route from `S` to `RV`
>  - Build an onion with the route, specifying the trimmed partial onion
>    as payload, along with the usual parameters, for `RV`
>  - Initiate payment with the constructed onion
>
> Upon receiving an incoming HTLC with a partial onion the rendez-vous
> node `RV` then just does the following:
>
>  - Verify all parameters as usual
>  - Extract the partial onion
>  - Use the ephemeral key from the partial onion to generate the shared
>    secret and the prefill key
>  - Generate the prefill stream and insert it in the correct place,
>    before the HMAC. This reconstitutes the original routing packet
>  - Swap out the original onion with the reconstituted onion and forward.
>
> My writeup [1] is an early draft, but I wanted to get it out early to
> give the discussion a basis to work off. I'll revisit it a couple of
> times before opening a PR, but feel free to shout at me if I have
> forgotten to consider something :-)
>
> Cheers,
> Christian
>
> [1]
> https://github.com/lightningnetwork/lightning-rfc/blob/rendez-vous/proposals/0001-rendez-vous.md
> [2] https://gist.github.com/cdecker/ec06452bc470749d9f6d2de73651c5fd
>
> Bastien TEINTURIER via Lightning-dev
> <lightning-dev at lists.linuxfoundation.org> writes:
> > Good morning list,
> >
> > After exploring decoys [1], which is a cheap way of doing route blinding,
> > I'm turning back to exploring rendezvous.
> > The previous mails on the mailing list mentioned that there was a
> > technicality
> > to make the HMACs check out, but didn't provide a lot of details.
> > The issue is that the filler generation needs to take into account some
> hops
> > that will be added *later*, by the payer.
> >
> > However it is quite easy to work-around, with a few space trade-offs.
> > Let's consider a typical rendezvous setup, where Alice wants to be paid
> via
> > rendezvous Bob, and Carol wants to pay that invoice:
> >
> > Carol -> ... -> Bob -> ... -> Alice
> >
> > If Alice knows how many bytes Carol is going to use for her part of the
> > onion
> > payloads, Alice can easily take them into account when generating her
> > filler by
> > pre-pending the same amount of `0` bytes. It seems reasonable to impose a
> > fixed
> > number of onion bytes for each side of the rendezvous (650 each?) so
> Alice
> > would
> > know that amount.
> >
> > When Carol completes the onion with her part of the route, she simply
> needs
> > to
> > generate filler data for her part of the route following the normal
> Sphinx
> > protocol
> > and apply it to the onion she found in the invoice.
> >
> > But the tricky part is that she needs to give Bob a way of generating the
> > same
> > filler data to unapply it. Then all HMACs correctly check out.
> >
> > I see two ways of doing that:
> >
> > * Carol simply sends that filler (650 bytes), probably via a TLV in
> > `update_add_htlc`.
> > This means every intermediate hop needs to forward that, which is painful
> > and
> > potentially leaking too much data.
> > * Carol provides Bob with the rho keys used to generate her filler, and
> the
> > length
> > used by each hop. This leaks to Bob an upper bound on the number of hops
> > and the
> > number of bytes sent to each hop.
> >
> > Since shift-and-xor kind of crypto is hard to read as equations, but very
> > easy to
> > read as diagrams, I spent a bit of time doing beautiful ASCII art [2].
> > Don't hesitate
> > to have a look at it to find more details about how that works. You can
> > also print
> > that on t-shirts to look fancy at conferences. I also have some sample
> code
> > working
> > in eclair [3] for those who can read Scala without getting headaches.
> >
> > Are there other tricks we can use to reconcile both sides of the onion at
> > Bob's?
> > Maybe cdecker (or someone else) has an ace up his sleeve for me there? :)
> >
> > One important thing to note is that rendezvous on normal onions will be
> > costly to
> > integrate into invoices: it takes 1366 bytes to include one onion, and if
> > we want
> > to handle route failures or let the sender use multi-part, we will need
> to
> > have a
> > handful of pre-encrypted onions in the invoice (hence a few kB, which may
> > not be
> > practical for QR codes).
> >
> > But I did mention before that doing rendezvous on the trampoline onion
> > could have
> > better properties [4]. When doing that, having Carol transmit her filler
> > data only
> > to Bob, via the outer onion payload becomes practical and doesn't leak
> > information.
> > Multi-part would work with a single trampoline onion in the invoice (~500
> > bytes),
> > because nodes can do MPP between trampoline nodes thanks to the
> > onion-in-onion
> > construction. We simply need to decide the size of the trampoline onion
> to
> > allow
> > each side of the rendezvous to be able to insert a number of hops we're
> > comfortable
> > with. You can find more details in the "Rendezvous on a trampoline"
> section
> > of [2].
> >
> > I'm really interested in other approaches to making rendezvous work with
> > the HMACs
> > correctly checking out. If people on this list have drafts, intuitions or
> > random
> > thoughts about possible constructions, please share them, I'd be happy to
> > dive into
> > them to explore alternatives to the one I found, hoping we can make this
> > work and
> > provide this feature to our users in the near future.
> >
> > A small side-note on Hornet. Hornet does offer many features that I
> believe
> > we will
> > want in Lightning in the future. It may seem that doing a custom
> rendezvous
> > scheme
> > is a waste of time since we'll ditch it once/if we implement Hornet.
> While
> > that is
> > true in the long run, I believe that if we're able to find a rendezvous
> > scheme that
> > isn't too much work to implement, it makes sense to have something
> > available soon-ish.
> > Hornet will likely be a longer-term effort that we won't get as soon as
> > we'd like
> > (especially since it will probably require a network-wide update). But
> who
> > knows, maybe
> > we may see that we are trying to create many features that are already
> > built into Hornet
> > (rendezvous, directed message support, etc) and will decide to implement
> > Hornet sooner
> > than expected?
> >
> > Cheers,
> > Bastien
> >
> > [1]
> >
> https://lists.linuxfoundation.org/pipermail/lightning-dev/2020-January/002435.html
> > [2] https://gist.github.com/t-bast/ab42a7f52eb2e73105557957c8359601
> > [3] https://github.com/ACINQ/eclair/tree/sphinx-rendezvous
> > [4]
> >
> https://lists.linuxfoundation.org/pipermail/lightning-dev/2019-October/002237.html
> > _______________________________________________
> > Lightning-dev mailing list
> > Lightning-dev at lists.linuxfoundation.org
> > https://lists.linuxfoundation.org/mailman/listinfo/lightning-dev
>
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